Low hum from amp

Thanks to all of you for your response-I appreciate it!
Mike, yes it's coming from both the amp and the speakers but it's definitely starting with the amp.
Thanks again!

My amp is a Manley Stingray.
When I first power it up, the hum isn't audible until I'm about 30 minutes into listening.
Thank you dweller for the correction.

Thanks again everyone for the feedback.
jea48, thank you for the links.
It's 60Hz
Mind you, I can still listen to music because the hum is that soft, but it's still there.
And of course it's more audible when the amp's powered up but there's no music playing.

Thanks jea48

Appreciate the feedback.
Yes, checking tubes tomorrow.