Just swapped the power cord on a Buddy's MC-60 and used a 3 prong cord. Well guess what..... HHHUUUMMM....... Until I "Lift" the ground, then it ceases. Any ideas?

What's the condition of the amp itself? Has it been rebuilt? If not, then this needs to be addressed. If there's AC on the chassis due to faulty components, then you have one type of problem. If you're generating ground loops with other gear, then that's another sort of problem.

Without more data to go on, what was the situation before you swapped the AC line? And a bit more about the other components would be in order.....

Cheers,

David

What's the condition of the amp itself? Has it been rebuilt? If not, then this needs to be addressed. If there's AC on the chassis due to faulty components, then you have one type of problem. If you're generating ground loops with other gear, then that's another sort of problem.

Without more data to go on, what was the situation before you swapped the AC line? And a bit more about the other components would be in order.....

Cheers,

David

Hum is not there if the RCA is not connected. The Preamp is an AR LS-15 (mine). I have not tried it with another source. The LS-15 is grounded as well. It did not hum before. The only work done to it was the .22uf cap off the phase inv was replaced. No pwr supp upgrades. All components are plugged into the same 20 amp H-N-G circuit. With the ground lifted, I have
.575 VAC from the chassis to earth/electrical ground. Since there is no "Death Cap" in the 60's, how would A/C get on the chassis outside of a bad Power x-former?

This is a VERY GOOD exorcise as now I know NOT to put a 3 prong plug on mine........ :D