(02-20-2016, 09:10 AM)AEB-1 Wrote: I'm short on time this weekend but I did the tube swap and no improvement. Then to be sure there isn't anything funny with the "E" supply, I added a filter capacitor at R6 and Gnd. Also no improvement. I'm more convinced it is filament to cathode hum. Next weekend I will sacrifice a board and add a DC rectifier/filter for V1, V2 filaments. The main problem is lack of physical room for a filter cap on the board.
the cap doesn't have to be huge. the voltage rating can be like 16 volts, and you could use two caps, one on each socket... if you wanted to use smaller values.
The DC filament supply doesn't have to be completely free of ripple either. a little ripple won't be audible compared to the full AC heaters.
I am interested to see how this goes. I've always wondered if it would help, but never bothered to try.