a tube primer for guitar and bass players - Guitar Amplifier Blueprinting

A TUBE PRIMER FOR GUITAR AND BASS PLAYERS 

WHAT TUBES SHOULD I USE IN MY GUITAR OR BASS AMPLIFIER? 

Part 1 of 2 

Guitar Amplifier Blueprinting 

Myles S. Rose www.guitaramplifierblueprinting.com 

THIS DOCUMENT WAS FORMATTED FOR “LANDSCAPE” RATHER THAN “PORTRAIT” AS IT WAS INTENDED TO BE 

ABLE TO BE VIEWED ON A COMPUTER. ON COMPUTERS, MOST SCREENS ARE WIDER THAN HIGHER. THERE ARE 

ALSO NUMEROUS LINKS WITH ADDITIONAL INFORMATION PROVIDED. THIS WILL PRINT OUT NICELY USING 

ACROBAT. 

What are the differences between tubes? 

What is a phase inverter? What about a matched phase inverter? 

What is matching? Static versus dynamic matching? 

Aren’t all tubes really the same? 

Is 100 watts twice as loud as 50 watts? How much power do I need? 

These questions and more are covered in this document 

1

Version 3.30 9/23/04 part 1 of 2 

BEFORE WE EVEN GET STARTED … IF YOU JUST WANT A FEW BASICS AND HATE TO READ ....................................................... 4 

BRIEF TUBE AND SOLID STATE DIFFERENCES ........................................................................................................................................ 4 

DITTO – THE SWISS ARMY KNIFE OF THE RECORDING STUDIO AND FOR LIVE PERFORMANCE TO THE HOUSE SYSTEM......... 5 

MY PERSONAL FAVORITE OR MOST IMPRESSIVE AMPLIFIERS............................................................................................................. 5 

MATCHING IN BRIEF .................................................................................................................................................................................... 12 

BIASING IN BRIEF ........................................................................................................................................................................................ 13 

WHAT TUBES SHOULD I USE IN MY AMPLIFIER? - A MODERN DAY TUBE SELECTION PRIMER.................................................... 13 

PREAMP TUBES ........................................................................................................................................................................................... 17 

SAG – SPECIAL APPLICATIONS GROUP (SPECIFICALLY SELECTED TUBES FOR SPECIFIC NEEDS) ............................................ 17 

THE BASICS OF PREAMP TUBE DIFFERENCES ...................................................................................................................................... 19 

12AX7 (ECC83, 7025, 12AT7, 12AY7, 12AU7, AND MANY OTHERS) PIN CONNECTIONS..................................................................... 21 

12AX7 RATINGS............................................................................................................................................................................................ 21 

12AX7 SHORT DATA .................................................................................................................................................................................... 21 

CHARACTERISTIC NOMENCLATURE USED BY TUBE ENGINEERS ...................................................................................................... 22 

PREAMP TUBE OVERALL CHART OF SOME NOS AND MANY CURRENT TUBES ............................................................................... 32 

ADDITIONAL SMALL SIGNAL TUBE TESTS AND REFERENCE .............................................................................................................. 35 

RECENT TUBE TEST RESULTS .................................................................................................................................................................. 37 

THE OUTPUT TUBES – THE PENTODES AND BEAM PENTODES .......................................................................................................... 46 

BIASING – A NEEDED CONSIDERATION WHEN REPLACING OR CHANGING POWER TUBES .......................................................... 47 

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6V6 OUTPUT TUBES .................................................................................................................................................................................... 49 

6L6 / 5881 / KT-66 TYPES............................................................................................................................................................................. 52 

EL34 – E34LS – AND 6CA7 TYPES ............................................................................................................................................................. 59 

MISCELLANEOUS AND OTHER OUTPUT TUBE TYPES........................................................................................................................... 64 

6L6 / KT66 / 5881 / EL-34 TUBE OUTPUT CURRENT OUTPUT COMPARISONS..................................................................................... 66 

POWER OUTPUT COMPARISON TEST RESULTS OF VARIOUS OUTPUT TUBES ................................................................................ 67 

6550 / KT-88 TUBES...................................................................................................................................................................................... 70 

POWER COMPARISON CHART OF CURRENT OUTPUT FOR THE KT-88 / 6550 FAMILIES.................................................................. 77 

THE EL-84 TUBES......................................................................................................................................................................................... 78 

TECHTIP ON EL-84 AMPS............................................................................................................................................................................ 78 

OUTPUT POWER CHART FOR EL-84 FAMILY ........................................................................................................................................... 82 

7027 AND 7591 TUBES................................................................................................................................................................................. 85 

RECTIFIERS – VACUUM TUBE AND SOLID STATE .................................................................................................................................. 85 

THE GT RATING SYSTEM – AN EXPLANATION OF “DISTORTION RATING” ALSO KNOWN AT TIMES AS “HARDNESS RATING” 92 

CONVERSION INFORMATION – GT NUMBERING, FENDER COLOR CODES AND MESA COLOR CODES......................................... 92 

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Before we even get started … if you just want a few basics and hate to read 

There are three items that are the foundation of most of what is in this document. Tube and solid state differences, matching, and bias. I 

will cover these briefly. 

Brief Tube and solid state differences 

Much is available on this subject, but from a very high level there are a few points worth mentioning. Some folks feel that solid state and 

digital components have greater frequency response than tube devices. I suppose this may be true in some area out in areas that are so 

far out in the frequency spectrum that it’s crazy, but for our purposes, this is not the case. Remember, there are tube devices operating in 

the gigahertz range, way beyond what we need in the world of audio. There are devices such as the DITTO, which have a bandwidth of 6 

Hz – 200 kHz which are tube units, as an example, in the audio business. 

Solid state devices have sort of an “on or off” aspect to their operation. This is something akin to one being in a dark room, with their eyes 

closed, while another in the room flips the light switch on and off. You cannot see the light, but know that something is going on, as you 

hear the switch being flipped. This is true in the digital realm, and is sometimes called “hash” or has other terms, for this by-product if 

digital products. You think this is no big deal or does not matter? Thankfully today, there are very powerful tools available to mere mortal 

men, only available in the past to folks that worked at places like Bell Labs. Pro Tools, a popular recording suite, can illustrate this pretty 

easily. Look at any digitally recorded aspect of one of your recordings. Drum tracks are great for this. Zoom in on one short area of a 

sample of the sound. You will see a lot of information there. Now, borrow something like a DITTO box or Brick. “THE BRICK” is 

something of a DITTO but has onboard phantom power so you may use it for microphones also. THE BRICK shares some of the ViPRE 

technology. More can be found on THE BRICK at http://www.groovetubes.com/product.cfm?Product_ID=2084 

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DITTO – The Swiss Army Knife of the recording studio and for live performance to the house system 

Take the original track, reroute it through the DITTO, and reproduce it to another unused 

track area. 

Now listen to the difference. Look at the difference. You will notice that the DITTO rerecorded 

track lost NONE of the original material, has more dynamic range, sounds much 

more open and alive, and yet has less on the display. 

Why is there less on the display? Zoom in on the “missing” part of the waveform. 

LISTEN to only that aspect of the sound. I think you found it was clicks or noise, or 

whatever you want to call it … but in any case, stuff you did not want and stuff that did not 

help your sound. Try plugging a POD XT into one of these before you go to the board or 

the amp. You may be surprised 

Tubes do not have the “on-off” aspect. They are always conducting and flowing the electron flow. This is one of their differences. This is 

not better or worse, it is different. Before you think or buy the fact that sticking a tube in the front end of a digital or solid state amp is 

going to turn it into a tube amp, think again. It may give you everything you are looking for, and that may be just what you wanted, tube or 

no tube. 

My Personal Favorite or Most Impressive Amplifiers 

Being a big fan of Aspen Pittman’s “The Tube Amp Book” for almost two decades, one of the first sections of the book that caught my 

interest was Aspen’s Top Ten amps of all time section. Today, there is a wider selection of amps than ever before, and although many of 

the original amps are still considered the classics, some of todays amps are what I consider to be the classics of today, and probably 

tomorrow. I picked these amps for various reasons, but some of the aspects are unique features, design, and superior build quality. I will 

explain some of my thinking as I go along. I do not own all of these amps by the way, but have had access to them and they impressed 

me enough to stick in my mind. I know many will complain as there are not amps such as the Matchless DC30 and many other great amps 

here that they feel should be here. These are amps that a higher number of folks out there can afford, and amp that in their price range 

give up nothing in the way of tone and build quality compared in cases to amps with two or three times their retail cost. 

This book is primarily a tube document, but I could not keep myself from saying a little more about the amplifiers themselves. 

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HIGH POWER MODERN PCB CHANNEL SWITCHER AT A STEAL $$$$$ – THE RIVERA KNUCKLEHEAD REVERB 55 AND 100 

WATT AMPS 

This amp is one of the few, if not the only, that has two differently 

voiced channels, one British and one American. Most other amps 

just alter the gain on the channels. The amps are built using aspects 

such as threaded inserts in the cabinets, and machine bolts, rather 

than just self tapping wood screws. I have NEVER had any client 

Rivera amp fail due to design or workmanship. VERY flexible front 

end with great EQ and tone controls. Sub level control if you use a 

sub system. 

With the Rivera Head Master controller, total control of the amp is 

just a tap away. 

The amps are really nicely priced compared to a lot of things out 

there that might do as much and are not built as well. 

To my way of thinking, Rivera amps are the best made production 

channel switching amplifiers today; heads or combos using PCB 

construction. 

For more info: 

http://www.rivera.com/prod_khseries.htm 

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THE ‘MARSHALL” STYLE AMPS in the non-master volume style 

Dr Z Delta 88 

There are a number of great “non-master” volume “Marshall” type amps out 

there. This is one I have found that gives the tone and feel of the EL34 and 

6550 Marshall amps but is not just a copy or modification of the original 

design. Much more pedal friendly than the originals and with a very simple 

front end and over 100 watts on tap via a duet of KT88 output tubes these 

amps are worth a test drive. 

More things than I can really say here. 

http://www.guitaramplifierblueprinting.com/doctorz88.html 

“VOX AC-30” AMP - THE VOX THING AND MORE WITH SIMPLICTY AND EVEN MORE VERSITILITY 

Dr. Z Mazerati and new Prescription Extra Strength 

Lots of clean headroom if you need it, lots of AC-30 tones it you need 

that, only one tone and one volume, and trust me, that is more than 

enough. It also seems to be enough for Brad Paisley who chose this 

amp to replace his own vintage AC-30. If you want more controls and a 

more “conventional amp”, then look at the Dr. Z Maz 38 which is voiced 

more like a Fender Tweed Bassman / Marshall Plexi, but can do the 

Vox AC 30 stuff as well. Dr. Z can even optimize the Maz 38 in more or 

less of either direction. Read the Guitar Player Magazine March 2004 

review at: http://www.guitaramplifierblueprinting.com/gp032004.pdf 

http://www.drzamps.com/mazerati.html 

http://www.drzamps.com/maz38.html 

http://www.guitaramplifierblueprinting.com/zpaisley-rx.html 

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BLUES / SMALL CLUB AMPS – TWO STANDARDS OVER DECADES 

The Victoria 20112T (tweed deluxe) and Fender Deluxe Reverb 

The Fender Tweed Deluxe (on the right in the photo). 

There is little to be said about these Deluxes, and this Victoria is one of the 

very best. 

The phase inverter on the tweed amp is only one half of a 12AX7, and the 

tone control works directly on that triode. This lets one balance the ratio of 

preamp to output distortion just so, a feature that I miss on the Black Face 

Deluxe Reverb that came in later years. Then again, pick either of these 

amps, depending on taste! 

For a lot more info on this amp head to 

http://www.guitaramplifierblueprinting.com/Victoria1.html 

MY FAVORITE CLEAN AMP – FOR COUNTRY PICKERS, RHYTHM PLAYERS, BUT ALSO HUGE LEAD CRUNCH FOLKS ALA JOE 

WALSH / TED NUGENT 

The Dr. Z KT-45 for Fender Twin Reverb or Super Reverb fans 

This amp puts out close to 70 watts or more. Again, simple controls, 

but not limited tones. The EF86 in the front end has huge gain and is 

driven by the tone stack, so a very huge sound with massive sound 

stage image is possible with this amp, more so then with the old 

classic, the Fender Twin Reverb. These amp are currently with 

Brooks and Dunn, and many others. This is the amp Joe Walsh 

used on the Eagles “Hell freezes over tour”. 

http://www.guitaramplifierblueprinting.com/doctorzamps2a.html 

8

HUGE ROCK AMP COMBO FOR THE CLASSIC 1950S AND EARLY 1960S TONES 

Victoria 80212T and 50212T 

Massive clean tones, more headroom and larger transformers than a 100 

watt Hiwatt. The Tweed Twin is what you see being used by folks from 

Clapton to The Stones these days. This is one huge sounding amp, the 

sound of Rock and Roll. 

If this amp is too much for you, there is also a 50 watt version which 

frankly, is more than enough for most stages and outdoor venues. 

I know some folks are going to say, “what about the Victoria Bassman”? I 

don’t think there is any reason to list that amp in here, doesn’t everybody 

already have a tweed Bassman? 


MONSTER GAIN IN A SMALL PACKAGE – STUDIO MASTERPIECE – SMALL CLUB MASTERPIECE 

Dr. Z Carmen Ghia – “if I could have only one amp for the rest of 

my playing days, it would be a Ghia” … msr 

One tone, one volume, and can be pretty clean at less than 

10 o’clock on the volume with a decent amount of clean 

headroom, a bit louder than my Deluxe Reverb. 

The amp is rated at 18 watts … don’t let that fool you, look at 

the power transformer! Hand wired turret board, the best 

parts, and turning the volume past 10 o’clock the fun starts 

and the surprised looks never stop. A necessity in any studio. 

