Peavey XXX massive distortion

[Enzo]

If it didn't fix the problem, what is still wrong? Or are you referring to the preamp issue?

If the power amp still has a problem, earlier you said you were shorting out resistors or some such. That leads me to want to check for any damaged ones from that process. The phase inverter is right there. Check the DC voltages around that.

Plug your guitar into the FX return jack to test the power amp alone.

[thrgadgetman]

that is how I know it is a power amp issue not and not a preamp issue. Phase inverter Pin 1 322.5V, Pin 2 58.3V, Pin 3 84V, pin 6 344.9V, pin 7 70.2V plus pop and hum when I touch it, and pin 8 84.1V. After the capacitors there 7.5mV AC and 4.5mV AC.

[Enzo]

Pion 7 is the input pin so we expect noise when touched. Pin 2 has the .047 cap to the tail, so less sensitivity there.

I suspect your meter is loading the circuit. Pins 3 and 8 are wired together, the cathodes. In this case you measured to ground from the cathodes and got 84v, you then measured the grids and got 58 and 70v. That makes about 26v and 14v bias in each case. Now measure voltage directly from pin 3/8 to either 2 or 7 and see what bias voltage you get that way. Note the grid resistors are not symmetrical.

You have cured the red plating tube, exactly what are we trying to cure in the power amp now?

[thrgadgetman]

Pin 7 -1.39, Pin 2 -1.37. This is why I am stumped everything seem all right, but the guitar bypassing the preamp still has massive distortion. My original thought was that maybe a relay went bad and the clean channel was being mixed with the dirty channel. So I bypassed the preamp all together and still had the same sound of massive distortion. I replaced the output transformer and that didn't help so I am back to being confused as to why there is so much distortion when plugging directly into the power amp, and why if I switch 2 either dirty channel I get nothing not even hum. It seems like maybe too much capacitance or too much gain, but where does it come from and how do I check? Also why is there so much sustain and why does the dampening switch help?

[Enzo]

Yes, see how those bias readings even right out when your meter doesn't load things? Nothing wrong with your meter, by the way. My trusty Fluke would act the same way.


Wait. The power amp is distorted. Now you also say switching to either dirt channel kills the sound completely? How can the preamp channel afect the power amp?

Are we talking about two things here? If power amp distortion is the problem at hand, please don't add in preamp problems to the discussion until the power amp is fixed. Otherwise it is hard to keep track of what we are talking about. However, if this is actually what it does - dirt channel on preamp kill;s power amp, then we have a very strange problem. I can easily see that a dead dirt channel might kill any signal that WOULD have gone to the power amp, but I just can;t imagine how the dirt channel could kill sound that was applied directly to the power amp.

Plug the test signal into the power amp in jack and leave it there. Unless under that condition switching to the dirt channel STILL kills the sound, we need to cure the power amp before wondering about anything else.

Did we establish if you have a scope? Or a signal tracer?

When you apply a signal to the powr amp, how do you do it? Are you plugging into the FX return, or are you clipping right to the input of the phase inverter? REmember, when going in through the FX loop, the power amp inludes V3a and the master volume control, on the main board. So don't forget to check out that tube too.

Aha! Try this. Power off, measure resistance to ground from pins 3/8 of the phase splitter tube. DO you get about 27k like you should? Or do you get more like 90k? An open R119 would raise that value, cause distortion, and would be affected by the damper switch. Just a thought.

[thrgadgetman]

Sorry I thought it was all related.
I was clipping to the input of phase splitter until you taught me the FX return trick.
No switching does not affect the signal when I do this.
Yes we still have distortion and sustain on power amp.
Grounded to Pin 3,8 gives 26.9K
I do have scope and function generator Vellman PCSGU250

[Enzo]

OK, then one at a time. The powr amp. The Fx return is good enough. If the same problem occurs with signal input to the FX return or clipped to the PI, then chances are the interveneing stage of V3 is not at fault.

So apply a steady test signal to the FX return. 1kHz is standard, but it doesn;t matter. I personally prefer to use 100Hz, simply because I can let 100Hz run a while without it piercing my ears like 1kHz does. I believe the master volume will still function in this setup. Signal level? I don;t know, maybe a volt? Now, scope the output - what does the distortion look like? Oh, and for this work, disconnect the feedback wire. The negative feedback from the output feeds back to the phase splitter. I believe it unplugs at the little board with the impedance switch. One end or the other ought to unplug.

SO with good signal applied, scope it at the master volume, at pin 7 of the phase inverter, at each plate of the PI, at the signal end of R107,112, and at pin 5 of each power tube. Which places look healthy and which not?


OOh ooh, here's one, if this pays off, you owe me a beer: Make sure the ground wire and the feedback wire on the output jack and impedance switch board are not reversed. There appears to be a push-on connector right next to the damping switch - that is the feedback wire. The ground wire goes on the push-on terminal over by the slave jack. Both wires come from the power tube board. The feedback wire comes from near the bias control and the phase splitter tube. The ground wire connects way over on the other end of the power tube board next to V8 socket.

[thrgadgetman]

I owe you a beer no matter what?
You can't possible mix those 2 wires there physically no long enough.
Everything looked good on the scope at all location so I checked the output wire from transformer at it is square with the left slightly higher with a small bump.
Douple checking everything I think I may of fryed the circuit board. When rechecking pin 5 of v7 I heard pop and sparks then blue fizz for a second got power turned off during firework show circuit board all black at that location now.

[thrgadgetman]

Pulled the two inner tubes out for safety and turned on
still works still same problem.

[thrgadgetman]

Thats what I get for working in a hurry and tired. Got lucky though I Probed pin 3 not 5 and the probe must of got to close to pin 2 to start the arching. Anyways it looks like the arching stayed on the surface and the black cleaned up good and everything seems to be functioning as normal. Enzo here is a pic of the distortion on the scope.

misc - Jon-Paul Jones' Photos

[Enzo]

Scoped where, the output? Looks like extremely overdriven signal that has clipped. If that is really 52vRMS at the speaker, my god...


Did you try the amp with the feedback line disconnected?

[thrgadgetman]

That was with the feedback disconnected and scoped from the 8Ohm output right where it plugs into the output circuit card. I had 24V on 2 tubes and 27V on the other 2 tubes on pin 5 (the real pin 5 )

[thrgadgetman]

so not touching anything for a few days because I can't figure it out things look better still same issue, but much better (strange).
Here is scope of pin 5 1V 115HZ.
misc - Jon-Paul Jones' Photos
and output:
misc - Jon-Paul Jones' Photos
hope all this meens something to someone.
I can buy new boards from peavey, but don't want to buy ones I don't need.