Peavey Triple X head problems

[guitardog]

Hey folks, I have posted here before and got some excellent guidance from Enzo and others. I am hoping you can help again. I recently acquired a Peavey Triple X head that is not working as it should. I will explain the symptoms and what actions I have taken. The amp appears to be stock, and is running 4 6l6s and 4 12ax7s. The amp does power on and produce sound, but it is weak (I have a another XXX driving an identical cabinet so I know what it should be able to do). Turning up the volume controls will increase volume, but the sound is "poppy" and muddy and not even close to the volume of my other XXX. All 3 channels function, with the foot switch and all with the same low power condition. I removed all tubes, and the amp from the cabinet. Visual inspection noted nothing obviously burned or damaged. I swapped out the power tubes and there was no change in sound. I did however notice one of the tubes red plating a little. I pulled those tubes back out and deoxed all tube sockets, jacks and ribbon cable connectors and adjusted the BIAS to obtain 55 v at test probe port. This didn't change the sound but the red plating went away. To isolate the problem to the pre-amp or the power amp, I parked a VTM 60 on top of the XXX and connected the effects out of the VTM to the effects return of the XXX. Plugged a guitar into the VTM and it sounded about the same. Practically no noticable difference. I then reversed the connections and went from the effects send of the XXX to the effects return of the VTM. Wham, I have 60 watts of full tube power filling the room. After this, I am confident that the problem is in the power section, but I don't think that the problem is with the power tubes because it behaved the same with 2 different sets. (I do intend on changing all of the tubes, but not until I correct the problem.) I am competent with a multimeter and a soldering iron, but I am not sure where to look first. Any suggestions would be appreciated. Thanks!

[tedmich]

You inspected all molex connectors for a burned pin/trace? SGRs all test AOK? Could also be a bad PI 12ax7 as this would cripple your PA.

[Cygnus X1]

Jack Darr taught me to change tubes first, not later.

[Amp Kat]

I agree that ^^^ the tubes could very well be the problem and usually are. Those symptoms have power tubes written all over them however if you really think it's a different socket problem try 1 known good tube and go down the row and see if each socket will bias up right and play right. The amp will play with one tube.

[guitardog]

Status update; I found an open SGR and another that had already been changed (who ever did it took the easy route and didn't remove the board and soldered the new resistor on the bottom. I removed and replaced all SGR and the power has been restored to the amp. Over all it sounds pretty good, but I get some loud popping and hissing when I switch to stand by so I am going to replace all of the filter capacitors (waiting on them now) and re-tube the beast. I think I will be in good shape after that. Thanks for the help!

[Enzo]

When a power tube fails it often takes out a screen grid resistor too. So finding a replaced resistor just means someone had a tube fail at some point in the past.

I often recommend replacing those resistors on the foil side so as to avoid having to dismount the whole thing. The circuit doesn't care where the resistors are placed.

[guitardog]

I agree with soldering in on the foil side, and on this board doing so requires it to be removed because some of the traces are on top, some on bottom.