Hum- help needed

Try an outlet on a different circuit (not just another outlet))

You don't mention if you moved the L amp to the R speaker, but I suppose you did.

Do you have a halogen lamp nearby? It's possible a lamp transformer is interacting with one of the amps, but not with the other.

Do the L and R amps use two different AC outlets, or do they share one outlet?

It looks like you swapped power cords, yes? Maybe the ground is disconnected on one of them?

Tubes may be the problem, but each tube needs to be swapped one at a time rather than the whole set at once. Just in case you didn't do this. I did notice that in your sequence, the hum shifted from the R amp to the L amp (number 5 in the list) AFTER you switched the tubes from L to R. Did you swap the tubes back to their original amps before observing number 5 on your list? If not, I'd say it could be a tube...and probably a power tube.

My system in the past has had a hum with nothing connected except the amps, but then the hum went away when ICs were connected. Especially in the case of phono ICs not being connected.

Hum is a PITA to track down. Good luck.

Shadorne- amps are VAC PA 90C,four chassis mono blocks.
WWShull- the "quiet" one can only be heard with the ear to the driver, but the other one is significantly louder.
Zenblaster- good point:
I forgot to mention, when I took the amps to my technician- they are both dead quiet.
I moved "humming" amp to another floor, effectively different circuit in my own home- quiet.
Tvad- L and R amps were connected with the same power cord to the same outlet.
I simply physically moved "humming" R amp. to the L speaker and hum dissapeared,without changing anything else, but when the same power cord reconnected to the L amp. and the same L speaker, the L amp began to hum.
It hummed BEFORE and AFTER I swapped the tubes, and hummed again after tubes were in the original position, in other words, tubes swapping didn't affect the hum.

It hummed BEFORE and AFTER I swapped the tubes, and hummed again after tubes were in the original position, in other words, tubes swapping didn't affect the hum.
Maril555 (System | Threads | Answers)

Understood. Good luck.

FWIW...when I have had a hum in one tube amp and not in the other, it was because either a power tube required re-biasing, or a power tube was bad.

If you changed the "bad humming" amp to a different circuit and it was quiet then I would suspect that the outlet you are using for your stereo may not be properly grounded or may be wired improperly. Any electronics store should sell a simple outlet tester for less than $10 that will instantly tell you if the outlet is properly grounded and wired correctly...