Phew! LOT to unpick there. I am glad Mr L that you have understood that the AC30 is CATHODE biased, a circuit mode that 'may' contribute to its "classic rock" tone. I shall come to that later.
Pulling two valves of 4 in a fixed biased amp such that the current through the OPT remain balanced is the usual procedure. Zero current in one half of the OPT usually results in horrendous hum! This is one of the greater benefits of Push Pull operation.
Now, thinking of PPop, the idea that the OP valves are handling half the waveform is a myth. That is just an historic way used to explain the circuit's operation (put a scope on the grids of each OP valve and you DON'T see a rectified waveform!) The output valves are effectively put in series and this in theory quadruples the power output of a single valve. This is the same principle used in "bridge mode" transistor amps.
"Class AB" means that neither OPV conducts for the whole signal cycle BUT! This is level dependent!! Take a 50W, FB amp, each EL34 say is dissipating maybe 12-18W thus AS IT STANDS it is the rough equivalent of a 15W TOTALLY class A EL84 amp and will deliver very clean power up to 15, maybe 20W. As the signal swing gets larger however the negative going part will begin to 'cut' off one valve in each cycle but the 'other' valve is being driven even harder and the transformer 'sorts out' the power and you get a more distorted but bigger signal. If you just had ONE valve left of four then you would get about 6-8W. That is still pretty loud through a guitar speaker! But again, likely it would hium like a bastard!
But...pulling bottles changes the optimum load the valve need to "see" so power and distortion levels cannot be predicted. Nor can the safety of the amp, it is often said a 2 to one "mismatch" is safe but not for every design and overvoltage spikes can wreck costly transformers. There is a BIG difference between what valve manufactures consider safe ratings and what risks player will take. Nobody is going to sue YOU if you do something daft to your amp and wreck it!
Cathode bias and "tone". There is almost always a large capacitor across Rk, usually 100mfd often bigger. When you drive a CB amp hard the voltage on Rk will rise tending to reduce anode (plate) current and therefore power. This voltage will be stored on the cap and build up and so the harder you drive the amp the more it cuts off and distorts...we like this! FB amp on the other hand can be driven ever harder and are only limited by the HT voltage. A 50W FB amp can deliver 70-80W as a rule and even 100W of distorted mayhem in the limit. The AC30 usually cannot manage much beyond 40W.
Another big factor in amp tone is the output resistance of the amp (and NFB) but these old eyes are giving out. Apologies for typos!
Dave.
