Hum in Ampeg B-15N at Power-UP

[OldFenderPlayer]

When I first power up my 1970s Ampeg B-15N there is a hum - it's very noticeable but the amp seems to still play fine. The hum goes away after a few minutes and afterwards the amp is completely quiet.

I will have a qualified tech look at the head, so I'm hoping to educate myself before I take it to the shop.

Any opinions on what the source of the hum might be?

[rickdog]

When was the last time it had new output tubes? The push-pull output stage of a tube amp is naturally hum-canceling as long as the "push" side and the "pull" side are in good balance. Old, tired tubes may not balance well until they warm up thoroughly.

When was the last time it was recapped? A weak power supply capacitor may make the hum worse; the tubes have to balance all that much better to cancel it effectively.

[OldFenderPlayer]

Thanks for the input Rickdog. I've only owned the amp for a year, so I don't know the age of the tubes or if it has ever been recapped. The hum showed up just within the past few weeks.