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I have an early 60’s Ampeg M15 guitar amp – a pet project. Replaced a blown OT but having hum problems that I can’t trace. Since it is completely dependent on volume level, I thought it was in the preamp stage. Don’t see any hum on the B+ voltages there or on the grids. There is about 20v p-p ripple on the 6L6 power tube B+ using HV scope probe, but none on the plate voltages for previous stage. I replaced that cap but ripple is still there.

Any systematic way to trace down the source?