5150 Head post gain volume has no effect

[samino]

I have owned my 5150 head for about 12 years ... never had any issues with it until now. I get no volume change when I turn the post gain knob on either channel. The volume is really really low. I can can hear changes to the volume and tone when I turn the pre gain, presence and resonance controls.

I have tried the effect loop trick with a spare cable from send to return and there was no change. I have also replace the power and preamp tubes with brand new ones and still no change.

Any ideas on what else it could be?

[Enzo]

Unlikely they both went bad. But a bad HIGH control in the EQ could kill the signal.

Try this. Plug the guitar into the FX return. The preamp controls will now have no effect, but is the signal strong and clean out the speaker? That tested the power amp. Now plug guitar into the regular input jack out front. Rin a spare cord from the FX send to some other amp so we can hear what the FX send sounds like. Is it OK or does it have the problem still? You must have a speaker connected to teh 5150 at all times.

[samino]

I did try that too actually. I plugged the guitar straight into the FX return and I got a much louder signal there than just plugging the guitar into the front of the amp. I was clean and strong as you said.

I can try the send to another amp and see what that sound like.

So when you say a bad high EQ ... you think that maybe its the pot or some other component? If its as simple as replacing the pot that would be sweet. Keep in mind this is the original 5150 with the shared EQ for both channels, why it was designed like that I will never know but o well, so its not as unlikely as you might think.

[Enzo]

I meant the treble control itself, or the solder to it. I know it may be hard to listen to, but do the tone countrols sound like they are working? All the sound comes out the treble control on the way to the post volume controls. SO a bad or unsoldered treble control can kill the sound. The bass and mid controls can cartainly affect things but not cut it out completely like the treble can. This is just one possibility of many though.

And of course try patching a spare cord from FX send to FX return and see if that helps.