My tube amp causes isolation transformer to buzz

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. 

Oh, and a variac isn't an "isolation transformer," it's an autoformer where the pickup wiper is on the same coil.

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. 

It does sound like you might have a ground loop, but testing on a variac isn't how it's normally diagnosed.

Disconnect all inputs from the amp, see if the hum persists. Alternatively, lift the ground with a cheater plug (temporarily only).

Variacs are not built with the intention to be particularly quiet. The only one that was to my understanding was made by MacIntosh back in the 1960s.


Also something to be aware of with any isolation transformer- 50% of rated capacity is a safe limit. The transformer can make significant distortion if loaded above 50%. That distortion can cause diodes and transformers downstream to become noisy.


I think Erik might be on to something. If you have a ground loop, its not uncommon for an isolation transformer to fix it. But its usually better if you find what is causing the ground loop and fix that. Even if they don't hum or buzz, ground loops degrade the sound overall by increasing intermodulations.