Peavey Bandit Transtube 112 Preamp hums

[Just Mike]

But the power amp works fine when I plug in an external preamp into the pwr amp in.
Any ideas where I should start?
Just for reference, I've built a few tube amps, but those little bug thingies are kinda foreign to me.
Thanks

[Jazz P Bass]

Peavey transtube

The first thing to do is get a schematic.

Call Peavey, they are very helpful people.
Customer Support Representatives toll free at 877-732-8391/ ext#1386

Post it when you get it.