vt60 making noise

[tomtech]

Got an ampeg vt60 112 combo. Its making a scratchy noise when turned on as the tubes come alive. the noise seems to be there most of the time. That is to say, it does not make the noise when cold and after warm up it goes away. Nor does it develop the noise after a long period of being turned on, its there from the start. It appears to be only coming through when on channel C or channel B. Channel A seems OK. this seems odd to me since when I look at the schematic it looks like all active components are shared in the signal path and only passive components are unique to the channels. Any thoughts or guidance appreciated. turning all the vol controls down all the way does no affect the noise. So it must be coming from something after the early preamp stage. ?? I was able to change out the 12ax7's and the 6L6's with some others I have and the prob persisted so I dont think its tube noise (though I have heard tubes make this noise in the past). The only tube I have not changed out is the 12au7 cause I dont have one. Thanks.

[stokes]

To try to find the stage it is occuring in,pull V1 and turn the amp on.If the noise is there shut it down and replace the tube.Now pull V2 and turn it on,if the noise is still there replace and move to V3 and so on.Since it doesnt happen when the amp is warm,I suggest waiting a few minutes before moving to the next stage.It is also important to progress from V1 to 2 to 3 etc.If you start at V3 and the noise stops,it doesnt tell you if the problem is in V1 or 2.Once you determine which is the offending stage,it makes it easier to narrow down and locate the problem.

[g1]

Do you have the VT-60 schematic? If so please upload it here.
I think everyone usually refers to the VT120 schematic, but the correct one for the VT60 would be nice.
There must be something about CH.A that is not common to the other channels, and that would help to isolate the problem.

[drewl]

Edit: I thought this was a Peavey VT, so nevermind.

Pull preamp tubes one at a time to try and find the faulty stage as mentioned.

[Jazz P Bass]

Yes, do what Stokes suggests to narroe it down.
If pulling the phase inverter tube kills the noise, you may have a faulty solder connection, a bad plate resistor or a leaky coupling cap.

[stokes]

Okay,I misread "and after warm up it goes away".So dont worry about waiting between pulling each stage.A copy of the schem would be good,I have the VT40 but not the 60

[Jazz P Bass]

VT 120 is as close as it gets.