Peavey Musician 400 (1970's)

[Oldskool]

Hope someone can help.  When I power this beast up it works fine for a few minutes and then I start to hear a "roar" like the reverb spring is moving.  I can cut some of it out by adjusting the bass.  Turning the reverb all the way down has no effect so I'm not sure it's a reverb problem.  Nothing gets hot and amp plays fine but it has that annoying roaring sound.  Any suggestions?
Thanks.

[teemuk]

A quick way to test if its feedbacking through reverb springs is to punch the amp; this should momentarily stop the "roaring".

[LJ King]

If there is no feedback with the reverb control at zero then it can't be feeding back through the reverb springs - there is no other path through to the preamp output but the reverb control.

There were (at least) two models of 400 Musicians during the 1970s. Does your amp have a master volume and reverb control... along with both series and parallel input jacks?

[nashvillebill]

I have an old Series 400 bass head that has the exact same power amp design as the Musician 400, I'm pretty sure.  One of my output transistors started to go, and I began to hear a noise kinda like a reverb...but it wasn't really a roar....The transistors are 2N3772 I believe, they're relatively cheap from Mouser.

Can you look at the amp's signal path--I'm not familiar enough with the features of this particular amp--and use a send and return to isolate whether the noise is in the preamp stage or in the power amp?