100%. You do have to keep on top of the bias. Generally a good practice regardless of your amp anyway.46346 wrote: ↑Sun Aug 01, 2021 1:38 amthat Dual Prof was a 4x6L6 power tube bias-tremolo'd amp... i had read that theory you mentioned as well, but it *can* be done! you gotta pay more attention to your bias level, though.
my latest guess is that in the brown/blonde Tolex era, Fender developed the awesome Harmonic Vibrato, and put that in their top pro amps. and the cheaper amps kept the cheaper bias-trem circuits. same thing with the blackface era - the new optical method went into the pro amps, and the student models kept the cheaper method. maybe it was just Leo's economics?
I think that harmonic vibrato went away due to packaging and expense as much as anything else. A Super or Pro needed three (?) tubes to run the harmonic vibrato circuit, and IIRC they only used half of one of those tubes. For a similar amount of space in a chassis they could put a simpler tremolo and reverb in the Vibroverb. As much as people really like the swirly, almost Uni-Vibe quality of harmonic vibrato, at the time I imagine Leo et al seeing the Vibroverb as achieving essentially the same goals and them some with a simpler circuit. Fast forward to the black panel era and they can make the vibrato even simpler again. The fact that there's nuance between the tonalities of these three vibrato/tremolo circuits, and that people have their preferences between them is likely incidental to the original aims when building these amps.
That said, I've got time of all three circuits, and I reckon all have their strengths. The recent resurgence of interest in harmonic vibrato, and pitch-shift circuits in general has produced some great pedals if nothing else.