Peavey Triple XXX Head has strange problem Please Help

[CoopDaKill]

Hi everyone. I have a problem with my Triple XXX Head. Here's the problem, I'm having with this amp. When footswitch is not connected, the volume is very low. Channel switching works, as it should. When I plug in the footswitch, the volume is fine and footswitch works all functions. I tried lots of things, such as, turning off the amp before connecting or disconnecting the footswitch. I'm pretty good with a soldering iron, schematics, and repair. I don't claim to be a pro. But this one has me bum-fuzzled. Bad relay? Bad component? I have the schematic. IDK? Any help will be greatly appreciated.

Sorry I've not been on here in a while. My wife has been very ill.

[Enzo]

When the FS is not connected, it automatically engages the FX loop. Take a spare cord and plug it from FX send to FX return. ANy help?

[ncjoyota]

I have a problem similar, but i dont think its the effects loop.. It went quiet sometimes, and now it has lost all input volume all together.. I thought it might be the tubes, so I got eurotubes to send me a set of JJ KT77's an EC83's well it wasnt the tubes either.. I biased the power tubes to 37.2mA and still no sound.. then I noticed the middle preamp tube wasnt glowing.. so i took the chassis out and the socket has no voltage running to it... while the other preamp sockets have about 340v running thru them.. I cant see any cold solder joints nor any burnt resistors.. I am stumped.. Im not a electronic guru, so I dont want to get to fooling around too much inside this amp. any suggestions?

[Enzo]

Resistors don't always burn up. They can be bad and look no different.

[Chuck H]

If the tube isn't glowing that's a lack of filament voltage. If there is no high voltage that's another problem. If the tube isn't glowing AND you have no high voltage that's two problems. Perhaps unrelated.

[ncjoyota]

well as far as I can tell none of the pins have any voltage running thru them on the middle preamp tube.. so it has to be somthing large that is affecting the whole tube plate.. but somthing that only affects that one tube plate.. because all the rest of them still work.

[Enzo]

I think there is some confusion here.

First the tube heater is totally separate from the plate voltage. The preamp tubes on the XXX are wired in series across the +/-18vDC rails. And oddly, they grounded the center tap on the center tube, which they could easily have not bothered to do. If the two sides of the center tube were dark, then so would the other two preamps tubes be. If they are glowing, then teh center tube must be as well. Turn off the room lights and look closely at that tube to see if the heaters are not just hiding down inside.

Meanwhile, something large? I don't think so. The plate is only one pin of a triode. so pins 1 and 6 on a 12AX7. If one of those has zero volts on it, it means either the B+ supply is gone, and none of the tubes has it, or it means the plate resistor has opened or been disconnected from the supply.

And if the plate has no voltage, the tube has nothing to conduct, so the grid and cathode voltages will also be zero. ALl for lack of a single resistor.

[ncjoyota]

I cut all the lights off, and got it in the dark and I noticed the tube on the far left wasnt fulling lighting up like rest either.. I drew a paint diagram.. how do I find out which resistor it is thats deadning the tubes?

[Chuck H]

how do I find out which resistor it is thats deadning the tubes?

At this point I would suggest a qualified repair shop.

[tedmich]

At this point I would suggest a qualified repair shop.

+1!

[Enzo]

Brightness is not important, if the tube is glowing at all, it is working. Well to be more accurate, the heater in that tube is working. The glowing heater tells us nothing at all about the rest of the tube function. There is no resistor involved with the tube heaters. They are wired directly from -18v to +18v, all three in series. Three 12v tube heaters in series needs 36v, and -18 to +18 is 36v.


If pins 1 and 6 of any 12AX7 have no B+ voltage, then that right there is a problem. And an open resistor is the most likely cause.

The voltages in this amp can KILL YOU. If you really don;t know what you are looking at, I agree it is probably time to take it to a qualified technician.

[ncjoyota]

yea I found a shop that is supposed to be really good about 80 miles away.. and they said bring it anytime, and they estimated the repair at $100- $120.. he said it sounds like a problem he has encountered before. dangit tho.. I went from some old dusty groovetubes, that I thought finally gave up... to some HOT JJ KT77's- JJ EC83S's and I was so looking forward to popping them in... biasing the 77's and melting faces.. and now... theres more I have to do.. GRRRR. I even ordered some new drumheads and a new snare screen in anticipation of the new tubes.

[Chuck H]

On the up side, there's probably nothing wrong with your old tubes. That's $$$ you won't need to spend when it's time to retube again.

[ncjoyota]

wellll.. they are about 8 yrs old.. withe moderate playtime on them.. so they were getting a little weak.. so when the amp basically died.. I figured it was the tubes.. but I will keeep them as spares..

by the way... is it possible to run a EC83 and a 12ax7 right next to eachother? say if one of my 83's die

[tedmich]

ecc83 is the European name for the (US) 12AX7 tube; they are for all intents and purposes identical.

Here are some old GE tubes I picked up with both labels: