Gain and Current in preamp tubes
I ran into numerous amplifier problems. ALL of these could have been avoided if some basic information was known or considered by the amp owner. I have said a lot on this subject, but it is worth repeating, maybe in a different way, as some folks out there are just not getting the message.
Gain and Output are NOT the same thing.
Today’s off the shelf preamp tubes are very inconsistent. You better get them from a vendor that actually tests them and has standards.
Today’s 12AX7 tubes do NOT have a gain of 100 generally, it is closer to 85. This is not that big of a deal for the most part, as most amp designers have designed around this in many cases.
THE BIG PROBLEM IS CURRENT OUTPUT. A 12AX7 tube at a reference test voltage and bias should be able to supply 1.2 milliamps of output current. 80%-90% of 12AX7s today fall far below this. It is common to see tubes in the 0.5-0.8 milliamp range. This is the cause of ALL my problems with folks this weekend.
Current …. think of this like there are two football players. They are the same height and can run the 50-yard dash in the same amount of time (think of this as gain), but one guy weighs 190 and the other weighs 285 (think of this as output).
Now, in a simple front end amp, such as a Fender Tweed era amp, or some of the high-end classic design amps, this is not as much of a factor. This is like the guys running down the field, and only encoutering one of two folks they are hit by. There will be an impact, but not as severe as if you are hit by six guys.
Channel switching complex front end amps (Bogner, Diesel, Rivera, Mesa, Marshall DSL/TSL, etc.) have lots of trace routing, lots of components, lots of pots, and tone controls. Every single component in the signal path is one more guy on the field, trying to stop you from getting to your goal. Even if you do manage to get to the goal, you are going to be pretty beat up and weak.
The phase inverter – The most important tube in your amp?
VERY FEW tube sellers can check for current. If they check anything at all, it might be gain (and not even real gain, but sort of a version of actual gain), or transconductance. To check this properly (output or gain) takes time and expensive equipment. Your preamp tubes would cost $25.00 each easily, in labor alone, to check these factors accurately and properly. There are a few folks out there that do have output current minimums, and this is a big help and much better than tubes that are checked for little more than if some noise comes out of them.
In any case, if you get a chance to try a proper output tube in your complex front end amp in the first gain stage (which feeds the rest of the stages too by the way), you may be surprised.