Dealing with transformer hum

I had a Belles 150a Hot Rod that annoyed me mightily with the same problem. The toroid transformer in these amps are very easily excited by noise pollution in the ac lines. I found that when fans or certain ac appliances would turn on in the house (not on the same circuit) the amp's toroid would start humming. If your hum is intermittent, you might see if there is any correlation between appliances/ac/etc. operation and the hum. Perhaps ac power treatment would help, or maybe you just have an amp that's going to hum. When I talked to Dave Belles, the owner of the company and designer, he basically offered no help and admitted that the amp was prone to this peculiarity.

Photon46 is right on the mark. I have a Belles 150a hotrod which also had an intermitant transformer hum which I eventually tracked down to a humidifier on the other side of my apartment. Took me forever to isolate this. Before I figured this out I did try an AC power conditioner which solved the problem although I did feel it constricted the amp somewhat. I should add, hum issue solved, that I think this is a great amplifier. Good luck!

As mentioned, many appliances set off the humming. Even though I have dedicated lines from a separate sub-panel, I still hear the hum whenever the microwave is used. Other motors and appliances do the same. Worst case, you may need to go to a line conditioner of some sort -- preferably one that attacks both small amounts of dc voltage that could be in the line (you could check that) and the spurious high frequency noise that is part of our AC system.