One channel on power amp hums on startup

I have an Apt 1 power amp that hums on only one channel at startup. Disconnected from the pre-amp it lasts for about 20 minutes and then fades away over the next 3-5 minutes. It is not too loud but it is annoying. It sounds like it is a 60 hertz hum. Any thoughts?

Considering its age it may need to have the electrolytic capacitors replaced.

I had them replaced about three years ago.

I’d inspect the solder joints for a cold solder joint staring with the left channel input stage. I’d also inspect the caps for bulges or leaks. Possibly you have one or more bad caps.