60 AND 120 cycle hums, one per channel

My ST-70 has developed a 60 cycle hum in the L channel, and a 120 cycle hum in the R. It was functioning well previously. I replace the EL34s a few months ago (maybe 15-20 hrs on them). I'm such a newbie ... where to start?

No vintage electronic repair in the area (Richmond VA), so I'd love to have this be tubes or ground related, but it will be what it will be.

Thanks for any suggestions ...

Jon

jklwood wrote:My ST-70 has developed a 60 cycle hum in the L channel, and a 120 cycle hum in the R. It was functioning well previously. I replace the EL34s a few months ago (maybe 15-20 hrs on them). I'm such a newbie ... where to start?

No vintage electronic repair in the area (Richmond VA), so I'd love to have this be tubes or ground related, but it will be what it will be.

Thanks for any suggestions ...

Jon

Revert the tube change. Is the problem over ?
Any things that happens may be a result of changing something. Remembering any changes
and correlate to symptoms might give the answer.

In general one should not change things ( like changing tubes or components) without
proper logging. "tube rolling" is an example of things that shuld not be done outside
a laboratory environment.

Jon, welcome. great forum. There WAS a shop in Rickmond; Colonial heights, i think.
Member Dave_in_va may chime in. He'll know. Good luck, Dale .

You can go to Action Electronics in Williamsburg

https://action-electronics.net

of you can try Joe Kolla, The Tube Amp Guy in Powhatan

https://www.thetubeampguy.rocks

Both guys are EEs and both have done work on my amps. Bob at Action has been in the same brick and mortar location for 52 years.

Edit: Joe Kolla restored and uses an ST 70.