Sovtek Mig-60 trouble

[Fartrider]

I've got a mig 60 since 1 1/2 year back. Had to replace power tubes after the first summer (lots of gigs) after that it has worked fine until now.

The problem i have is that after a while of playing through the high input i get a scratchy and fizzy sound that can't be dialed out by the tone controls.
The sound gets buzzier and weaker.
The signal from the HIGH input goes through a 12ax7 into the LOW.
It's hard for me to hear if the noise is coming from the first gain stage or if its later in the signal path.
Sometimes when i mute the strings and hit them hard i get like a crackling+whistling noise.
I've tried to replace all the preamp tubes and the problem didn't go away.

I want to believe that the power section is fine. It's still plenty loud and i can get the sound more "ballsy" if turn the MV up.
The noise is there where ever i set the MV at.

I can't find any signs of something broken inside (like burnt components or loose wires)

here's a schematic: http://www.schematicheaven.com/newamps/sovtek_mig60.pdf

Any suggestions?

[Zouto]

Have you checked/swapped/cleaned all preamp tubes and sockets?Also Sovtek input jacks tend to be lousy.I'd also check the ground connections.
Zouto

[Enzo]

I want to believe that the power section is fine

NEVER think up reasons NOT to check something. Try different power tubes.

[Fartrider]

I'll at least check the bias tonight. Maybe it's a bit off..

[52 Bill]

That "scratchy fizzy" description sounds like a parasitic oscillation. It could be caused by any number of things like bad lead dress, bad tubes, bad cap, etc.

Seeing as the output of the first gain stage is run to the front panel, make sure all wires going to and from the input jacks are properly shielded and are separated from one another.

The next thing that I'd try is to check the filter cap for the pre-amp stage.

[Fartrider]

So I took a second look inside. The grid resistors are 510k instead of 470. and the 33k (grid stopper) resistor connecting pin 1 and 7 on v2 is removed.

and i have some voltages on v1
anode - cathode v1.1 = 140v
cathode - grid = 1v

anode - cathode v1.2 = 140v
cathode - grid = 0.2v (normal?)

i also adjusted bias on the power tubes, it was a bit off.

the problem is still there.
guess i´ll have to take a look at those preamp filter capacitors too.