I have a Gibson skylark amp that has a very loud hum even when nothing is plugged into it. Turn the volume to half and the speaker actually vibrates like crazy. A noise gate doesn’t do much for it. Once a guitar is plugged into it it’s not even usable with the amount of hum. I’ve tried an outlet right off my panel and still does it. HELP !!!
The first thing I would check would be the filter capacitors in the power supply of the amp. If I'm not mistaken, these amps were made back in the sixties and often capacitors of that age require replacement, but it could also just be a defective solder connection in that same circuit causing the capacitor to be left out of the circuit. Any technician worth his salt can fix it for you in minutes.