There's something wrong w/ this amps PI stage. Power amp is good. When the amp is turned on it may or may not oscillate. As soon as the Presence knob is turned it begins. The Presence does affect the pitch of the oscillation. It goes from a high squeal to a motorboating sound. I've replaced the PI tube, replaced the presence pot, measured all resistors and continuity between components, DC voltages checkout good, I've replaced C24 which is the filter cap for that stage, grounding the middle lug of the treble pot does not kill the oscillation, connecting both PI grids kills the oscillation, all 10k dropping resistors checkout fine. Disconnecting the negative feedback loop makes it worse.
![Confused](https://music-electronics-forum.com/core/images/smilies/confused.png)
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