Conjunctive tone filter and fixed bias phase inverter … you 

won’t see these in other amps 

http://www.guitaramplifierblueprinting.com/doctorzamps3.html 

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JUST GREAT ALL AROUND, SUPER AMP(S) IF YOU CAN ONLY HAVE ONE AMP 

Okay, I cannot pick one, as this is for the great all around amp. 

The Rivera Quiana is a departure from Paul Rivera, as it uses 6L6 output 

tubes rather than EL-34 tubes. This amp does Fender on channel 2 and 

high gain Brit (sort of the Bluesbreaker thing on Channel 1. A lot of 

versatility and features and the 2x12 configuration will fill any venue. 

http://www.rivera.com/prod_latest.htm 

The Dr. Z Maz 18 (MAZ Jr.) 

Hand built, PTP, best parts and construction, ultra versatile, compact, light, 

loud, reliable in the worst situations, and at a price that is generally less than 

½ of what many amps hand built amps of this class sell for. 

A lot of output and great reverb too, in an easy to move package. This amp 

has a different sort of flair than the Rivera, quicker to distort as it is based on 

a duet of EL84 tubes. These amps are very loud, much louder than their 

power rating suggests. These are also one of the few EL-84 based amps 

that can do the Vox sound AND the Fender sounds. This amp should also 

be in one of the other sections above, as one of the best club and blues 

amps. This is perhaps one of the most versatile amps anywhere …. Rock, 

jazz, blues … and pedal friendly to boot. This amp is also very portable. 

1x12, 2x10, and head. 

http://www.drzamps.com/maz18.html 

The Dr. Z Z-28 (not shown here but can be seen from the below link) 

All the great Fender tones and more. This amp has replaced many of my 

Fender amps for those applications. The Z-28 puts out more level than one 

would expect though. Think of the amp as bigger than a Black Face Deluxe. 

It easily keeps up with a BF Super Reverb. 

http://www.guitaramplifierblueprinting.com/doctorzamps2a.html 

10

Great designs with something worth considering – MASTER BUILT, PTP TURRET BOARD, ATTENTION TO DETAIL AND GREAT 

TONES AND FEEL. 

The Mako amps 

Hand wired, PTP, turret board, channel switchers. 

Hand built PTP channel switching amps. An amp where there is no room 

for improvement. Even the transformers are done in house. There are not 

a lot of PTP wired channel switching amps out there, only one other 

crosses my mind. 

These have a waiting list, and are generally only sold direct unless things 

have changed, but they are worth the wait. The wood cabinet in the photo 

I stole here from the Mako Website was a custom cabinet. 

These folks also have some of the best speaker cabinets in the world, from 

my point of view. 

http://www.makoamplification.com 

A BRITISH FLAVOR / USA HAND BUILT AMERICAN STANDOUT – 60 WATT MASTERPIECE – A GREAT ROCK OR CLEAN AMP 

The Victoria Sovereign is not really a master volume amp. 

With an EF86 in V1 (huge clean headroom or gain) an 

amazing reverb driven by a 6BM8 Pentode/Triode and 60 

watts easily out of a set of even 25 watt EL-34 tubes such 

as Svetlanas, this is an amp that is a real masterpiece. 

There is too much to say here, but you can read a detailed 

PDF format review at 

http://www.guitaramplifierblueprinting.com/Sovereign.pdf 

There is also more at: 


11

Matching in brief 

There is a lot more on matching deeper in this document. Let me just briefly say here, that folks need matched output tubes and at times, 

matched triodes. If you have what is termed a Class A amp, such as a Vox AC-30, Matchless, Fender Blues Jr., and think you got lucky as 

there is no bias adjustment, you are wrong. These amps are what is termed cathode biased, but are not what are called single ended. 

They have two or four output tubes, and operate in push-pull, just as the class AB brothers. What gave the Vox AC-30 the reputation for 

being unreliable, was the heat that was generated, the lack of ventilation, and a few other aspects. Balanced output tubes are critical in 

these amps. They are idling at close to 100% idle dissipation, much higher than their AB counterparts. An unbalance in the output set is 

akin to two riders on a tandem bike. If one is tired or slow and does not contribute 50% to the effort … no more, no less, the other rider will 

tire more quickly due to additional work effort required. This is the same with tubes, one tube or duet is working harder than the other, and 

this causes a few problems. The first aspect is the sound most would surmise. Yes, this is true, a lack of sustain, less power, less clarity. 

There is another important side effect. Heat. Lots of heat that is not nearly as high as when balanced tubes are used. The tubes of the 

past were not matched at times. Vox AC-30 amps were known at times to also catch on fire and have reliability issues. It was common for 

groups using these amps to have more than one, and “matter of factly” just wheel one in to replace the one that failed on stage. Don’t 

believe it? Read a bit or ask around some of the folks from the sixties. Look in the Vox Book where this is mentioned in passing. Lower 

chassis temperatures also give longer life to the resistors and capacitors that also reside in the amp’s chassis, extending their life. There 

are big differences in the ways tubes are matched, and this is covered later in this document. 

From an Internet post I answered in regard to “matching” preamp tubes, specifically the phase inverter:: 

quote: 

Originally posted by proudmore 

Hey Myles, for a PI, what brand of tube would be the best? What should I look for, other than matched triodes? Should 

be a high gain tube, high output..? Does it matter? 

The brand of the tube or tube type is not really critical. Some runs have more balanced tubes than others. 

The PI is not part of the tone generation stage, so it is picked for specs and performance, tone is not an issue. 

You want proper gain, but proper output current is MUCH more important. This is why simple gain matching or transconductance 

matching is pretty good, but will not yield the same results as a tube that has proper current drive (most are about 1/2 of what is 

spec) for today's tubes on average, or at least 3% down for the most part. 

Even more important is the tubes curves and rise time. In some amp designs that balance (actually they average) the output or gain 

of the two sides ... they cannot average rise time. Sort of think of it in a way, that is you have a rise time of 20 "counts" on one side 

and 40 on the other, now you have a wobbly window of 20-40 counts overall, or a 20 count spread. This is a lot less articulate and 

defined than a count of 0-2 or so. This is where pretty sophisticated test rigs come into play, and cannot be done with conventional 

tube "matching" equipment. 

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Biasing in brief 

Bias your amp properly every time output tubes are replaced, if not of the same distortion rating, type, and maker. The bias is something 

like the idle of your car in a simple way. Setting your bias so the amp runs really hot is something like using this faulty logic to set the idle 

on your car to 5000 rpm. You want your bias set for great tone at low, mid, and high levels … not just at high levels. Bias effects tube life, 

tone, playability, dynamics, and other areas. You also need to bias the amp properly. This does NOT mean pulling the old tubes out, 

slapping in the new ones, and THEN trying to bias. This will only lead to problems, or at best, take a big chunk out of the life of your new 

tube set. A proper bias method is described later in this document. 

What Tubes Should I Use In My Amplifier? - A Modern Day Tube Selection Primer 

Today in 2003, there is a lot of misconception on where tubes come from, how they actually perform, and what their characteristics are. 

Information that was valid only a year or two ago, is not always valid today. “Chinese tubes are less powerful than the Russian tubes”. 

“Chinese tubes do not hold up to high voltages”. “Matching is matching”, or “there is no need to match tubes”. This information gets 

passed along, and becomes something of gospel, when most of it is just plain rubbish. 

In the case of the Chinese tube issue, a few years back, this generally was the case. Today, the typical Chinese power tube is 15% 

stronger in many cases than its Russian counterpart. The Chinese have great tooling, and money. They are putting a lot of resources into 

tube manufacture. The Chinese 12AX7 as one example, is the most consistent tube in quality and specifications when compared to a lab 

spec 12AX7. 

As you proceed with this paper, examples and characteristics of many tubes will be given. These are not bits of data collected from 

magazine clippings, Internet spatter, or hear say. This data comes from my personal time sitting with thousands of tubes, one at a time, 

with very high end test equipment – not simple tube testers. 

Amps come in all types, sizes, styles, etc. You cannot turn a Marshall into a Fender, or visa versa, no matter how hard you try. If you 

“need “ both of these sounds, the bottom line is, you need one of each of these amps. Some folks like Paul Rivera, have two different 

voicings in the same amp. This is pretty unique, as most folks just change the gain structure on the various channels, with the same 

voicing. 

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A 60 watt Sovereign amp from Victoria Amplifier – very versatile with it’s variable front 

end which uses an EF86 in the front end, a 6BM8 in the reverb circuit, a cross between 

a Plexi era Marshall and Tweed Bassman. This is not a master volume amp. You can 

turn the gain all the way down and it’s pure Plexi era. Turn the gain up, and it moves 

through JCM 800, 900, 2000, and beyond Mesa Rectifier territory. This is one example 

of an amp that is very unique, and built, and very unique. I have a full write-up on this 

amp someplace on my personal website. If you would like a copy of the review sent to 

you via email, just drop me a note. You can also learn more at 

http://www.guitaramplifierblueprinting.com/Victoria1.html for Victoria in general. A 

review of the Sovereign is at http://www.guitaramplifierblueprinting.com/Sovereign.pdf 

Some of the proven classic amps ……… Fender Black Face 

Deluxe Reverb and the Tweed Deluxe (in this case a Victoria 

20112T) Perhaps to some folks thinking, the best small club 

and blues amps around. 

This Victoria Deluxe is simply, one of the most amazing and 

touch sensitive amps one will ever play. This is just one of 

Victoria’s amps in an extensive lineup. To see more, head to 

www.victoriaamp.com 

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The Dr. Z Carmen Ghia. Sure, the good Doctor makes lots of amps, but this is one of the most 

amazing pieces of gear around. This is a studio amp masterpiece and also a super club amp, Dr. Z 

is one of the folks out there that copies nobody. His amps are all very unique and built to the very 

highest standards of construction. His Route 66 has won awards that were not political, they were 

from real players. The Z-28, MAZ 18 and 38, and other models each do their target aspects VERY 

WELL. For much more on Dr. Z amps, internal photos, comments, pricing and more, head to 

http://www.guitaramplifierblueprinting.com/doctorzamps.html 

If you are one of those folks that thinks that wattage = manhood … 

this older amp may be your cup of tea. The top section is two 60 

watt EL34 based amp sections, and the bottom slave is 8 GE 6550 

tubes, for a total of about a ½ a kilowatt of tube power. This amps 

runs with four 2x12 cabs and four 4x12 cabs. For more on Rivera, 

and their amps (which are a lot more current that this particular 

example, head to 

http://www.guitaramplifierblueprinting.com/rivera.html 

There are amps for every taste and need out there! This is a very 

old Rivera model, no longer produced. For their new and great 

amps see www.rivera.com 

Some of my other personal favorite folks out there can be seen in 

the amps / guitars area of my website at 

http://www.guitaramplifierblueprinting.com/ampvendor.html 

15

This is a THD Univalve. This is a single ended class A 

amplifier that one can change the tubes in with no 

adjustment necessary. There are a lot of other features 

on this amp. This amp is a great tool in the studio or for 

smaller club live work. 

Some folks think of amps such as Vox AX-30 amps as 

class A amps. Although this is somewhat accurate, they 

are really cathode biased push-pull amps. The Univalve 

is a true class A amp with no feedback loop. 

The THD UV is a MUST for any studio. 

For more information go to www.thdelectronics.com 

Two Doctor Z amps, perhaps on each end of his product 

line. The red one is a Carmen Ghia, one of the coolest 

amps around and a great studio and recording amp. The 

tan amp is a KT-45 with loads of clean headroom, but 

with it’s tone controls pushing a pentode front end (an 

EF-86, this amp is capable of a lot of gain too. The pair 

of these amps will cover just about any music base. 

Dr. Z has a LOT more amps, all of original design, and all 

built hand wired to extremely high standards. 

For more see www.drzamps.com 

16

Preamp Tubes 

Please note that the tests on tubes were done on raw factory samples. Some tube vendors test and/or select or grade, and some do not. 

Many rely on the end user’s reluctance to return a single tube such as a preamp tube for problems, so just send their tubes out with no 

testing or minimal testing. A preamp tube that is not microphonic when first installed, can become microphonic in short order after a few 

heat up and cool down cycles with expansion and contraction. This is one reason to know and trust your vendor. Some companies such 

as Groove Tubes have a very long warranty on tubes, in the case of preamp tubes, six months. Some reasons for the cost of GT premium 

tubes are due to the labor intensive testing process (each tube, one at a time, in place in high gain amps and tested for various attributes), 

and their cost for the tubes initially when buying from the major tube factories. In many cases, the GT reject rate is in excess of 60%. The 

data below, are for the raw factory samples, not GT tested examples. This is the way they come from the factory, and the sort of 

performance you can expect if your vendor does not test for gain, output, noise and microphonics. In the case of power tubes, gas 

leakage, grid leakage, and low vacuum are also test parameters that require expensive and complex test equipment which very few 

vendors have in house. 

The complete list of Groove Tubes preamp tubes can be seen at: 

http://www.groovetubes.com/tubes_list.cfm?ObjectGroup_ID=107 

SAG – Special Applications Group (specifically selected tubes for specific needs) 

As you will learn throughout this document, all tubes, new or NOS are subject to very wide variations in characteristics. As an example, a 

12AX7 is supposed to have as spec, a gain of 100, a transconductance of 1600 and a current output of 1.2 milliamps at a reference test 

voltage and bias. Tubes will typically vary +/- 50% or more, even ones tested from most vendors. For GT preamp tubes, this spread is 

much narrower, as the testing acceptance margins are tighter. Still, you do not know what you are actually getting in any case. 

One problem that kept coming into play, was a player would decide to retube the preamp section of his or her amp. The amp sounded 

worse in some cases with the new tubes. It was noted that perhaps the old tube had a current output of 1.0 milliamps, while the new tube 

only had a current output of 0.7 milliamps. This was a 30% drop in actual power off the first gain stage. Searching through tubes until one 

was found that met or exceeded the specs of the original tube would solve the problem and bring the amp back to life. 

