I'm looking at doing a push-pull amp using 6S4S triodes and some Electra-Print output transformers. All of the circuits I've seen so far use cathode bias. Is theere any fundamental problem with using fixed bias if the bias source is low impedance? I was thinking of using source followers to drive the grids. I expect I'll get the usual snarky remarks about polluting the vacuum's precious bodily fluids with nasty silicon, but is there any real objection to using fixed bias with directly heated triodes? I'll be using some homebrew SMPS circuits to heat the filaments.