Splawn hum problem

[ElPedro]

Working on a Splawn Nitro head for a buddy of mine. His complaint is that there is a ton of hum in the amp. There sure is! He said this problem started sort of suddenly.

There is no hum in the amp with power tubes out, so I don't think the hum is being generated directly by the output transformer. The hum gets louder as you turn up he volume and add in gain stages.

I have tried different power tubes (he's running 6l6), and different preamp tubes. The bias is sitting at about 72% dissipation.

I tested all of the filter caps and they look good. They are within about +6% or lower of spec.

a visual inspection of the other components didn't reveal any obvious issues.

I had to hit the road for a business trip before doing anything else, so I thought I'd run this by you all and see if you had any ideas.

Thanks!

ElPedro