I need help with a theory. If I want my power tubes to work less hard, that is put out less power so I can have my volume knob at 12 o'clock instead of where it is now, 9 o'clock, would I turn down the bias on each tube? Right now I have the "loudness" control on so I can get the good tone I like on the lower volume which is how I listen most of the time. My thinking is that if I can crank up the volume, I can turn off the loudness control and get closer to a more natural sound, closer to flat frequency (besides, I have an equalizer in my stereo system which is where I dial in compensations for room acoustics, etc.). I know this theory works on guitar amplifiers. It's hard to get good tone at low volumes on guitar amps. I assume the same principal is at work on a Fisher 500-C. Or is the tone directly tied to the power tube output such that I'll lose tone if I dial down the bias? I'm pretty sure tone is handled by a different stage in a tube amp and the power amp (tubes) section is just amplifying whatever the preamp tubes are doing. I mean this is just a math exercise to lower the watts, right?
(later that day) - I dialed the cathode bias down to 31mA. On one of the tubes I couldn't get it to go any lower. The pot was turning, but the voltage was not going down. So I set all 4 power tubes to 31mA. It seemed to me that the right channel was lower in volume than the left channel although it could well have been the radio station. Of course I didn't think to check other radio stations...DUH ! It also seemed that one fo the power tubes was significantly less bright than the others even though the cathode biases were close. Anyway, now I'm running a slightly lower wattage. By the way, I measured the voltage at tube V14 pin 4 where the Filament out from the SDS board goes and it measured -22.3 vdc. I thought it is supposed to be a little more negative but close enough. Comments?