From just the amp or the speakers as well?
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If it's coming from the amp -- the physical amp cabinet -- and not the speakers, it may be the power transformer humming. This occurs when there's a little DC offset on the line (not uncommon). A cure is available as the AVA HumDinger. I have three; they work great.
greh OP131 posts
When I first power it up, the hum isn’t audible until I’m about 30 minutes into listening.
That could be caused by a filter cap in the power supply going bad. Does the hum sound like this?
Or does the hum sound like this?
@ greh OP
If you had said the hum sounded like 120Hz then that would indicate an electrolytic filter cap/s in the DC power supply.
60Hz..... My first thought would be a ground loop hum, except where you said the hum doesn’t start until after 30 minutes after powering up and listening to the amp.
To me a ground loop could care less how long the amp is powered up.
I would suggest you need to first isolate the amp from any front end equipment. Disconnect all ICs from the inputs of the amp. Turn on the amp and listen for the 60Hz hum from power on to after 30 minutes. If no hum the hum is caused by a front end source.
If after 30 minutes the hum starts then it has to be the amp.
You never said the 60Hz hum was only coming from one channel so I assume it is coming from both channels.
I ran across this video that I found interesting. Could the 60Hz hum be caused by a bad small signal tube? Maybe, if the heater/filament voltage is AC.