It was found that things could be made even better, actually moving up or down in various tube characteristic parameters, to dial in to a 

particular player’s taste of needs. Tubes with more or less gain, more or less current, faster or slower rise time (more or less 

compression), and other factors were “tweeked” as the player went from one tube to another. This is one of the cornerstones of amplifier 

blueprinting. 

Matched phase inverters also came to pass in this process, there is more on this later. 

17

One of the biggest benefits of SAG tubes, is that the end user KNOWS what is in the amp. They can replicate this later, or go “up or down” 

in various parameters. 

For very critical requirements, the rise time of the matched phase inverter is also optimized to the characteristics of the output tube set. 

Most GT offered tubes can be processed through the SAG (Special Applications Group). It costs about $10.00 per tube to have one found, 

and in some cases, some tubes may not be able to be accommodated in short order. Some tubes from some vendors, have such a high 

reject rate, or extremely bad characteristics, that one would need to go through over 100 of them to find one example. 

You may contact Myles Rose for specific requirements. Some of the more common SAG tubes are available off the GT website or may be 

special ordered through any Groove Tubes dealer. 

There is more on the SAG on the Groove Tubes website at http://www.groovetubes.com/product.cfm?Product_ID=1773 where one can 

also add the SAG labor cost to any of the tubes in the GT product line. 

Some of the more common SAG tubes generally in stock are: 

SAG-AT7-MPI - matched phase inverter (on a tracer, not just statically matched) generally used in Fender Black Face, Silver Face, and 

early Tolex era amps. 

http://www.groovetubes.com/product.cfm?Product_ID=1722 

SAG-AX7-MPI - matched phase inverter (on a tracer, not just statically matched). Used on most amplifiers 


SAG-FHG – Fender High Gain Kit for Black Face and Silver Face era amps, many of the newer amps also. 


SAG-FST – Fender Soft Touch Kit – same as above, also used for any 12AX7 front end based amp where a slower rise time is desired, a 

quieter tube, more clean headroom, and a higher ratio of output tube distortion compared to stock V1 and V2 tubes in these amps. This 

was the old “SRV” trick, where he would use 5751 tubes in the first gain stage. Unfortunately, there are not a lot of great new 5751 tubes 

around today, so finding tubes with the same sort of characteristics was the goal here. 


SAG-MHG – Known as the “Marshall High Gain Kit”, this should really be called the high output kit. This kit is very popular in not only 

Marshall amps, but in Mesa, Bogner, Fender amps that need a more aggressive nature, Diezel, Rivera, and many other high gain amps. 

Most of these amps have very complex front end design. Having a tube that is down 30% or more on current drive ability will zap the life 

out of the best of any of them. Many folks love these with the included AX7 phase inverter in Fender Blackface and Silverface 

amps for an aggressive update on these amps. 


18

The basics of preamp tube differences 

The primary and most common family of preamp tubes is the 12AX7 / 7025 / 

ECC83 family. These are directly interchangeable in most guitar or bass 

amplifiers. Contrary to popular opinion, these are not the same sounding 

tubes in many ways. They each have their own sonic signature and sound 

characteristics. Below one can see the difference between a commonly used 

Sovtek 12AX7WA (the green line) and a long smooth plate 7025 (the blue 

line). The 7025 is a brighter tube, with more treble and high mid response. 

The 7025 was used in the Black Face Fender era, and was one aspect of the 

bright clear sound of these amps. 

There are many factors in a tubes character. In the case of preamp tubes, 

aspects such as gain (Mu), output, transconductance, and other factors, are 

all part of a bigger picture. Many people use the terms gain and output 

interchangeably. This is NOT correct. A tube can have high gain, but be very 

low in output current capability. This is a very common mistake today. In a 

clean front end preamp circuit, such as a tweed or blackface era amplifier, low 

output is less of a factor than in a modern channel switching amp with a 

complex circuit topology design. These newer amps perform their best if a 

high current drive tube is utilized. Today’s most common factory tube, the 

Sovtek 12AX7WA, is picked for many reasons. It is not expensive, it is sturdy 

(so the amp makers have less warranty return), and it is generally low on gain 

and output. The latter characteristic makes the amps more quiet with less 

background noise, and this helps cover up other manufacturing or design 

shortcuts. 

Today’s tubes are VERY inconsistent. You may re-tube your amp, just to find that is sounds worse than before the replacement. Why? 

As one example, a “spec” 12AX7 should have a gain of 100 for a “typical” tube based on specs set many decades ago. It should have a 

transconductance of 1600, and an output of 1.2 milliamps. If your V1 (first gain stage tube) had an output of say 1.0 milliamps before it 

was replaced, and the new tube had an output of 0.8 milliamps, right there you just lost 20% in your front end. Remember, this signal gets 

amplified now down the rest of the amplifier chain. 1.2 milliamps is the “spec” number we look for in a 12AX7 / ECC83 / 7025. 

Unfortunately today, the vast majority of the tubes we see made newly are on average, much less, and at times 0.7 milliamps on average. 

This is almost a loss of 50% of our expected output. Poor plate materials that are inconsistent, fast work, incomplete vacuum pump down 

procedures, cathode coatings with high percentages of impurities, and more. These all contribute to today’s inconsistency. These are 

some of the areas where the Chinese are improving faster than in other geographic areas. 

19

The most important tube(s) in the preamp section are in the first stages, the tone and gain stages. Changing V1 in many amps will yield 

the most results. Tubes used for current drivers as their primary function, such as reverb drivers or effects loop drivers, do little in most 

amps to change the tone. This is generally not part of the tone or gain stage. The exception to this is the phase inverter or driver, of the 

power tubes. There will be more on this subject later in this paper. 

Moving on to the tubes 

What we are looking for are specific factors in testing, and other factors in sonic aspects. Rather than list all the technical data for each 

tube, where I have this data, I will provide additional information for gain, output, and consistency. The consistency factor I will call QA for 

quality. For this “spec”, the lower number, the better. A perfect tube factory run would have 0% inconsistency. A factory run where the 

tubes were +/- 20% off target factory expected specifications, would have a number of “20”. The standard specs that are the target 

numbers for gain is 100, and output is 1.2 milliamps, in the case of the 12AX7/ECC83/7025 family. Output will be shown as a percentage 

of factory expected target. If a tube has 1.2 milliamps of output, it’s Output would be 100%. 

The most important factor in all of the above, is know and trust your tube vendor. Tubes need to be tested for microphonics, low output, 

and noise. With today’s material inconsistency, a tube that is quiet today may become microphonic after a few hours of use, so a vendor 

with a good warranty is one that one may wish to have in your corner. 

NOTE: This test data is from bulk factory batches of large quantities of tubes, at times 1000 tubes of each type from each factory run. This illustrates 

how random these tubes are, and shows what a chance you may be taking with untested tubes in general. These tubes, after testing using good 

methods, will narrow this inconsistency considerably. In one Tube Company’s case, Groove Tubes, their reject rate on factory tubes can exceed 50%. 

You can sometimes easily tell how well a tube has been tested easily … price. When there is a lot of test time and labor, and half of your stock hits the 

trash bin, the end retail cost has to be higher. The data below was collected at one part of late 2002 and early 2003. For the most current data, see the 

updated information elsewhere in this document. 

20

12AX7 (ECC83, 7025, 12AT7, 12AY7, 12AU7, and many others) pin connections 

12AX7 Ratings 

12AX7 Short Data 

12AX7 (ECC83, 7025, 12AT7, 12AY7, 12AU7, and many others) pin connections. 

12AX7 ratings 

12AX7 short data 

Note the Mu, or sometimes called gain, in a “spec” tube has 

a target of 100. Few tubes made today will reach this, most 

falling below 90. 

21

NOTE: THESE TYPES OF CHARTS WILL BE USED THROUGHOUT THIS DOCUMENT. HERE IS A LEGEND SO ONE 

MAY KNOW WHAT THE COLUMN HEADINGS MEAN. 

SEE BELOW 

Characteristic nomenclature used by tube engineers 

22

Keep in mind, the specs on the tubes below was, at the time of testing. This is an average of many factory runs. These 

numbers are always changing from factory run to factory run. The most current data which is more detailed, can be found 

elsewhere in this document. 

23

th 

12AX7-C – Chinese 9 generation tooling 

Gain: 93 Output: 92% QA: 17% 

This is possibly the best of the current 12AX7 family. This is a warm and linear tube. It is suited to 

rock, blues, and jazz. It is perhaps the most versatile of the 12AX7/ ECC83/7025 family. There 

are many Chinese variants, and many tube vendors sell older tooling versions. 


This is perhaps the most linear and most warm of new 12AX7 / ECC83 / 7025 tube production. It 

is also the most close in specs and the QA from the Shuguang (not to be confused with Sino), is 

higher than any current production tube in this family. The Chinese are pouring money into new 

tooling that is of very close tolerances, and has the money to do so. This is not true in the case of 

other companies generally. 

This is a great all around tube for 60s rock, blues, and is a favorite of jazz players or players that 

wish to tone down a harsh amp. 

12AX7-R – Sovtek 12AX7WA and Sovtek 12AX7WC 


This tube comes from the Reflektor factory in Russia. It is perhaps the most commonly used tube 

by many amp makers. It is reliable, and quiet. The main reason it is quiet, is it’s generally lower 

gain and output. 


This tube is the “old standby”. It is the most commonly used preamp tube. It is lower in gain and 

current output than most of the others in this family. This is a long lasting tube, and amp makers 

like it (and have influenced it’s production specs) to have lower gain and output, so their amps 

will pass their own QA easier, and the amps will be quiet. It is a good all around tube, but there 

are others that are sonic improvements with more detail and harmonic content. 

The newest (2004) Sovtek 12AX7WC looks the same to the eye but is a VERY different tube 

than the WA. The WC has all the “proper numbers”. It’s gain, current, and transconductance are 

right on original NOS 12AX7 spec. A job well done by Sovtek on this tube. 

24

12AX7-R2 – Sovtek 12AX7LPS 


This tube is made in the Reflektor factory and sold under the Sovtek name. It is also known as the 

Groove Tubes 12AX7R2. It is a long plate structure, that is brighter than a 12AX7WA, 12AX7R3 

(EH), or ECC83. It is not as bright as a 7025. It’s long plate structure can be more prone to 

microphonics is some combo amplifiers. 


A newer tube with a long plate structure, these can tend to be microphonic in combo amps more often 

than the shorter plate tubes. These are brighter in most amps than the 12AX7WA or 12AX7C. 

These need to be selected by a good vendor that really tests these tubes, as the factory out of spec 

range is very high in my own testing, they are somewhat more inconsistent than many other tubes, 

and can become microphonic in short order. Their curves are not as smooth or linear than the 

12AX7C. 

12AX7-R3 – Electro Harmonix 12AX7EH 


This tube is made in the Reflektor factory on newer tooling and also sold as the Electro Harmonix 

12AX7EH. It is a shorter plate structure, a linear tube. It is not quite as warm or linear as the 

12AX7C, but is just about as versatile. It is a quiet tube, and works nicely in many amplifiers. It is a 

touch brighter than the 12AX7C, but not as bright as a 12AX7R2 (LPS) or 7025. One of the photos 

shows a factory tube on a Kaye Audio Labs tester, before it goes through any GT processing. This 

is one aspect of the factory testing I perform at Guitar Amplifier Blueprinting. From noise data here, 

these tubes move on to other test equipment such as curve tracers. I go though thousands of 

samples each month from all the tube factories collecting the raw data to see how close or 

consistent to specs they come. 


25

The 12AX7R3 is the second most consistent tube made today in this family, just 

th 

about a tie with the 12AX7C (9 generation). This tube is great in Rivera, Bogner, 

and Diezel amps of my clients. 

This is typically how factory bulk tubes come to the tube vendors from the factory. This 

happens to be 200 Sovtek 12AX7LPS tubes (2 layers of 100 each). On average, the reject 

rate on most new tubes can run 50% and higher if the tube seller has tight specs. Many 

vendors just ship whatever they buy, and hope for the best from the end user as far as them 

not complaining. 

After looking at a LOT of tubes, my advice is; trust your vendor. 

26

ECC83-S - from the JJ factory 


This is the sound of Marshall and the British era amps. A stronger mid range response, with a bit of roll 

off on the high end compared to many other tubes (but not the 12AX7WA). The BLUE line shows the 

typical response curve of the ECC83 in an average tested sample compared to the reference 12AX7WA 

(as the WA is a common tube in so many amps today). This tube may be brighter or darker depending 

on amplifier. This is due to the higher output of this tube and it’s ability to “push” signal with it’s high 

current through complex front end circuits where other tubes show their limitations. 

ECC83’s need to be purchased from a trusted tube vendor that tests tubes pretty extensively, as they 

can be very inconsistent. A good one is a find for sure. These are the masters of current drive in MOST, 

but not all factory batches. In modern amps such as a Bogner, Diezel, Rivera, or Mesa Recto series, 

these are the tubes that will push signal through those complex front ends. 


Also limited SAG in High Gain Kits: http://www.groovetubes.com/product.cfm?Product_ID=1737 

The curves of an ECC83 in blue, compared to a Sovtek 12AX7WA in green. Notice the 

increased response across the range, with an even stronger mid range response. 

27

7025-Y from the Ei factory 

Gain: 90 Output: 46% QA: 25% - 60% 

Long smooth plates, the most bright of the current tubes. The graph on this tube is at the start of this 

paper. These are the characteristic sound of the Fender Tolex years. These are very articulate. In 

V1 and V2 of a Fender Tolex era amp, these give the original sound signature. These tend to be too 

bright in Marshall Plexi era amps for some tastes when used with a Strat or Tele, but if you tend to 

load these amps down with pedals in the front end, they can help resolve the loss of treble from the 

pedals. Long plates can tend to be microphonic in combo amps. The Ei factory was the OEM for 

many “premium” tubes folks in the past, such as Siemens and Telefunken at times, so if the plate 

structure looks familiar, this may be a clue. 


