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

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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