My tube amp causes isolation transformer to buzz

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. 

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. 

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”

Geoff,

I wish this was as easy as you think it may be.

Here is what I did:

Step 1: unplugged everything in room but amp. Went to house circuit breaker, turned off entire house but room with amp. Remember, the only thing hooked up in house is the amp.

Step 2: Amp is plugged into power outlet - only the amp - no interconnects are connected to amp - no other components are even plugged in. Speaker cables are connected to loudspeakers. Amp on, no sound from speakers, almost completely quiet with barely a hit of the buzz.

step 3: plug in interconnects to amp, but NOT to preamp = buzz - relatively loud. Remember, nothing else in house has power but amp.

step 4: plug in interconnects to amp and to preamp = buzz - much quieter bit still audible, can be heard from sitting position. Also, Amp and Preamp plugged into same outlet and a ground wire connected to chassis of amp and preamp.

step 5: plug amp into Variac - amp still connected to preamp via interconnect = buzz almost completely gone. I have to put ear right next to speaker to hear it.

Ground is good at outlets, that is confirmed and cables are shielded.

I guess what I’m saying is it’s easier said than done