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FWIW, I have a '66 or '67 (very late BF) that had some very similar problems. Bass channel was microphonic, mainly V2. PI had weird voltages, something was pulling one side down but I'm not sure what. And it had an intermittent, very light hum. This was both before and after I modded it (typical Bassman mods: adjust+balance bias circuit, regular BF Fender PA and PI, removed those 220K feedback resistors, cap across stand-by switch, 3-prong plug, no death cap, etc.).
I tried removing the top board, sucking all the solder out and resoldering everything; a lot of work for naught...no change.
Then I decided to just gut it and rebuild it with G10. This solved all the problems and it sounded great...until that stupid hum came back after playing it for 2 days. I can (usually) make it go away by pounding on it so I'm thinking it might be one of the few solder joints I haven't touched or perhaps an old CC resistor I didn't replace.
I made the preamp to be: BF Normal and a tweeked 6G6B. That tweeked 6G6B channel is my favorite for clean.
Oh yea, mine had mostly the brown drops for caps but there were a few blue tubular caps (the blue caps were retained and used in the rebuild).
I tried removing the top board, sucking all the solder out and resoldering everything; a lot of work for naught...no change.
Then I decided to just gut it and rebuild it with G10. This solved all the problems and it sounded great...until that stupid hum came back after playing it for 2 days. I can (usually) make it go away by pounding on it so I'm thinking it might be one of the few solder joints I haven't touched or perhaps an old CC resistor I didn't replace.
I made the preamp to be: BF Normal and a tweeked 6G6B. That tweeked 6G6B channel is my favorite for clean.
Oh yea, mine had mostly the brown drops for caps but there were a few blue tubular caps (the blue caps were retained and used in the rebuild).
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