So, I've had this hellbeast of an amp on my bench for months tracking down problem after problem. I'm down to one last problem that has stumped me. The gain channel is oscillating, and for the life of me I cannot figure out why. Here's a schematic:
Here's what I've deduced so far:
So... what am I missing? My thought now is that it's just RFI at V2A, but if that were the case wouldn't it be present on the clean channel at V1A too?
Here's what I've deduced so far:
- The first point I can see it on my scope is at the plate output of V2B. Obviously it must be present prior, because Grounding V2B's grid input makes it go away, therefore I'm thinking it's between V2A and V2B that it's starting.
- On the note of the above, switching to the clean channel (which grounds V2A) also causes the oscillation to disappear.
- I'm aware there are no grid stoppers in this amp. Adding a 47k stopper at V2B made no difference, and doing so on V2A is more or less impossible (the trace is on the bottom of the preamp tube board, and I'm not keen on cutting traces if I don't have to).
- The cables from the input jack to the preamp tube board as well as the one from the main board after the relay between clean and dirty are shielded cables, and I've checked their integrity.
- The power supply is not the source of the issues - the supply side of the plate resistors is clean.
- All grid leaks check out both in terms of value and their connection to ground.
- The oscillation is not present on the clean channel, at all.
So... what am I missing? My thought now is that it's just RFI at V2A, but if that were the case wouldn't it be present on the clean channel at V1A too?
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