I was also noticing transformer hum. I checked the mounting hardware and
found that the transformer was secure. I plug my amp into a 20-amp nominal
120-v circuit. When the transformer continued to hum, I checked the voltage
at the wall receptacle. The voltage would, intermittantly dip to less than
105-v and lower. It was under this low voltage excursions that the
transformer hum occurred. It my case, it turned out the culprit was a
faulty circuit breaker at the service panel. Replacing the breaker solved
the problem. I would recommend checking the input voltage when you hear the
hum.
73 de K6SBA
David in Santa Barbara, CA
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