Peavey Session 400 problem

[aentresz]

Hi all, I've just bought a Session 400. It sounds loud and clear, but there's this fizzy distortion happening at the same time in the background.... any ideas as to what's going on? it sounds like you're running a clean amp, and a high gain amp (quietly) at the same time.....  :loco

[Enzo]

The real old one, like from 1974?

First, isolate the problem.  Disconnect the speaker from the amp chassis,and connect some other speaker to the amp.  Is it still there?   Is it more noticable at low volumes and kinda disappears at loud volumes?   I am looking here for a speaker with a rubbing voice coil.

Likewise, with the speaker disconnected from the amp, you could connect some other amplifier to the speaker and see if it sounds OK.

The amp is 35-40 years old, that means things like filter caps are drying out.  That can cause little sonic symptoms like you describe.  A cap job cures that.

The old preamp is all transistor, which is fine, but it is strewn with small 2uf 35v caps in the signal path.  They almost always need replacing at this point, and could cause such problems as you describe.

So if we can narrow it down to preamp, power supply, or speaker, that would let us decide what to do about it.

[lapsteelman]

You might want to do a search on the Steel Guitar Forum. This amp is popular with pedal steel players and they have a pretty good electronics section on the fourm over there.
I would be suspicious of the power supply caps also.