My tube amp causes isolation transformer to buzz

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.

The full specs on the power consumption of the Classic 60 are:

105-125VAC 60Hz (210-250VAC 50Hz) 500W at rated output, 700W maximum, 370W at idle.

I suspect that the 700 watt number doesn't reflect normal usage, but rather is approximately based on the current rating of the 6.25 amp mains fuse that is used in the design. (6.25 amps x 120 volts = 750 watts). And you are most likely using the amp's rated output power capability only rarely, meaning that the amp would be drawing significantly less than 500 watts most and perhaps all of the time.  So I suspect that you are not exceeding the 50% guideline Ralph cited above (corresponding to 400 watts for the particular Variac) by much if at all.

A basic question, though: Does this Variac have a built-in voltmeter? Or if not, how have you adjusted its output voltage?

Regards,
-- Al

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. 

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