JCM900 50W master Volume hum

[Mik]

Hello,

I bought this amp one week ago for 200 EURO. I already changed the tubes and HV filter caps but it's still there. I also changed the bridge rectifier and the TL072 IC. The hum increases with more gain level. Please help I wan't to play this monster. I 'am shure there must something be wrong in the preamp
stage. The amp is absolutely loud enough so there is no problem.
When I play over a external preamp in the return jack the hum level is normal.

Please Help,

Mik

 

[coldsteal2]

Well that narrows down the problem that its your preamp
im not a tech so dont know what to say other than take
it to an amp tech at a music store and have them check it out.
a hum sounds like a short of some kind so could mean a capacitor
is out or something.