These are VERY inconsistent from the Ei factory. Over at GT, their reject rate can exceed 60% and I 

saw one batch go through test where Groove Tubes tossed 90% of them. These are the tubes that 

many other vendors just pass on along that do not test, so beware. When you find a great one of 

these though, they are magic. They are very close to the original long smooth plate Telefunkens, so 

close in fact, that some folks screen these with Telefunken logos and counterfeit them. Look for a 

diamond molded into the base of real Telefunkens to be sure when buying NOS. 

These are some NOS 5751 tubes. The famed GE 5 star is on the left, another NOS 

in the middle, and the RCA Command Series black box plate is on the right. 

28

5751 New Manufacture 

Expected Gain 70 Expected Output 1.2 milliamps 

Used in V1 by some folks like SRV to change the ratio of preamp distortion to power tube distortion. Current manufacture from Russia is 

so far outside of NOS sample specs on curve traces that this tube is not at all like NOS 5751 versions. Perhaps down the road things will 

change, but in 2002-2003 this is the case. If you use these new 5751’s and like them, that is great, but they are not a vintage 5751. When 

there is a good new 5751 available, it will be noted here in an update. 

12AT7-Y Ei / 12AT7-C / 12AT7-A USA NOS JAN spec 

Expected gain 60-70 Expected Output 10.0 milliamps 

These are commonly used as phase inverters and reverb drivers. In the first gain stage of an amplifier, they will drop the gain a bit, make 

some amps a bit quieter in background noise, and yield more clean headroom. This is also one way to change the percentage of output 

distortion to preamp distortion in non master volume amps when used in the first gain stage. One of the differences between the later 

Fender amps and the Marshall JTM-45 / Fender tweed era amps, was the use of the 12AT7 in the Fender tolex era amps in the phase 

inverter position, rather than the original 12AX7. The 6201M is a selected low noise tube for use in some microphones. It is an NOS JAN 

selected tube. As of May 2003, GT will generally be offering the 12AT7A NOS as it’s 12AT7 if marked as 12AT7 without the Y or C 

designator. 

12AT7-A http://www.groovetubes.com/product.cfm?Product_ID=1589 

12AT7-Y http://www.groovetubes.com/product.cfm?Product_ID=1590 (currently not in stock – replaced by 12AT7A) 

12AT7-C http://www.groovetubes.com/product.cfm?Product_ID=1591 (currently not in stock – replaced by 12AT7A) 

6201M – This is a low noise selected tube generally used in studio gear or microphones. 


12AY7 – various makers / 6072M NOS JAN Spec selected low noise 

Expected Gain 44 Expected Output 3.0 milliamps 

This tube was the first gain stage of many Fender tweed era amps for many years. It has about ½ the gain of a 12AX7, but much greater 

current output. In modern Marshall type amps such as a DSL or TSL series, this tube in V2 will bring the touch, feel, and sonic qualities, 

closer to Plexi era Marshall amps if this is an objective. This tube is also known as a 6072 or 6072M, a selected low noise version used in 

studio equipment and some microphones. These are the common V1 in Fender Tweed era amps in many models. 

12AY7 http://www.groovetubes.com/product.cfm?Product_ID=1779 

6072M http://www.groovetubes.com/product.cfm?Product_ID=1215 

29

12AU7-Y – Ei 

Expected Gain 16-18 Expected Output 10-11 milliamps 

These are commonly used as phase inverters in McIntosh Hi-Fi amps, some Ampeg amps, and will yield the most clean headroom in many 

amps. In the latest Fender Pro Series amps such as the Pro Reverb, Concert, and Twin, this tube in V2 (or a 12AY7), will drop the gain of 

the gain channel where some feel the tone is of a less intense or “buzzy” character. This will then allow a much wider range in the sweep 

of the volume and gain controls of this channel. 


To the left is a GT EF86 adapter. This was developed as new production EF86 tubes are very inconsistent, and 

can be noisy, or hard to find at times. For a group on tour using an older Vox amp, a Matchless, Victoria 

Sovereign, Bad Cat, or Dr. Z amp, these can be a life saver. These allow a 12AX7 to be used in the place of the 

EF86 / 6267 pentode tube. 

This adapter can be seen at http://www.groovetubes.com/product.cfm?Product_ID=2044 

EF86 – 6267 

Expected Gain 400-2000 and higher Expected Output 3.0 milliamps QA: 50% + 

A 9 pin pentode, used in the first gain stage of some newer amps such as the Matchless, Bad Cat, Dr. Z, and others. The current versions 

of this tube from the Russian factory, are very inconsistent. Their output is way above spec, sometimes as high as 50% over what is to be 

expected. They have transconductance that is not close to the original tubes 80% of the time in factory sample tests. These factors make 

finding a low noise version of this tube very difficult. These need to be closely screened and tested by a tube supplier that has very 

sophisticated test gear that is generally out of reach for most Internet tube vendors or smaller vendors. 


30

The famed 7199 gold pin of the past. 

The GT 12AX7M “Mullard” reissue was shown at the NAMM 2004 show and has been hard to keep in stock 

since. The M is not as bright as the ECC83S and many folks have chosen this tube as a replacement in 

Fender Blackface/Silverface amps in V1 and V2 and in Marshall amps. Dr. Z is using some of these as 

phase inverters for their strong current drive. 

More info at http://www.groovetubes.com/product.cfm?Product_ID=2100 

31

Preamp Tube overall chart of some NOS and many current tubes 

Tube Tolerance Ave output 

% of mA 

expected 

Ave TC 

(gm) 

Guitar Amplifier Blueprinting 1600 

Reference small signal tubes - in some cases, single tube tests. Items marked * were three to five tube test averages 

TC % Average gp Average 

Gain 

GE 6072A 1970s Date Code * 2.6% 96% 1710 1750 42/44 

GE 6072A Black Plate 1963 * 1.9% 100% 1750 1750 44/44 

GE 12AX7WA (1980s) * 8.7% 92% 1520 1600 98/100 

GE 5751 (1950s) 2.5% 98% 1190 1200 69/70 

JAN/Philips 12AT7WC 1980s * 11.6% 105% 5780 5500 72/70 

RCA 12AX7 NOS 1950s-60s 8.8% 99% 1620 1600 100/100 

RCA 5751 Black Plate (1950s) * 3.7% 96% 1251 1200 69/70 

RCA 7025/12AX7 NOS 1960s * 6.6% 97% 1580 1600 97/100 

Telefunken ECC83/12AX7 - diamond base * 11.6% 102% 1680 1600 102/100 

RCA 12AU7 NOS Cleartop * 2.1% 96% 2180 2200 18/18 

12AX7 Telefunken smooth plate new Dynaco 12.6% 102% 1610 1600 98/100 

12AX7/ECC83 Telefunken Diamond Bottom Ribbed Plates * 2.7% 94% 1680 1600 105/100 

12AX7 MINIWATT / SUPER RADIOTRON AUSTRALIA 1960s 0.8% 100% 1606 1600 100/100 

12AX7 Amperex Bugle Boy Holland LONG PLATE 1950s * (10) 4.4% 112% 1640 1600 105/100 

12AX7 AMPEREX BUGLE BOY HOLLAND 1967 * 6.1% 109% 1580 1600 101/100 

7025 Amperex Holland orange Globe Logo 1971 7.7% 94% 1690 1600 92/100 

CV4004 Brimar 1961 4.5% 92% 1570 1600 91/100 

CV4035 / 12AX7 Brimar NOS flying leads 9.8% 108% 1740 1600 112/100 

7025 GE ribbed plates 1950s * 2.2% 100% 1610 1600 99/100 

7025 GE ribbed plates 1960s 3.7% 100% 1590 1600 102/100 

M8137/CV4004/12AX7 Mullard Box Plates * 11.2% 92% 1640 1600 92/100 

12AX7 Raytheon black ribbed plates square getter halo 1950s * 5.4% 94% 1520 1600 100/100 

JRC-12AX7 RCA Black Plates 1954 * 4.9% 101% 1590 1600 100/100 

12AX7A RCA gray ribbed plates 1960's-1970's * 12.6% 93% 1660 1600 96/100 

ECC83/12AX7 Siemens long plates early 1960s * 9.9% 95% 1620 1600 104/100 

12AX7 Sylvania Gray Plate square getter halo 1958, 1959 * 8.2% 102% 1710 1600 101/100 

12AX7R3 Electro Harmonix (10/7/02) 25.0% 95% 1507 94.19% #DIV/0! 

32

12AX7R3 Electro Harmonix (10/29/02) 16.7% 85% 1382 86.38% 0.0162 85.31 

12AX7R3 Electro Harmonix (12/12/02) 16.7% 83% 1403 87.69% 0.0161 87.14 

12AX7C - Chinese (10/28/02) 25.0% 83% 1461 91.31% 0.0160 91.31 

12AX7C - Chinese (12/13/02) 16.7% 92% 1588 99.25% 0.0170 93.41 

ECC83 JJ (10/07/02) 33.0% 93% 1467 91.69% #DIV/0! 

ECC83 JJ (11/07/02) 66.7% 113% 1664 104.00% 0.0195 85.33 

ECC83 JJ (12/13/02) 58.3% 112% 1604 100.25% 0.0189 84.87 

12AX7R - Sovtek 12AX7WA 10/7/02 41.7% 78% 1190 74.38% #DIV/0! 

12AX7R - Sovtek 12AX7WA - (10/28/02 batch) (note a) 41.7% 88% 1293 80.81% 0.0147 87.96 

12AX7R - Sovtek 12AX7WA - (12/13/02 batch) 41.7% 78% 1180 73.75% 0.0137 86.13 

12AX7R – Sovtek 12AX7WA (5/16/03) - This batch was stronger 

than the prior groups, but was more inconsistent. There was 

generally a very large spread between the A and B sides of most 

of these dual triodes so these are NOT recommended for phase 

inverter use. 

Average output – 1.1 mA (closer to spec of 1.2 and better than the 

past) 91% of expected output. Good by today’s new tube 

production from all vendors, and better than their past production 

of this tube. Output ranged from 67% of what was expected as 

spec to 125% of what is expected. Very inconsistent current 

output. 

Average gain – 82.99 (spec 100) Good by today’s production 

standards in new made tubes. 

Average TC – 1278 (1600 spec) – slow rise times, not all that 

great for high articulation players such as metal folks or folks that 

play harmonics. Lack of harmonic content and detail. 

Tolerance range – 58% Still not good at all. There is a lack of 

consistency from one tube to the next, near the bottom of the 

batch of vendors for the most part. 

More noisy than past production – due to more proper output and 

gain. 

33

50.0% 83% 1469 91.81% #DIV/0! 

12AX7R2 Sovtek LPS (10/7 batch) 

12AX7R2 Sovtek LPS (10/23 batch) 67.0% 70% 1418 88.63% #DIV/0! 

12AX7R2 Sovtek LPS (10/31 batch) 75.0% 83% 1505 94.06% 0.0182 82.69 

12AX7R2 Sovtek LPS (12/12 batch) 42.0% 97% 1557 97.31% 0.0186 83.71 

7025 – Ei (10/31 batch) 33.3% 87.50% 1419 88.69% 0.0155 91.55 

7025 – Ei (12/12 batch) 25.0% 46.67% 1064 66.50% 0.0118 90.17 

7025 – Ei (4/03 batch) Output = 1.1 / TC = 1490 / Gain = 93.1 / QV 

= 33% - Nice gain this run, current way up too, great batch. 

Svetlana 12AX7 4/8/03 42.0% 72.50% 1369 85.56% 90.66 

JJ 12AT7 / ECC81 4/7/03 - output = 8.9/ 10.0 gain = 58.8 / 60.5 TC = 4711 / 5500 

34

Additional Small Signal Tube Tests and Reference 

Tube Tolerance Ave output 

% of mA 

expected 

Ave TC 

(gm) 

Guitar Amplifier Blueprinting 1600 

Reference small signal tubes - in some cases, single tube tests. Items marked * were three to five tube test averages 

TC % Average gp Average 

Gain 

GE 6072A 1970s Date Code * 2.6% 96% 1710 1750 42/44 

GE 6072A Black Plate 1963 * 1.9% 100% 1750 1750 44/44 

GE 12AX7WA (1980s) * 8.7% 92% 1520 1600 98/100 

GE 5751 (1950s) 2.5% 98% 1190 1200 69/70 

JAN/Philips 12AT7WC 1980s * 11.6% 105% 5780 5500 72/70 

RCA 12AX7 NOS 1950s-60s 8.8% 99% 1620 1600 100/100 

RCA 5751 Black Plate (1950s) * 3.7% 96% 1251 1200 69/70 

RCA 7025/12AX7 NOS 1960s * 6.6% 97% 1580 1600 97/100 

Telefunken ECC83/12AX7 - diamond base * 11.6% 102% 1680 1600 102/100 

RCA 12AU7 NOS Cleartop * 2.1% 96% 2180 2200 18/18 

12AX7 Telefunken smooth plate new Dynaco 12.6% 102% 1610 1600 98/100 

12AX7/ECC83 Telefunken Diamond Bottom Ribbed Plates * 2.7% 94% 1680 1600 105/100 

12AX7 MINIWATT / SUPER RADIOTRON AUSTRALIA 1960s 0.8% 100% 1606 1600 100/100 

12AX7 Amperex Bugle Boy Holland LONG PLATE 1950s * (10) 4.4% 112% 1640 1600 105/100 

12AX7 AMPEREX BUGLE BOY HOLLAND 1967 * 6.1% 109% 1580 1600 101/100 

7025 Amperex Holland orange Globe Logo 1971 7.7% 94% 1690 1600 92/100 

CV4004 Brimar 1961 4.5% 92% 1570 1600 91/100 

CV4035 / 12AX7 Brimar NOS flying leads 9.8% 108% 1740 1600 112/100 

7025 GE ribbed plates 1950s * 2.2% 100% 1610 1600 99/100 

7025 GE ribbed plates 1960s 3.7% 100% 1590 1600 102/100 

M8137/CV4004/12AX7 Mullard Box Plates * 11.2% 92% 1640 1600 92/100 

12AX7 Raytheon black ribbed plates square getter halo 1950s * 5.4% 94% 1520 1600 100/100 

JRC-12AX7 RCA Black Plates 1954 * 4.9% 101% 1590 1600 100/100 

12AX7A RCA gray ribbed plates 1960's-1970's * 12.6% 93% 1660 1600 96/100 

ECC83/12AX7 Siemens long plates early 1960s * 9.9% 95% 1620 1600 104/100 

12AX7 Sylvania Gray Plate square getter halo 1958, 1959 * 8.2% 102% 1710 1600 101/100 

Tesla ECC802S 10.9 of 10.6 2153 2200 16 / 17 

Tesla ECC82 11.1 of 10.6 2601 2200 18.8 / 17 

35

12AX7WA General Electric JAN NOS Green Printing 12/85 batch. 50 samples tested 5/21/03: 

All very quiet. Some popping on turn on, but after the tube warms up this stops. More gain than current production 12AX7 tubes. A touch 

more output than industry spec, far above current production 12AX7s from any current factory. A very nice tube, in a high percentage of 

samples, very close matches on two sides. 

