I'm beginning to explore designs for a push-pull power amplifier with two pentodes as the output stage. Having looked at many schematics online, I've noticed a variety of bias strategies ranging from self-bias, mixed fixed / self bias, and fixed bias output stages. I have some questions relating to some of the practical implications of each strategy. More specifically, I'm interested in implementing a self-biasing scheme. My questions are
1) How important is it to have a "matched pair" of tubes for each output channel?
2) If I choose to use a matched pair of tubes, what type of adjustment mechanism do I need in place to ensure that the tubes have the same quiescent bias point? Should I put a trimpot in series with the cathode resistor for each tube so that I can adjust the tubes to have the same quiescent cathode voltage?
3) In the contest of a stereo amplifier (as opposed to a guitar amp), is there a hard logic to using fixed-bias v self-bias?
A final unrelated question: what do black glass tubes look like when they're illuminated? I've found some cheaper Russian tubes which feature black-glass envelopes.
|