Here's what I've done so far after reading other posts and please note that this is with the amp powered on, standby on, but volume completely down:
- Checked the continuity of all ground points in the schematic, and they all seemed to check out.
- I moved the PT center tap wire and bias ground wire over to the ground lug of the filter caps, which Sluckey recommended in another post.
- Added a 10k grid stopper to V1b and a 1.5k grid stopper to V1a (the recommended value was 860R, but 1k5 was as close as I could get with the part I have on hand)
- Swapped the preamp tubes around, and got no change.
- I pulled V1 and that did in fact kill the noise, but I'm not sure how to use that finding.
- I used a chopstick to move wires and tap on components. No change with wires, but I found that tapping on C8 and C9 was audible through the speaker. None of the other caps or resistors had this behavior. Is this expected?
Let me know what else I can provide, and thanks in advance!