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My tube amp causes isolation transformer to buzz

So I have an ARC Classic 60 tube amp that was causing a hum/buzz on my speakers regardless of volume.  In an effort to eliminate variables I plugged the amp into a Variac Transformer.  The sound from the speakers was almost completely eliminated, however once the power was turned on at the amp the transformer, which was otherwise silent, started to buzz.  Not super loud but audibly.  My amp draws 350 watts at idle. My Variac can handle up to 800 watts. 
So my question is, does applying a load to a transformer cause it to buzz, or can the amp cause the transformer to buzz?
last_lemming
last_lemming OP
639 posts
 
There is a gage on the front that as you increase the voltage goes up. It is labeled in “volts” and has a green “safe” zone (approx 105-120v).
Is this what you mean?

I originally bought the Variac when I replaced the main caps in amp so I could slowly bring up the voltage the first time it was turned on.


Btw. My room is very small and I don’t use that much power (watts) to power my speakers.

also the Variac is temporary, though I am thinking about a 1000w transformer. I cannot find the ground loop. 
I’ve gone as far as turning everything off at the elec panel and still get it. 
almarg's avatar
almarg
9,670 posts
 
There is a gage on the front that as you increase the voltage goes up. It is labeled in “volts” and has a green “safe” zone (approx 105-120v).
Is this what you mean?
Yes.

I don’t know what the accuracy of that particular gauge/meter is, of course, and personally I would want to verify the voltage with a good digital multimeter. But I’d feel confident that it is accurate enough for safe operation of the amp, although perhaps not accurate enough to be fully optimal in terms of sonics.

Good luck. Regards,
-- Al
last_lemming OP
639 posts
 
So can someone explain the “why” to this statement:

“The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy”


atmasphere's avatar
atmasphere
12,070 posts
 
“The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy”
Sure. Harmonics are the result of distortion. The big nasty is the 5th harmonic (300Hz) which is the one that causes the problems. Any transformer will make more distortion as you put more energy through it, since they are inductive devices. Fluke Instruments published a paper on this topic back in the 1990s; I'll have to poke around to see if I can find my copy, but I think its online too. Here's one I found that deals with this topic in greater depth:
http://support.fluke.com/find-sales/Download/Asset/1260362_6001_ENG_K_W.PDF
geoffkait's avatar
geoffkait
23,365 posts
 
It’s probably obvious but transformers generate magnetic fields that produce....you guessed it, distortion!
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