Test Specs: 

Average Output/TC 1.2 1434 102.50%

Recent tube test results 

DATE TUBE B+ 

volts 

BIAS 

volts 

mA 

actual 

TC (gm) 

actual 

Expected 1.2 1600 100 

gp Gain Comments 

6/19/03 12AX7EH 250 -2 1.0 1450 0.017 85.29 

1.0 1440 0.017 84.71 

0.9 1380 0.016 86.25 

1.1 1490 0.017 87.65 

1.1 1510 0.018 83.89 

1.1 1530 0.017 90.00 

0.9 1400 0.017 82.35 

1.1 1560 0.018 86.67 

0.8 1330 0.015 88.67 

1.0 1470 0.016 91.88 

Current TC Gain 

Averages 1.0 1456 86.73

DATE TUBE B+ BIAS mA TC (gm) gp Gain Comments 

volts volts actual actual 

Expected 1.2 1600 100 

6/18/03 Sovtek 12AX7WA 250 -2 1.2 1420 0.017 83.53 

1.3 1440 0.017 84.71 

1.3 1410 0.016 88.13 

1.2 1360 0.015 90.67 

1.0 1250 0.015 83.33 

1.2 1360 0.016 85.00 

1.1 1330 0.016 83.13 

1.3 1450 0.017 85.29 

0.9 1240 0.015 82.67 

1.3 1520 0.017 89.41 

Output TC Gain 

Averages 1.2 1378 85.59

Fairly wide current spread, so beware as one tube will 

work differently in the same amp as another. Gain is 

consistent, but consistently low. This will be a quiet 

tube but will not have a lot of gain. 

39



Expected 10.0 5500 60.6 

6/26/03 12AT7 JAN NOS 250 -2 12.3 6140 0.096 63.96 

7.6 4590 0.068 67.50 

9.3 5550 0.078 71.15 

8.6 5710 0.075 76.13 

10.5 5470 0.084 65.12 

6.3 4000 0.065 61.54 

9.6 4890 0.085 57.53 

10.9 5640 0.092 61.30 

5.3 3250 0.056 58.04 

9.0 5140 0.08 64.25 


Averages 8.9 5038 64.65



Expected 1.2 1600 100 

7/16/03 12AX7C Chinese 250 -2 1.2 1630 0.018 90.56 

1.0 1460 0.016 91.25 

1.2 1620 0.018 90.00 

1.0 1510 0.016 94.38 

1.2 1670 0.018 92.78 

1.0 1550 0.017 91.18 

1.1 1610 0.018 89.44 

1.1 1580 0.017 92.94 

1.2 1690 0.018 93.89 

1.2 1670 0.018 92.78 


Averages 1.1 1599 91.92

B+ BIAS mA TC (gm) 

DATE TUBE 

volts volts actual actual gp Gain Comments 

Expected 1.2 1600 100 

10/15/2003 12AX7B Chinese 250 -2 0.8 1150 0.013 88.46 Construction visually looks the same as older 

1.5dB greater noise on average than 

0.9 1300 0.014 92.86 12AX7WA/G 

1.2 1670 0.019 87.89 Wider current QA spread than new 12AX7WA/G 

0.9 1510 0.016 94.38 Wide plate resistance specs. Quality declined 

0.8 1270 0.014 90.71 Wide TC specs - quality declined 

1.0 1530 0.017 90.00 Lower percentages of matched triodes A/B side 

Quality overall has dropped over all previous 

0.7 1210 0.013 93.08 runs 

Current output down 10%-15% over previous 

1.0 1460 0.016 91.25 runs 

Still better gain than 12AX7R/G 12AX7R2 

0.8 1360 0.015 90.67 12AX7R3 

0.9 1440 0.016 90.00 


Averages 0.9 1390 90.93


TUBE 


Expected 1.2 1600 100 

10/15/2003 

12AX7R - Sovtek 

12AX7WA/G 

Fender STR 250 -2 1.0 1230 0.014 87.86 

1.0 1190 0.014 85.00 

1.0 1200 0.013 92.31 

1.2 1340 0.015 89.33 

1.1 1300 0.015 86.67 

1.0 1210 0.014 86.43 

1.2 1340 0.016 83.75 

1.2 1350 0.0155 87.10 

1.2 1370 0.015 91.33 

1.0 1200 0.014 85.71 


Averages 1.1 1273 87.55


DATE TUBE 


Expected 1.2 1600 85 Gain in this circuit. 

8/10/2004 8th Mullard run 250 -2 1.1 1500 0.0185 81.08 Most consistant batch yet. 

1.2 1650 0.02 82.50 Good quality run 

1.2 1610 0.02 80.50 Gain up over last batches 

1.1 1500 0.018 83.33 

1.3 1590 0.019 83.68 Plate resistance vs TC on target 

1.1 1400 0.017 82.35 Mica assembly best yet 

1.2 1530 0.0185 82.70 Current great - better than R2, R3 

1.3 1620 0.019 85.26 

1.3 1430 0.0175 81.71 

1.2 1460 0.018 81.11 


Averages 1.2 1529 82.42


DATE TUBE 


Expected 1.2 1600 100 

1/20/2004 Sovtek 12AX7WC 250 -2 1.1 1350 0.015 90.00 #1 - I did not find a problem 

0311 datecode 1.1 1330 0.015 88.67 

1.2 1470 0.016 91.88 #2 

Tubes tested AFTER 1.4 1560 0.017 91.76 

accepted by GT tests 1.1 1340 0.015 89.33 #3 

1.2 1430 0.015 95.33 

1.4 1550 0.017 91.18 #4 

1.2 1430 0.015 95.33 

1.1 1350 0.015 90.00 #5 

1.2 1430 0.015 95.33 


Averages 1.2 1424 91.88

The Output Tubes – The Pentodes and Beam Pentodes 

A typical pentode trace. 

Many people believe that a 6L6 is a 6L6, an EL-34 is an EL-34, etc. This is far from the case. Each of these tubes have different sonic 

features. Many folks ask the difference between an EL-34 and 6L6. In a nutshell, I guess one can say that the EL-34 has more of an 

articulate mid-high and high end articulate response. If you like a lot of high end harmonic articulation and distortion with a high degree of 

articulation, than perhaps this would be the preferred tube. In the case of the 6L6, there are stronger lows and mid-lows, and for mass 

overall output, this may be the taste for you. Some folks love the 6550 and KT-88 tubes in amps if they get most of their tone and 

distortion from pedals or effects. These tubes yield a lot of clean headroom. 

Many ask me which is “more powerful” between the EL-34 and 6L6. Some feel the EL-34 is stronger, due to the 100 watt Marshall or 

Hiwatt type amps of the past. This is not the case in reality. These amps at times had higher plate voltages, so at times produced more 

wattage for a given tube type or tube set. A decade or so ago, Fender had an amp called the “75” that produced about 75 watts out of a 

duet of 6L6’s, as one example. 

There will be charts as we progress, that show the same types of tubes used in the same circuits using the same plate voltages and bias 

voltages. The output is in milliamps. A tube with a higher reading, is more powerful than a tube with a lower reading. It is as simple as 

that. This does not mean a higher number is “better” … there are many other aspects regarding tone. This is just a simple power 

measurement for the folks asking me which tube is stronger. This does not indicate or mean that a higher current tube is better in any way. 

46

This is simply a basic power output scale. There are many other qualities that are the aspects of the tone and sonic signature of an output 

tube. 

The complete list of Groove Tubes output tubes can be seen at http://www.groovetubes.com/tubes_list.cfm?ObjectGroup_ID=108 

The links below on each tube when shown, will be for a duet, a matched pair of two tubes. The link just above has the complete list, where 

one can find duets, quads, and single tubes for use in single ended class A amplifiers as one example. Before we get into the details of 

output or power tubes, a few words on biasing may be in order. 

Biasing – a needed consideration when replacing or changing power tubes 

Biasing output tubes is critical to great tone and long tube life. Some folks feel that setting the bias so the tubes run very hot, will make the 

amp sound better or be more powerful. This is not the case. Think of the bias much like setting the idle on your car. You can set the idle 

to 4000 rpm, and the car might run fine at high speeds, but mid and low speed drive ability will suffer. You want an amp to sound great at 

all levels, don’t you? 

Bias may be set by a few methods. One is the crossover notch method. This requires a signal generator, load, and scope. This method is 

not repeatable, as it is the technician’s “take” on the waveform that is being judged. The second and most popular method used today, and 

by most amp makers, is the current draw method. Here, an adapter is placed between the output tube and it’s socket, converting the 

current to an output voltage in a 1 to 1 ratio. You set a conventional DVM meter to the 200 millivolt scale, and read your current directly – 

one milliamp = one millivolt. In a typical Fender type amp using 6L6 output tubes, this will be around 30 milliamps or so, depending on 

plate voltage. These tools are sold under many names such as bias tool, bias probe, bias rite, bias king, etc. Some have meters attached, 

some do not. I personally prefer the tool with no meter attached as it is less complex, less costly, and takes less space in my toolbox. 

After all, I already have a DVM that I carry anyway, and need one to check other voltages. 

Adjusting bias on most amps is fast and easy, and these tools quickly pay for themselves the first two or three times used. An example of 

the Groove Tubes tools can be found, with or without a separate meter on the website page at 

http://www.groovetubes.com/groovetoys.cfm?Category_ID=30 

Many folks do not know what the maximum output of a given tube may be. In fact, some tubes of the same family are very different. An 

example of this is the EL-34. Most EL-34 tubes have a stated maximum plate dissipation of 25 watts. In the case of the Groove Tubes 

E34LS tube, this is different. This is a 30 watt tube, quite a bit stronger. This is a proprietary tube design made in the JJ factory on GT 

tooling. Looking at the JJ version and the GT version side by side, you will see heat sink wings on the GT version – more plate mass. 

Tossing a set of these E34LS tubes into an amp that is no PROPERLY biased, is asking for trouble, and almost guaranteed. When I say 

PROPERLY biased, I am not only speaking of the current draw, but the method used to set up the amp initially for a tube change. Just 

plugging the E34LS in and “chasing” bias is a mistake. In the time it takes to get the bias in the proper range, you may blow fuses, ruin the 

tubes, or take many hours off their life. 

47

The proper way to do this, is first install your bias probe on one of the old tubes, and set it’s level to the lowest possible milliamp draw. You 

may also just take your meter on the DC scale (another reason for a separate meter), and set this to the HIGHEST negative voltage. This 

means that –52 volts is the more preferred value than –49 volts as an example. 

Now, you can remove the old tubes with the amp off, install the new ones, and work up to the proper idle dissipation target. 

6L6 and 5881 tubes come in ranges from 19 watts to 30 watts. A KT-66 may have the same rating and be interchangeable with a 6L6, but 

in the same circuit, it may draw 10 more milliamps at load, so it’s bias may be very different than a 6L6. 

Different 6550 tubes and KT88 tubes may also vary. 

If you have something like Groove Tubes, you will not have to rebias if you use the SAME TUBE OF THE SAME RATING after properly 

setting the bias the first time. If the tube type changes though, a rebias is advised. “Same tube” here means just that. If you had a 6L6 

Svetlana, and you changed to a 6L6 JJ tube, these are NOT the same, even though both 6L6 tubes. Same tube means the same family, 

type, and manufacturer. 

Cathode biased amps (Class A) are thought of as self biasing, and this is accurate to a certain degree. The amp maker is expecting a tube 

within the original spec range to be used. Today, this varies more widely than ever before, and if a tube outside the “normal range” is 

used, the amp may run very hot, with tube life short at best, bad and harsh tone, and at worst, tube and output transformer failure. Using a 

tube with a mid range rating number, such as GT’s 4-7 ratings, will be the best choice in most cases. You can also use a bias tool (there 

are 9 pin adapters for EL-84 tubes) and check your dissipation too. Remember, in Class A amps, the tubes run close to or at 100% idle 

dissipation. If a tube has a maximum plate dissipation of 12 watts, such as an EL-84, a bit of math will be needed to be done to see what 

milliamp value should be as the maximum. If this is too high, just go to a lower range tube. It is that easy. As an example, if you have a 

#7 tube in an amp and it is running too hot (high millamps value), drop to a 5 or 6 and the value will drop into a more proper range. This is 

also a cool trick on fixed bias class A/B amps such as Hiwatt and Mesa Boogie amps. Mesa amps like mid range tubes in the 4-7 range. 

