I bought a used 6550 amp and wanted to get opinions on ways to improve it. I reverse engineered it and have already changed a few things such as lowering the supply to the phase inverter from 406 to a safer 373. The 12At7 is only rated for a plate voltage of 300v and the 285v cathode to anode was a little close for me. The 3 10k resistors around the balance pot also seemed odd. I was hoping somebody could explain the reason for doing so. It is a stereo amp and I have included schematic for 1 channel. I am currently considering changing the biasing circuit 1 of 2 ways.
1. Removing the 10k resistors and adding 10k pots (1 per tube) to allow individual tube bias (I know I can do that now with the balance pot).
2. Switching to cathode bias by grounding the 220k resistors and using LM317HV's 1 per tube as current sources which would allow switching to KT88's without re-biasing.
Thanks for taking the time and providing your input.