Thanks, Terry... Sorry, Amie! 
Not sure that the idea of an RC network in the earthy end of the grid leak to improve the smoothing of the bias on the triode's bias supply would give benefit, I'd suggest the reverse.
As an extreme, consider the triode grid hard-coupled to the cathode. If the pentode current wiggles a bit (noise) then the cathode voltage will wiggle, and the triode grid will be taken with it. So there will be no wiggle in the voltage between triode grid and cathode.
Consider the other extreme, the triode grid hard-coupled to 0V. If the pentode current wiggles a bit (noise) then the cathode voltage will wiggle, and as the triode grid is grounded, the full wiggle will appear between triode grid and cathode. This will be in such a phase to reinforce the original wiggle, hence positive feedback and oscillation is likely
Increasing the capacitor between triode anode and pentode grid will aggravate the condition unless the cathode decoupling capacitor is increased with it. But as the output transformer is easily the limiting factor in the bass response, there probably is not really much point in going above 2.2nF. It's easy to verify, though.
(02-10-2019, 09:47 AM)Terry Wrote: What I suggested was putting an RC network in the earthy end of the grid leak to improve the smoothing of the bias on the triode's bias supply, effectively making it independent of the pentode bias, even though both will still be derived from the same source.
Not sure that the idea of an RC network in the earthy end of the grid leak to improve the smoothing of the bias on the triode's bias supply would give benefit, I'd suggest the reverse.
As an extreme, consider the triode grid hard-coupled to the cathode. If the pentode current wiggles a bit (noise) then the cathode voltage will wiggle, and the triode grid will be taken with it. So there will be no wiggle in the voltage between triode grid and cathode.
Consider the other extreme, the triode grid hard-coupled to 0V. If the pentode current wiggles a bit (noise) then the cathode voltage will wiggle, and as the triode grid is grounded, the full wiggle will appear between triode grid and cathode. This will be in such a phase to reinforce the original wiggle, hence positive feedback and oscillation is likely
Increasing the capacitor between triode anode and pentode grid will aggravate the condition unless the cathode decoupling capacitor is increased with it. But as the output transformer is easily the limiting factor in the bass response, there probably is not really much point in going above 2.2nF. It's easy to verify, though.
