After rebuilding the amp and things working flawlessly for quite a while, the intermittent pop has returned. I had all ready swapped the preamp and drive tubes during the last troubleshooting, so doubtful that was the issue. Before digging into the underside of the chassis again,I noticed that a tap on the 6AQ5 tube itself would reliably reproduce the pop. I had been focusing only on the audio path last time, but it looks like my issue might have been a failing tube all along. After replacing that tube, the issue has disappeared again. Now this could be some wiring of connection getting moved while changing the tube, but it really seems like the tube was the issue.
Just wanted to post this hear for FYI and to help grow the troubleshooting on the forum…
