Another thing to go back to is the RCA data sheets for a 12AX7 showing average plate ratings out past 400V.ElectronAvalanche wrote: ↑Wed May 27, 2020 3:22 pm Update:
I installed a 22k (2W) resistor in series between B+ and the CF driver anodes and dropped the voltage to 380V (B+: 435V), this calculates to 2.5mA for the CF driver. I measured the bias and the output tubes lost 1mA.
I do not hear much of a difference between the high CF driver voltage and the lowered one and assume this will not make much of a difference or only at extremely high volumes.
BTW: the plates measure -47V when on standby, and this voltages slowly drops to -27V over time. Of course putting the amp in operate mode (standby off) results in the measured 380V on the plates.
So now I think I can play the amp with less anxiety that the CF driver tube may malfunction. Installing a 12AT7 would probably be even more cautious, albeit I am not really sure 12AT7 have a higher max Va-Vk rating.
Electron
A lot of these tubes will handle the voltage if they're set up ok. Jim Kelley took advantage of this pushing 6V6's beyond what most would do.
A VHT 2150 KT88 power amp would be another schematic i'd like to take a gander at, as I think you'd find some similarities in there as well...
A lot of interesting and valid inputs on this topic, so I'm definitely following..!!!