Elsewhere in this document is a conversion scale to covert the Mesa and Fender color codes to rating numbers. Mesa amps typically are 

OVERBIASED, meaning they run cold. This is part of the reason that many feel they sound a bit grainy. On my own Mark 1, the stock idle 

with Mesa mid color (range) tubes is only 38%. What we are looking for generally is 50-70%. Using a GT #7 tube in the amp brings this 

into proper range. Cold running (overbiased) amps sound grainy, lack power, and definition. Underbiased amps (hot running) amps sound 

harsh, do not perform as well at low and medium levels, and have shorter tube life. 

48

Maximum plate dissipation on tubes will be found in tube data sheets or in manuals such as the RCA tube manuals. As a quick point of 

reference on a few common tubes: 

EL-84 12 watts 

6V6GT 14 watts 

5881 19-30 watts depending on type 

EL-34 / KT77 25 watts 

6L6 25-30 watts depending on type 

E34LS 30 watts 

KT-88 35-42 watts depending on type 

6550 35-44 watts depending on type 

6V6 output tubes 

49

6V6 Types 

Some of the classic RCA 6V6 tubes to the left. 

6V6R - a newer Russian 6V6 that is from the same tooling as the Electro Harmonix 6V6. This tube holds up well to plate voltages of 450+ 

volts, and will do very well in amps such as the Fender Deluxe Reverb. 


50

6V6C - a new 6V6 off new tooling that is holding up to higher plate voltages nicely and has a sound more folks are starting to prefer over 

the 6V6R as time passes during 2002-2003. 


A GE black plate on the left and a GE gray plate 

on the right. Some folks think that the black 

plates sound better or perform better than the 

gray plates. I will leave this to your own 

judgment. 

Comparison of new and NOS tube tubes in a mid rating (bear in mind, the “NOS spec” number we are looking for here 

is 45.0 milliamps below for a 6V6 

TUBE Output in 

Milliamps 

6V6 ( #6 rating) 

6V6A USA NOS RCA Blackplate 43.7 

6V6GT Tung Sol 45.4 

6V6 Chinese 47.2 

6V6R Russian EH 51.9 

51

6L6 / 5881 / KT-66 Types 

The shorter bottle (but very strong) 5881 

6L6-CB - Chinese, softer vacuum, coke bottle shape. Quicker to distort, with a warm soft tone. 

A great blues tube for smaller venues when you want to tone it down a bit. In a 50 watt Fender 

type amp, in a Groove Tubes 1-3 rating, the typical Super Reverb will start to break nicely at “3”, 

with a humbucker guitar at “10”. 

Sometimes these are not easily available and being phased out by the factories in China. If you 

purchase these, be sure they are tested. If you want the sound and character of these, but 

cannot find the coke bottle tubes, some of the straight bottle Chinese in a 1-3 rating will a close 

result. There are also though, straight bottle Chinese tubes, that have higher output than many 

of the current (2003) Russian offerings, so be sure you know what you are getting. 


52

This is the 6L6C or straight bottle Chinese tube. There are MANY variants of this tube. There are 

versions from Shuguang (the major tube factory in China with the highest quality), and Sino ( a co-op in 

a manner of speaking, of many smaller factories. 

In the past, Chinese output tubes developed about 20% less output than their Russian counterparts from 

Sovtek and Svetlana. In 2003, this is no longer the case, and their power for some of their tubes are 

15% or so ABOVE that of the average Russian 6L6 offering in many cases. 

In mid 2002, tubes from Shuguang, also put to rest the issue of Chinese tubes not handing higher plate 

voltages. The Chinese have new tooling, and are pouring a lot of money into tube production. 

This tube in a 1-3 rating is a blues masterpiece. In a 4-7 rating, it is a upgrade from the stock Sovtek 

6L6 tubes offered as standard issue in many new amps. 

In the summer of 2003, there will be a new version of this 6L6 offered by Groove Tubes that will be built 

off their proprietary tooling to their specifications. This tube will be offered by no other tube supplier. 


6L6-B (also known as the R) - Russia, Reflector, the Sovtek 5881WXT, Fender 6L6GC. Sturdy tube, 

one of the physically most robust. Great for touring stuff that gets thrashed by heavy handed road 

crews. This is the stock tube in currently manufactured Fender mid and lower priced amps. A very nice 

general purpose tube. This is the most common supplied 6L6 in most production amps today. This is 

stock in all of Fender’s 6L6 based amps with the exceptions being their Custom Shop amps such as the 

2003 made Vibro King and Vibroverb amps. Vibro King amps prior to 2003 were shipped with this tube, 

New amps are shipped with the 6L6GE 


53

6L6-S - from the JJ factory but re-based and re-pinned by Groove Tubes if or when they come in with 

stamped pins with dipped pins that tear up some vintage sockets. The rolled polished pins are also 

tapered. 

A bit of an extended midrange that a lot of heavy rockers love. Something of a cross between an EL-34 

and a 6L6 in the mids. 

This tube is favored by some Fender folks looking for a bit of an edge over the stock tubes, or in a 

Blackface Twin Reverb, Bassman, Bandmaster, or Showman, will give a bit more aggressive nature. With 

ECC83s in V1 and V2 of these amps, they will come a bit closer to a British sort of edge or character. 

These tubes are very inconsistent at times from the factory so good testing is very important. They also are 

one of the few tubes that need an extended burn in period before testing, or the testing will be rendered 

useless as they change dramatically in many cases after the first 5-10 hours. 

If you buy these from a generic vendor, be sure to have your bias rechecked, and probably readjusted, after 

the first 50 hours or so of use. 

The GT supplied tubes go through an extensive burn in process, as well as testing process, and thus may 

cost a bit more than bargain brands of these tubes. 


54

6L6-R2 - The Svetlana 6L6. Prior to 2002, this was a great tube, something of a Sylvania STR-387 copy. 

Due to the manufacturing changes, possible lower vacuum and material inconsistencies, these are not the 

same as the tubes in the past. You need to get these from a vendor that will test for low vacuum, gas 

leakage, and grid leakage. This is costly and time consuming, so expect vendors that have the equipment 

to test for these things to charge a premium. 

These tubes were patterned off the Sylvania STR-387 that was a popular Fender 60s era tube. These are 

one of the finest 6L6 tubes sonically and also from a reliability standpoint. Just make sure yours are 

checked for low vacuum and gas issues during your vendor’s testing cycle. 


5881-A - NOS, Tung-Sol, JAN Philips 6L6WGB, or other NOS tubes at times, great in Fender 

Black Face amps for a lot of my Blueprinting clients. The tube used a lot in Black Face Fender 

amps as original. These amps also used the Sylvania STR-387 and GE. These may not be 

carried in the future as the new 6L6GE is preferred. As of maybe April of 2002, these are 

generally no longer stocked by Groove Tubes. The NOS tubes left are not in sufficient good 

supply to offer reliable matching using the GT system. These were replaced in the GT lineup 

with the short bottle NVM (new vintage manufacture GT-6L6GE USA tube. 


55

A GE version of the Tung Sol 5881 – probably made by Tung Sol perhaps as this has the 

same plate structure, the same mica spacers. 

KT-66-C - China, the KT-66 that folks like Valve Art and others sell as the KT-66 at times. 

Many times the Valve Art folks and others sell the Sino co-op tube which is not as high of 

quality as the other Chinese tube from the main factory in China. Good to about 450 volts 

B+ maximum. Don’t try to run this tube at higher plate voltages, you are only asking for 

trouble. 

The KT-66 is a full bodied tube, with stronger and more linear mids than most 6L6 tubes. 

The GEC (UK) KT-66 was used in the Marshall JTM-45, and the first 100 watt Marshall 

amps. It’s tone can be heard by Eric Clapton on the John Mayall Blues Breakers 

recordings. This is the tube that is standard issue in the Carr Amplifiers Mercury model 

The “C” version of this tube is not as close of a reproduction of the original GEC tube. It is 

also limited to about 450 plate volts to function in a reliable way. Some prefer it’s tone 

over the HP version though, such as Steve Carr in his Mercury amp. 


56

KT-66-HP - Russia, built under contract for GT to GT specs on Groove Tubes tooling. 

A copy of the GEC tube. Handles higher plate voltages than the KT-66C. Great to 525 volts, and the tube 

the Dr. Z Route 66 was designed around. The tube to be used in Marshall JTM-45. Also a great tube in 

Fender Black Face and Silver Face amps. 

The "KT" meant "Kinkless Tetrode, and this tube was developed to get rid of the 6L6 kink in the response 

curve. This was accomplished, and this is why the mids of this tube are smoother and more linear than a 

6L6 or 5881. This tube is standard issue in the Dr. Z Route 66 amplifier. 


This is the tube that I use in my own JTM-45 amp. 

These are new KT-66HP tubes before going through the GT (Groove Tubes) process. These 

are only available from GT, as this is proprietary tooling of theirs over in Russia. 

As you can see, the Russians are not all that consistent. These two tubes are from the same 

factory run. Their size is quite different visually, as were their specs. This is yet one more 

reason to trust your vendor. 

57

6L6-GE - made in the USA by the Groove Tubes factory with USA parts and labor. This was the 

tube used in the four Fender Showman heads at the 1968 Hollywood Bowl concert by Jimi 

Hendrix. (not always a Marshall guy). This tube has received a lot of praise in various reviews. 

One original set of these from February 2002, I have had running at 105% output, 24 hours a day, 

seven days a week. They currently have over 6700 hours on them, and they have only dropped 3- 

4 milliamps during this period. This is due to a very high vacuum, and USA parts, cathode 

coatings, etc. These are more expensive than most non USA 6L6’s due to materials and labor 

costs being much higher. Their cost is offset with a very long life, and sonic qualities. They 

typically have at least a 15 degree wider sound stage image in any amplifier that uses 6L6 tubes. 

The traces on this tube are duplicates of the traces of the original General Electric tubes. This is 

not surprising, as these are made on the original tooling from the last plant that made these in the 

USA, and they use the same formula for plate materials and coatings. The mica spacers come 

from the original source. I coined these tubes “NVM”, for New Vintage Manufacture”, before I 

came to Groove Tubes while I was doing the original run testing. This tube is the standard offering 

in Fender’s Custom Shop amps such as the Vibro King and new (2003) Vibroverb. 


6L6WGB China – Groove Tubes tooling 

A new tube (July 2003). Output is strong, over 80 mA where 72 would be the expected standard. 

The output puts these tubes at the top of the 6L6 list as shown in the below snapshot of the 6L6 / 

EL34 test results. 

Nice curves, handles high voltage to 475 plate volts with no problems. Static match is well within 

industry standards, meaning testing and matching by any vendor would be necessary as usual. 

Dynamic traces are about average on these unmatched samples, again, GT matching will turn 

these into a nice finished product. 

6L6GCR Shuguang 5/03 76.6 

JJ 7027 78.3 

6L6GCM STR Ruby April 03 79.4 

KT-66 Genelex GEC 80.4 

6L6S JJ 81.7 

6L6WGB GT tooling China 82.6 


58

EL34 – E34LS – and 6CA7 types 

59

To the left are the RCA 6CA7 tubes. Even though a replacement for the EL-34 and marked as such, they are 

different tubes than the EL-34. They have a much stronger vacuum, an active beam forming element, a 

bigger bottle, and handle in generally, higher voltages. 

The GE version is preferred by myself over the other USA versions from a sonic point of view and testing. 

There are more “6CA7” tubes showing up all the time these days. They are currently nothing more than big 

bottle, low vacuum, EL-34 tubes. They generally have even less output than the same maker’s EL-34 

offerings. 

GE6CA7 – A strong beam pentode with an active beam forming element. Some folks think of this as an EL-34, but this is a stronger tube 

in all aspects. This is a drop in replacement for an EL-34, but a rebias is advised. This tube has a much higher vacuum than an EL-34, 

and much stronger construction. It is currently being developed for re-release as the Groove Tubes 6L6GE had been released. Look for a 

possible late summer 2003 release. The initial tests on this tube show very strong results. These will run at over 800 plate volts. There 

are current numerous “6CA7” tubes being made today, which are little more than the same plates and materials of a makers EL-34 in a 

larger bottle. In most cases, these do not even have the performance of the same companies EL-34 offerings. These do not trace or 

perform in any way as the original USA GE 6CA7 of the past. This tube is now in development by Groove Tubes for possible release in the 

summer of 2003. The original GE tooling from GE in Kentucky was purchased by Aspen Pittman of GT, and is up and running. The 

6L6GE was Aspen’s first GE tube reissue. There are more to follow, this 6CA7 next in the list. 

60

EL34C - Chinese EL-34 (25 watt tube). This latest ChineseEL-34 has closer response curves and 

performance of the Siemens EL-34's used in the late 60's and early to mid 70's Marshall amps. 

This is a great tube in Marshall model 1987 and model 1959 amps. 


EL-34-R - Reflector (Russian) EL-34 (25 watt tube) sometimes sold as the EH tube. Pretty linear and 

Mullard like in some aspects of its character. Not as close to the Siemens tube in Marshall amps or in the 

curve traces. The current Chinese offerings are more close to the original Siemens tubes in a Marshall 

amp. 


61

EL34R2 - Svetlana (Russian) 25 watt tube, used a lot as standard in Rivera amps. This is a stronger 

tube than the R or older C in the pre 2002 factory runs. The 2002 factory runs were down about 15% 

on power. Most current EL-34 types show better curves and more linear response than the 2002- 

2003 versions of this tube due to changes at the factory. If you have a vendor that can test for low 

vacuum and gas leakage, then this is a super tube. 


E34LS - JJ factory but with a GT developed heat sink assembly extruded onto the plate assembly. Hold up a JJ 

and a GT and you can see the difference without any sort of equipment (with the naked eye its easy). 

This is a 30 watt tube and bias should be checked and adjusted when one uses these. Very high output and 

strong mids. Used by Gibbons, Perry, and Walsh to name a few. This is the stock EL-34 amp tube for Bad Cat 

and Matchless. The JJ version of this tube is a 25 watt tube. This tube cannot be dropped in to an amp without a 

rebias. It is a much stronger tube. 

