New build noise troubleshooting help needed

[markedwardsims]

Hey all, I am working on my first build, an Express (Kelly '90 schematic) with JJ 6V6 power tubes. I was able to get advice on solving a wiring mistake, and now I'm on to the task of troubleshooting some noise problems. I'm getting some oscillation, which comes and goes as I move, so I'm hoping that the chassis cover I ordered will solve that. I'd like to focus on the noise. I'm not yet educated enough to use the right words to describe it as noise, buzz, or hiss so I'm posting a couple of sound clips to help.

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?

I read that a shielded cable from the volume pot to V1a grid can help but I have not yet tried that.

Let me know what else I can provide, and thanks in advance!

[markedwardsims]

It looks like the chassis cover has solved that oscillation issue, so that's good news.

After fiddling with the preamp tubes and following Rob Robinette's guidance (https://robrobinette.com/Tube_Guitar_Am ... ird_Noises) it seems the noise/buzz is eliminated when I pull the PI tube. I traded tunes around to ensure it wasn't the tube itself. I'll continue to look at the PI circuit, recheck wiring, grounds, and all that good stuff but am open to any more scientific troubleshooting techniques.

Thanks!

[markedwardsims]

I made a bit of progress here and am feeling a bit foolish looking back. I looked at the schematic and saw all of the filter caps going to ground, so when I glued the stack together, I made a nice little grid with the leads and tied them all together and grounded to the chassis. I looked at the layout and thought "ground is ground, right?". Not right. The filter cap for B+3 being tied in with the others seems to explain why pulling the PI tube stopped the noise.

I hope this helps someone else. Will test out tomorrow and see where it's at.