Hum coming from ST-70 amp itself

[dcriner]

I've just noticed a low-pitched hum, probably 60 Hz? It's not coming out of the speakers, but rather directly from the amp itself.

This "clone" amp has a power xfmr from Triode Electronics. I'm planning to pull off the cage, and tighten the thru-bolts on the power xfmr. Any other ideas?

[Ty_Bower]

You're probably on the right track. The power transformer in my ST35 has a mechanical hum, too. It's not loud, but one of those things that is difficult to ignore once you know it is there.

You might try to isolate the transformer from the chassis using rubber grommets or something. Try to minimize the opportunity for the humming transformer to couple to the chassis, resulting in a humming chassis too.

[Blackburn Audio]

Tighten up the nuts on the inside of the amp that hold the PT in place, best to use locknuts here. Using the rubber grommets as Ty suggested will help as well. Normally the new clones don't vibrate to much so this should fix it up if you are still getting a buzzing chassis check the amplifier out to make sure something isn't drawing excessive current from the PT and causing it to vibrate excessively.

-Matt at Triodeelectronics.com

[dcriner]

I tightened the four hold-down nuts fastening the xfmr to the chassis, and the hum is OK now. The thru-bolts holding the laminations together were already tight. I didn't add lock washers, but if the hum returns in the future, I'll add them. Thanks, Matt. So, Ty, I didn't have to try your grommet suggestion.

It was a pure 60-Hz hum not a buzz. I think a buzz would suggest loose laminations?