In a typical Marshall 50 watt amp, seeing 70-80 watts is not uncommon. This tube has much stronger mids and 

lows than the “normal” EL-34. If you are looking for that gut punch power, than this may be the tube for you. If 

you want your British amp to have it’s original sound and character, then stay with one of the 25 watt EL-34 tubes. 


62

The Groove Tubes GT-6CA7GE 

“The GT6CA7GE is the classic American-style EL34 tube made in the exact likeness of the famed 

General Electric 6CA7 tube of the 60's. This was the tube we sold to Eddie Van Halen for his Marshalls 

used on the first two Van Halen albums (often cited for his best tone on record).” 

The Groove Tubes 6CA7GE is a USA-style power pentode tube with similar specifications to the 

European style EL34 tube. However, the 6CA7 differs in design from the common EL34 types by the use 

of an active beam-forming electrode similar to that of the 6L6 and 6550. Groove Tubes purchased the 

actual GE production formulas, along with many of the original General Electric machines and vintage 

Plate materials via the closing of the GE tube factory several years ago. Using these original machines, 

the original components and their “secret processing recipe”, GT was able to authentically reproduce the 

classic GE power tube. Pittman says, “The 6CA7GE will appeal to players using Marshall EL34-style 

amps (and the many Marshall impersonators), providing a whole new sonic signature option that will 

certainly please a wide range of musical playing styles. If I had to simplify the description, the GT6CA7GE 

is somewhere between a Mullard EL34 and a GE 6L6…more ‘chest' and certainly more power than any 

EL34 tube. Plus, they are directly interchangeable in all EL34 circuits (with proper rebiasing, as always). 

This tube has a bias point between an EL-34 and 6550 (a bit more than a 6L6 actually). It’s power output 

falls between an EL-34 and 6550. This tube has a true active beam forming element which is not present 

on the EL-34 which is one of the major differences. 


63

Miscellaneous and other output tube types 

The 1980s JAN Philips 6L6WGB 

This is also known as a 5881 a lot of the time. This is a military grade heavy duty short bottle 

tube, that looks very similar to the GE, less similar when compared to the Tung Sol 5881. 

This is fantastic sounding tube, and the tube I use in some of my 6L6 based amps. 

The Philips 6L6GC 

This is similar in many ways to the Sylvania STR387 that is a favorite of many Fender folks. 

64

The RCA Gray Plate. 

65

6L6 / KT66 / 5881 / EL-34 Tube output current output comparisons 

The “numbers” we are looking for here, for an average NOS sample run on a 6L6, is 72.0 milliamps as an example. In the case of the 

GE’s the factory originals ran from 72-78 milliamps for the average sample tube as per the original GE spec. 

If you want to know which tube is the most powerful in a given circuit, this chart below will answer that question. 

6/30/03 – Chinese 6L6GCS PM tubes – stronger output than 6L6S on average. 

These are an older somewhat modified coke bottle shape with an old mica spacer 

design and a fanout wire to base design. A dual bottom getter design. The plates 

are not aligned to the bases with any sort of consistency, meaning the plate 

orientation to the key slot of the tube base is not consistent. 

There were three duets of this tube tested. The average set, marked with a #74 

sticker from the distributor was the final test set. The static match was only fair, 

even though the tubes carried the same “matching” number. The dynamic match 

was not very good, and in fact, at plate voltages above 410 volts, started to be on 

the edge of possible problems with current fall off. These tubes prefer voltages 

under 400 plate volts. 

Testing for burn in and stability, initial measurements on one duet was recorded at 

83.0 and 84.8 mA respectively. The upper voltage curves (410+) were not stable. 

After a one hour burn-in the measurements were respectively were 83.4 and 83.6 

mA statically, with a bit nicer curve traces, thus indicating that burn in may improve 

performance and consistency. These tubes would benefit from a bias check after 

the first 20 hours of use in a guitar or bass amplifier. 

66

Power output comparison test results of various output tubes 

Many of today's amps can 

switch power tubes via a bias 

switch of some sort or other 

method. This chart compares 

some of the same broad families 

of tubes using the same 

voltages, circuit, and bias for a 

power comparison 

COMMON FAMILY (#5 

RATING) 

6L6 / EL-34 / KT-66 / 5881 

6550 / KT-88 

For 6L6 types, the standard 

expected reference output at 

1957 RCA test settings 

should be 72.0 milliamps. 

The EL-34 tubes tested here 

were tested at 6L6 voltages 

and bias settings to show 

their performance in the 

same circuit. In an circuit set 

up to test EL-34 tubes 

specifically, the expected 

output would be 100 mA, 

higher due to lower bias 

voltage, and higher plate 

voltages. 

CURRENT 

IN mA 

7591 Russian 58.4 

6L6GCMSTR Ruby Tubes 

(older) 68.1 

6L6C Chinese 69.0 

67

6L6B Sovtek (5881 WXT) 71.0 

6L6CB Chinese Coke Bottle 71.6 

6L6R2 Svetlana October 2002 72.3 

6L6GE GT - Original run 72.7 

6L6R2 Svetlana pre 2002 73.5 

5881 JAN Philips 74.8 

5881 Chinese 4/03 75.2 

5881 NOS Tung Sol 76.1 

6L6GE GT November 02 76.2 

6L6GCR Shuguang 5/03 76.6 

JJ 7027 78.3 

6L6GCM STR Ruby April 03 79.4 


6L6S JJ 81.7 

6L6WGB GT tooling China 82.6 


6L6GCS PM China 6/30/03 84.0 

KT-66HP Russia 85.3 

EL34R2 Svetlana (pre 02 date) 86.3 

EL34 Svet/Sovtek 0211 

datecode 87.2 

GT-6L6GE 1/04 prod - new 

plate smaller bottle / higher 

vacuum 87.9 

EL34R EH / Reflector 89.0 

EL-34 Siemens marked Mesa 89.8 

EL34C China 91.0 

KT-66 China 91.6 

EL-34 Mullard 1970's 93.1 

E34Ls JJ / GT tooling 30W 97.1 

GT 6CA7GE 5/19/04 run 97.9 

GE NOS Ref 6CA7 108.2 

68

RCA 7027A NOS 

A big KT-90 

69

6550 / KT-88 Tubes 

70

The KT-88C 

A very strong KT88, with multiple getters on the sides and on the top. 

This is actually the strongest of the KT88 family, and not in character 

with the original GEC Gold Lion tubes which were not quite as strong, 

but more linear. 

For something like a Hi-Fi amp, the SV version may be preferred. 


71

This is a duet of the KT88SV tubes made in the JJ factory, 

once again on Groove Tubes developed proprietary 

tooling. If you compare this with the JJ sold version, you 

will notice that the GT version has four massive heat sink 

wings on a more massive plate structure. 

These are smooth and linear, tracing as the original GEC 

Gold lion tubes. These are very strong, and preferred by 

high end audio folks, or guitar players going for massive 

levels of clean headroom … allowing their tone to come 

from their effects and pedal boards. 

These are very long lasting tubes, even in severe service. 

Notice than unlike the Chinese KT88 version, this tune 

does not have side getter flashes. 

These are the Svetlana 6550. These are 

popular in Sunn amps, many Marshall amps, 

and amps looking for clean headroom. These 

are a good solid offering, but in mid 2002 

dropped a bit in power when compared to the 

newer 6550 offerings from China. 

72

Sorry, but I just HAD to show a photo of an 

NOS 6146 tube. 

These were the original Ampeg SVT output 

tube, and to me, just magic. High voltage 

comes in the top to the plates, not from the 

base. VERY VERY strong tubes, to say the 

least. 

6550A - USA NOS GE no longer in stock. The strongest of all the 6550's. This is the longest lasting of any of the 6550 family. 

73

6550-C - Chinese and a fun tube, as you can get these to distort in amps where the 6550A would not. 

These are in two types, the coke bottle shape (distorts faster), and the straight bottle with more power. 

Typically used in amps where a lot of power or clean headroom is desired. 

These may be either the coke bottle or straight bottle, depending on stock. You may want to inquire, as the 

coke bottle has a softer vacuum. 

The straight bottle is a great tube, and has more output. If you want a 6550 that will distort nicely instead of 

staying clean right until the end, this may be your tube. Just order it in a 1-5 rating or so. In ratings of 5-8, 

they will (straight bottle) keep up or exceed the power output of the current Russian (2002-2003) tubes. 


74

6550-R - Russian 6550, takes longer to distort than the Chinese, maybe somewhere between the A and 

the current straight bottle C. 

This is a very clean tube. This is also a great Ampeg SVT tube. These are generally selected Svetlana or 

Sovtek tubes. Svetlana tubes generally have a brown base material in the metal base, and Sovtek tubes 

generally have black base material. These are picked and tested for characteristics rather than by maker. 


KT-88-C2 - Chinese KT-88 – Nice and strong This tube finds a home in amps such as the Park 75, 

Marshall Major, and 6550 type amplifiers. 

This tube does not follow the original GEC KT88 curves. It is far stronger than the original KT88 from 

GEC, so if you are looking for a tube to replace that particular tube, be sure to check your bias, as these 

are much stronger than the common new KT88 tubes from most makers. For a Marshall owner, or amp 

owner using 6550 tubes, that wants even more power from the amp, this is the tube. 

These tubes are VERY pure inside. The have multiple getters, both top and side flashed, so they are 

also a very long lasting tube. 


75

KT-88-SV - JJ factory made off Groove Tubes tooling. Different plate assembly than the JJ version, with 

a large heat sinks welded to the plate. It may have a different base, pins, at times if necessary. 

There is also a KT-88 that is a JJ tube that is stronger by a wide margin than the original Gold Lion GEC 

KT-88. It biases differently. 

The KT-88SV (think of the "V" as "vintage"), where the output of this tube and design was redone to copy 

the GEC Gold Lion. The SV traces as the original GEC tube traced and is great in McIntosh amps, Park 

75's, and Marshall Major amps. 

This tube is NOT the same as the JJ version, even though produced in the same factory. This is a GT 

designed tube off Groove Tubes tooling, and is not sold by any other vendor that does not sell GT items. 

This is the most musical of the KT-88 tubes for high end audio applications also. 


76

Power comparison chart of current output for the KT-88 / 6550 families 

KT88 / 6550 / KT90 family 

6550/KT-88 bias raised to 

reduce current. Much additional 

power and clean headroom over 

6L6 / EL34 if power supply has 

the capacity. At 6L6/EL34 

voltages, typically 120-140mA + 

#5 rating 

CURRENT 

IN mA 

KT88 Svetlana 4/8/03 85.0 

KT88 Svetlana 4/8/03 87.0 

6550R Svetlana 92.5 

KT-88SV JJ / GT 97.3 

KT-88C China 103.6 

6550C China 107.3 

KT-90 Ei 114.8 

77

The EL-84 Tubes 

TECHTIP ON EL-84 AMPS 

Before I get to the EL-4 tubes, I would like to address a few points. 

Typically, EL-84 based amps are cathode biased. (There is at least one exception I can think of here off the top of my head, a Fender 

Tremolux from the Black Face era, that for one model used 6BQ5 (EL-84) tubes in a class A/B configuration). 

Cathode biased amps typically run hot from the start, as the output tubes are always running full bore, even when you are playing quietly, 

or not playing at all. Added to this factor, is that most of these amps also run their output tubes at voltages in excess, and at times FAR in 

excess, of the original design limitations of the tube. 

These amps are typically called Class A amps at times, but when there is more than one output tube, these amps run in push-pull 

configuration, just as typical class A/B amps. 

Adding to the hard life of these tubes, amps such as the Vox AC-30 series, are not well ventilated. The stories of these amps being less 

than reliable, or even stories of them starting on fire, are not all that unfounded. 

When tubes are not matched, they work against each other. This is explained elsewhere in this book, but to briefly illustrate, it is like two 

riders on one of those tandem bikes. One is me at 165 pounds, where my main form of exercise is speaking while standing, and the other 

is a 165 pound Olympic class rider with .02% body fat! I will do nothing but drag the other rider down. Mismatched tubes produce more 

heat too, a lot more. If you have access to one of the remote thermometers, check you output transformer temperature on your “class A” 

amp after running for 20 minutes. Then check it with a dynamically matched set of output tubes. You will see at least a 20 degree drop, 

and the side benefit of better tone, level, sustain etc., are icing on the cake. This temperature drop will lengthen the life of parts like your 

capacitors buried in the chassis too, and the other components. Look inside a well used AC-30 at the discoloration from heat on the 

components as another example. 

The reason I am pointing this out here, is these amps have a somewhat DIFFERENT requirement than their 6L6 and EL-34 brothers that 

run in Class A/B fixed bias. Some folks feel that “matching is matching”. While this can be argued, these EL-84 based amps really do 

require, from my experience, DYNAMICALLY matched tubes, and will benefit strongly using these over STATICALLY matched tubes. The 

differences on these two matching methods is explained elsewhere in this book. 

In typical Vox AC-30 and other amps using EL-84 tubes three hours per night, four nights a week in tour, I have found that typical life 

where the amp sounds best can be as little as 100 hours with statically matched tubes. It increases to 300 hours with dynamically matched 

tubes, and the temperature drops considerably. If you have a EL-84 amp with more than 500 hours on the tubes, I can guarantee that 

they need to be changed right now, and you will hear the result. 

78

A pair of the famed Telefunken NOS EL84 tubes. 

Bugle Boy / Amperex are also sought after. 

In my personal experience, there seems to be less difference in many makes of EL84 

tubes, and many makers made them for each other. Remember, Philips owned 

Mullard, and the Brits also made tubes for USA folks and visa versa, so this got very 

complicated. 

In some amps, these NOS types jump up and grab you. In other amps, they don’t 

show as much of a change. In Hi-Fi amps they work very differently than they do in 

guitar amps. 

Currently, the JJ EL84 is my most used new EL84 tube, and my personal favorite for 

my own amps and my clients amps. Make sure they are closely matched in “class A 

amps”, which are generally really push-pull cathode biased amps, that do prefer 

matched output sets. 

