Hey guys - First post! Warning...Don't do, or listen to anything I say.Markystang wrote:How different would this be for an HT-20?
The 40 looks to be "Fixed bias" which applies a fixed amount of negative voltage via PR2 (not as in "fixed" that you can't change) and the balance PR1 allows you to get the bias closer to the same on each tube in the event they aren't "matched" very well. Some old Fenders and others used to only have a balance control and instead of the adjustable pot for bias just made the value such that it was so cold "everything" worked.
The 20 is cathode biased, which is, and isn't adjustable, depending on your ability and knowledge
. It will autobias itself to a point that's determined by a cathode resistor, which is usually chosen to bias a wide range of "average" spec tube. Technically you should check to make sure your new tube is working within the accepted range, but by reading numerous posts, you know most people don't. So adjusting the biasing ability of the "self bias" is only accomplished by replacing the cathode resistor in this case.As far as the voltage not being adjustable, that's usually true. Some amps employ "power scaling" which will scale down the voltage to the amp to basically make it lower volume (and implemented differently for cathode of fixed bias). To a small point, the amount of bias will very slightly change the voltage at the tube because it is controlling how much the tube can draw. Keep in mind just going to a different town or location will vary the wall voltage going the amp, which WILL change your plate voltage. If it's major city at high draw and they're slammin' 125 volts that's going to make a difference compared to the far reaches might hit 115. This actually has killed filter caps in old amps, which were all based on about 110 wall voltage and when they get hit with modern 120+ it caused the amp to exceed the voltage rating of the caps.
Wow...sorry...that was kind of long. One last tidbit. Everybody gets hung up on "Must set an EL34 to xx ma bias". I think the more important thing is that you want to make sure you stay under the dissipation wattage rating of the tube (which is usually given as rating of 70% or less of rated dissipation in class AB amps, and can actually slighty exceed max in a true class A).
Hopefully in the next day or two I can get a couple of pics up of a little test I'm going to do on my HT 20 head with three different tubes to show how the cathode bias reacts to different tube (I'll show plate voltage, ma readings and calculated dissipation).
