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

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