05-21-2018, 01:17 PM
(05-19-2018, 04:56 PM)slider313 Wrote: Are you sure the cathode resistor on V1b is grounded to the pre-amp ground? I see it open to the switch until you ground it at the switch, turning the tremolo on.
OK. So it turns out that my description of the grounding scheme above was not entirely accurate. There is a wire that runs underneath the board that I hadn't noticed, so I think that both the cathode resister and the bypass cap are grounding to the input jack. No loop there.
This is still on my bench (I've been away for a week or so). There is another interesting piece of information. Actually, two:
1. The hum requires time to manifest (a couple minutes from a cold start). This kind of kills the whole ground loop hypothesis, in my opinion. I hadn't noticed this before.
2. The hum happens if I pull V1, but does not if I pull V2. This is super weird given the effect the trem has on it.
All the resistors on V2 test in range, but given the delay in observing the hum, I felt that it was likely that one of the plate or cathode resistors was behaving badly with heat, so I went ahead and replaced them. I didn't have time to really test it thoroughly, but it seemed better. Maybe.
I will continue to report.