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Hum- help needed

It's been very strange and I cannot think of a logical explanation.
It will require all your attention.
1. Two mono blocks plugged in. EVERYTING ELSE IS UNPLUGGED,
ICs disconnected- hum in the R channel (can be heard from
2-3 ft. away)
Hum in the L channel- order of magnitude lower, I would consider it "normal", can only be heard with the ear to the driver.
So I would think, it's not a ground loop (nothing else is plugged in, heard in R ch. only.
2. R Amp. moved to the L speaker (other amp unplugged altogether)- no hum.
L amp. (used to be quiet), connected to the same L speaker, with the same power cord- hum.
3. All tubes switched L to R- no difference.
4. R amp moved back to the R speaker (position, where it used to hum- no hum.
5. L amp (used to be quiet)- hums, no matter what.
To sum it-up:
I think it's not a ground loop (see #1),
It's not the amp.- (see # 2),
It's not a speaker- (see # 2),
It's not the tubes- (see # 3)

So, here is my question- what the hell is it?
maril555
maril555 OP
619 posts
 
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.
Post removed 
plato
1,543 posts
 
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...
Post removed 
maril555 OP
619 posts
 
Plato, Tvad- I agree with you, and also think, that the problem is with the outlet. Actually I have an electrician coming tomorrow to install two extra lines, and I will have him check the existing one, as well.
All that said and done, the most puzzling part to me is, that hum went from one amp to another. I can't even contemplate a reasonable explanation for this one.
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