80

EL-84R - Reflector Russia (as the Sovtek). Reliable, but not as articulate as the others. Darker sounding in 

most amps. The stock tube in most Fender, Vox, and many other EL-84 based amps. Not as bright as the 

EL-84-S, and not as articulate. If you liked the sound of your amp with these tubes it, then you may want to 

experiment with some others too, for a different tone which some prefer. 


EL84-Y - Ei – This is an older photo, and when I get some time I will update this. The current version of 

this tube in 2003, is easy to spot. It’s plates are silver, not the darker material as is shown in this photo 

that was used in earlier versions. 

Brighter than the S or R versions of EL84, these can be a bit unreliable in some amps if they are not 

checked for low vacuum or gas issues. Low vacuum will cause short tube life, and flat out failure at times, 

usually at what may seem the worst time. Many vendors sell these – be sure your vendor tests for gas 

leakage, grid leakage, low vacuum, and has a good warranty. A good tested set of these sound great 

though, so it may be worth the attempt to experiment. 


81

Output power chart for EL-84 family 

The “stock” NOS number here is 45-48 for reference. 

TUBE Current in 

Milliamps 

EL-84 

Chinese EL84 6/27/03 39.0 

EL-84 Sovtek 44.6 

EL-84S JJ 45.3 

Philips EL-84 46.5 

General Electric 6BQ5 48.0 

EL-84Y Ei 53.0 

Electro Harmonix 6/27/03 56.7 

EL-84S - JJ - The best of the current EL-84's by most folks thinking for higher power, more articulate mids 

and highs than the EL-84R. These are reliable, powerful and articulate. 

They are not as long lived as the “R”, and not as reliable, but that is usually due to their moderate 

inconsistency. Buy these from a vendor that does proper testing and they will be long lasting, articulate, 

and full of rich harmonic content. 

My personal favorite EL84 for most applications for the moment. There is a new Chinese offering under 

test that is showing some amazing results, and a new offering from Electro Harmonix is looking great and 

may give this tube a run for the money, and be even stronger. 


82

Chinese EL-84 from June 2003. 

Performed nicely, the first Chinese offering I have seen of this tube. It was the 

lowest in output of any of the current EL-84 tubes. This may be a good thing, as 

many cathode biased amps can run very hot, and this tube may help tone them 

down. 

The curves on this tube were typical. They static match was average, thus 

indicating matching is a requirement. Dynamic matching will be a bit more difficult 

without high end equipment. 

This tube ran reliably at Vox AC-30 voltages. 

Chinese EL-84 traces. 

83

New Electro Harmonix EL-84 (June 2003) 

This tube may be a winner. EH has a great 12AX7 and a great 6V6. They are also 

offering an octal preamp tube once more – hats off to EH. 

Nice curves - average dynamic match out of the box, will need matching. Static 

match close out of the box. Stronger than JJ EL-84 and may be preferred over 

Sovtek EL84 as well. A very nice tube, and another choice for folks with EL-84 

amps. Ran high voltages without failure way beyond specs. Handles voltage far in 

excess and was MUCH more reliable than the Ei EL-84. 

Highest power of all the current EL-84 tubes. Very consistent in output power. No 

duds in the batch. 

This tube may be the ticket for the biggest sounding EL-84, an honor that was held 

so long in regard to new made tubes, by the JJ EL-84. It is going to boil down to 

user tone preference. This tube is worth a try. 

The EH EL-84 traces. 

A bit hard to tell in this photo, but the two tubes are slightly off as far as maximum 

output (which would show up in a static match perhaps), but their dynamic curves 

were very smooth and linear. If you could see the scope and have one EH tube and 

one of the Ei tubes or the Sovtek EL-84 tubes, running at the same time, the 

difference would be very apparent. Perhaps on future tests I will show tube curve 

comparisons. 

In any case, this tube had great triode curves (as shown here) and pentode curves. 

They passed the torture high voltage and high current tests with flying colors. 

84

7027 and 7591 Tubes 

7027 – JJ – Used in amps in the past such as Ampeg. 


7591 - Reflector, Russia. – Used in amps in the past such as Ampeg. 


Rectifiers – vacuum tube and solid state 

NOS JAN Philips 5U4GB rectifiers. 

Rectifiers convert AC to DC, and many feel they do not contribute to the tone of the amp. 

They may not be in the tone path, but they do (to me at least) affect the feel and the 

output of the amp. Many of today’s rectifiers put out the proper voltage, but do not supply 

the current of the older versions. Less pure materials, faster production, lower vacuum, 

are all factors. The new rectifiers will generally not have the life of the older rectifiers of 

the same family or type, and may have more of a voltage drop at their maximum capacity 

or ratings. This may be a good trait for some folks looking to tone down an amp that is a 

bit too strong for their tastes. The increased sag will also give a different feel and 

compression. 

They may not impact tone, but they surely will impact touch and feel. 

85

These are some of the 1980s JAN Philips 5Y3 rectifiers. 

Many people have many opinions on rectifiers in vacuum tube guitar amplifiers. Some people feel that solid state rectifiers are more 

reliable. This is true in one sense, but gives vacuum tube rectifiers an "implied" unreliability. 

I found vacuum tube rectifiers rarely fail, although the Chinese 5U4 from 2000-2002 did have a high failure rate when new. In some cases 

at one point, a 50% new failure rate was not all that uncommon. If they do fail, it is usually a case of physical damage, or the rectifier being 

bad in the first place, and failing in newly made amplifiers where the rectifier failed due to bad manufacturing. In the case of the Chinese 

rectifiers, when new, I have seen a 50% failure rate. A bad rectifier will usually show itself in one of two ways. (1) Power light comes on, 

but NO sound at all comes from the amp. (2) When you flip the amp off standby and into play when all seemed fine, your fuse will blow. 

There are rectifiers with 4 pins and 5 pins, of the same type. The four pin rectifier may be indirectly heated, and not work well in some amp 

designs, so if a 5 pin rectifier comes out of your amp, put a 5 pin rectifier back in your amp. 

The rectifier basically converts the AC line current coming into your amplifier to the DC voltages that are needed. A solid state rectifier is 

easier to fit into an amp design, can provide more power, does not run as hot, and is less expensive than its tube counterparts. 

The Marshall JTM-45 used a tube rectifier, but when Marshall came out with the 50 watt version of the amp, a solid state rectifier was a 

change in the design. Most Fender amps over 40 watts use a solid state rectifier. 

If you have an older amp with a tube rectifier and want to replace it with a solid state replacement, be sure to first check your amp and 

make sure all the other portions of the amp are in good condition. The solid state rectifier is capable of higher voltages. 

86

As you will see in the charts, different rectifiers have different characteristics. In many amps, you can get a different sound and feel by 

replacing the rectifier with different types. Matchless is one example that lets the user choose various tube rectifiers as part of their 

features. 

A solid state rectifier will give very fast rise time and response as the voltages are produced very quickly. A vacuum tube rectifier will yield 

more to the player’s touch dynamics, sound warmer and less harsh by some folk’s feelings, and give the compression and sustain in a 

much different way than its solid state brother. 

When one initially hits a loud note or chord, with a tube rectifier, there is voltage sag, in some cases, a LOT of sag. As the note or chord 

starts to decay, the voltage then builds, and what you have in essence, is a built in compressor / sustain device. 

If you look at the charts, you will see how fast the voltage is developed with a solid state rectifier versus a tube rectifier. 

Using the same circuit, we replace the rectifier section. We used the same voltage input in all cases, but in the case of the 5Y3GT rectifier 

in the last test, our transformer output of 333v had to be reduced to 330v, as the 5Y3GT would have been pushed just a touch beyond its 

design limits. This change had negligible results on the final outcome. 

The tests were measured over a period of 500 milliseconds, or ½ a second. 

Using the commonly used configuration of 1N4007 diodes, its easy to see in comparison with the vacuum tube rectifiers, the difference in 

rise time to get the voltage we are after. It is a bit less apparent, but in this case with the solid state rectifier, we had 449.08 volts available 

for our B+ voltage, the highest voltage in the group. This is also something to keep in mind when you replace a tube rectifier with a solid 

state replacement, as your output tube bias will probably need to be checked. In the case of a class A amplifier such as a Vox AC-15, AC- 

30, a Matchless amp, most Carr amps, or others, the bias is automatically taken care of, but in older class A amps, you may also want to 

be concerned about the output transformers and capacitors if they are older. The amp will be running at higher voltages, and in some 

cases, much higher voltages as you will see later. 

In .01 seconds, with the solid state rectifier, we basically had full power. Our voltage was 449.08 as previously stated. 

Moving along, we substituted a 5AR4 vacuum tube rectifier. This is one of the strongest vacuum tube units used today. It took about two 

times as long, .02 seconds, for out maximum voltage to be developed, 413.9 volts. This is a fairly large drop from the earlier solid state 

rectifier. 

Going to a 5U4-G, commonly used in some Mesa Rectifier products, we found that it took .40 seconds to develop full voltage. This is a 

long time compared to the solid state device, and twice as long as the 5AR4. This is almost a half a second, and most people can hear 

something that is ½ a second long in duration or delay. Our high voltage was 357.06 volts, or almost 100 volts less than the solid state 

device. 

87

The 5R4-GYB, often used in place of the 5U4-G. About the same rise time as the 5U4-G above, but had a maximum voltage of 330.93 

volts. This is a nice rectifier in some Fender Deluxe type amps that have been made with everything from the little 5Y3 to a GZ34 in the 

past. Its a great blues rectifier in these amps. 

Lastly we tried the 5Y3GT. Rise time was almost a full half a second at .44 seconds. Our maximum voltage was 303 volts. 

Now you have a little more information on rectifiers, and how the changing of types in a rectifier equipped amplifier can change the sound 

of the amp, the power of the amp, and the characteristics of touch, feel, and sustain. 

In most cases, the NOS rectifiers are better than those of today. The Chinese ones still have a long way to go to reach the specs of those 

of the past. The Russian ones are the best of the new ones, and the JJ factory is currently working on a re-release of the GZ34 which 

shows promise. Mesa Boogie products currently use mostly the Chinese variety. 

As a safety note - you can generally go "down" but not always "up". This means you can generally put a 5U4 or 5Y3 in an amp that had a 

5AR4, but it is not recommended to put a 5AR4 or 5U4 in an amp that had a stock 5Y3. The increase in voltages may be too much for the 

other components in the amplifier. 

Rectifier substations / cross reference: 

GZ34=5AR4 GZ32=5V4 GZ31=5U4 GZ30=5Z4 

5Y3 -60 volts @ 125 mA 

5U4GB -50 volts @ 275 mA 

5U4 -44 volts @ 225 mA 

5V4 -25 volts @ 175 mA 

5AR4 -17 volts @ 225 mA 

It is worth noting, that comparing today’s rectifiers with some NOS offerings of the past, may 

show measurable differences. In the case of the GZ34 / 5AR4, in the same circuit, an NOS 

Mullard or USA unit, may have a lower voltage drop (meeting the –17 volts to the left here, 

while a new GZ34 may have the current but perhaps not the voltage. Thus the NOS rectifier 

may allow your amp to produce more wattage at maximum levels. 

Rectifier charts below – These are newer charts than those on my website from a later series of tests. The voltages may not 

correspond with the data in the text above, as the power transformer was changed slightly. In any case, the rise times and maximum 

voltages reached can be compared between the various rectifier types just as easily. 

88

5AR4 / GZ34 above – 416 volts max. 

5U4G above – 364 volts max. 

89

5U4GB above – 362 volts max. 

5V4G above – 380 volts max – Great in Dr. Z, Matchless, Bad Cat 

90

5Y3GT above – 322 volts max. 

6AX4GTB above – 387 volts max 

91

The GT Rating system – an explanation of “distortion rating” also known at times as “hardness 

rating” 

Basically, a #1 will distort sooner, and a #10 later. If, for example, with a mid range tube, say a #5, makes your amp start to break in the 

output section at a volume setting on the amp of "4", then with a lower number tube, like a #2, your amp would have a same sort of break 

into output distortion at say a volume setting of "3". With a higher tube, such as an #8, then you amp would stay clean to about perhaps "6" 

on the volume. 

High rating numbers are not more or less powerful, they just distort later. These are preferred by heavy rocker that want maximum clean 

output, as they get their distortion and tone from effects or pedals. These are not as touch dynamic. 

Low number tubes are very touch dynamic, and more suited for a lot of folks, for smaller venues and recording. These ratings are 

especially great in amps used to play blues or other touch sensitive styles. 

Most folks prefer the 4-7 range tubes, as they are the closest in character and touch to what the amplifier designer had in mind. They are 

also the most versatile. 

Conversion Information – GT numbering, Fender color codes and Mesa color codes 

Mesa to GT numbering system: 

Mesa vs Groove Tubes scale 

Red 4 

Yellow 4 

Green 

5 

Gray 5 

Blue 6 

White 

6 

Fender to GT numbering system: 

Fender Blue = GT 1-3 

Fender White = GT 4-7 

Fender Red = GT 8-10 

92

Myles Saunders Rose has worked in various development, design, support roles, projects including development work for Roland, 

Yamaha, and was Apple Computer's music consultant for the Vivarium project in the late 1980's. He currently has his own company, 

Guitar Amplifier Blueprinting, and also is the head of the SAG (Special Applications Group) at Groove Tubes and Tech Support at Groove 

Tubes. He started playing guitar at the age of 8, on equipment that is called vintage today.... when it was NEW. He will not generally 

admit that he played the accordion from the age of five ... but then again, there were only five or so channels on the television, and one 

was dominated my Lawrence Welk back in those days! 

Myles S. Rose - Guitar Amplifier Blueprinting - 2003 

http://www.guitaramplifierblueprinting.com 

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a tube primer for guitar and bass players - Guitar Amplifier Blueprinting

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