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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
millercarbon's avatar
millercarbon
12,531 posts
 
Here's a question: do you know how transformers work?
A certain amount of buzz is inherent in the design.

So the real question then is, what exactly are you hearing? Since the first "buzz" was eliminated by plugging into a different path that means it wasn't buzzing at all it was ground loop hum.

What you have now is probably very normal amount of transformer hum. But could also be DC offset. Or ground.

But yes, the answer to your question is yes.

I find it interesting that your 60 watt amp draws 350 watts at idle. Was it modded when you got it, or what did you do?
noromance's avatar
noromance
9,902 posts
 
It's buzzing under the relatively heavy load as the coil is energized by the alternating current. You could try the ubiquitous  red $110 20A 2000watt model. 
noromance's avatar
noromance
9,902 posts
 
Oh, and a variac isn't an "isolation transformer," it's an autoformer where the pickup wiper is on the same coil.
last_lemming OP
639 posts
 
Miller,

The unit is not modded. 350 watts is per spec.
it uses 700 watts at max output., again per ARC.

so the buzz being present in the Variac only when there is a load is normal? Because when I plugged a light into the Variac to see if I could replicate the buzz, did not buzz.  The light didn’t have a ground prong on the plug however..  though I don’t know if this means anything. 
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