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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
atmasphere's avatar
atmasphere
11,901 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!
erik_squires
16,311 posts
 
It’s probably obvious but transformers produce magnetic fields that produce....you guessed it, distortion!

I dunno, without magnetic fields there would be no loudspeakers, so I'm actually a big fan.
geoffkait's avatar
geoffkait
23,365 posts
 
Yuk, yuk! 😬
minorl
1,016 posts
 
I am confused when you say you can't find the cause of the ground loop.

Disconnect all inputs to the amp.  Turn on amp.  Noise through speakers?
yes? amp is the problem.  Try a different outlet or use a conditioner. No noise? Plug in input from pre-amp (with no other connections to pre-amp).  Noise?  yes, pre-amp or interconnection cables are the problem.  no noise?  not the pre-amp or cables from pre-amp to amp.

Plug a source into pre-amp.  Noise?  yes?  Source or interconnects from source to pre-amp the problem.  no noise?  plug different source and so on.

Check to see if outlet has ground connected.  

some cables aren't made very well, so better  cables with better shielding helps.

Also, don't run interconnect cables near power cables.

I agree with many hear that if it is a ground loop.  Find the cause and fix it.  masking the sound with cheaters (not ever a good idea because you actually lift the safety ground and now in the event of a fault, you, your kids, your pets, etc.  become a better path to ground).

take the time to find find the problem.

enjoy
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