Peavey 5150 Combo Preamp Signal

[bobloblaws]

I have a Peavey 5150 combo amp, 2 x 12, 2 x 6L6, 60 watts, here is the schematic.peavey_5150_combo.pdf

A few weeks ago I was measuring bias using the ammeter function on my DMM and I accidentlly touched one of the leads to the grounded reverb tank with the other lead connected to the OT center tap. Something evidently got fried as their is virtually no signal at the output. I've done obvious trouble shooting like swapping out the tubes, checking plate voltages, bias voltage, filament voltage etc. I actually have another of the same model amp which is handy for comparison purposes. Anyway, I can trace the input signal to the Rhythm Post and Lead Post pots but there is no signal at R28. I assumed the problem was the relay K1B so I went through the excruciating experience of removing the circuit board (this design is not tech friendly by any stretch) and swapped in a brand new relay. Alas, after putting everything back together the problem persists. So I'm leaning toward the 1M volume pots as the culprit(s) (the signal is not getting to R28 regardless of whether on rhythm or lead channel). Indeed, I get signal at the input legs of the pots but not the output legs. But before I dive in and change those pots there is something that give me pause. If I measure the in-circuit resistance between input and output legs it is comparable to what I see on my other 5150. So I don't know how much I can read into that given that the pots are in-circuit and the resistance is affected by other components, for example where the bass and treble pots are set at affects the resistance. The same holds true for the Rhythm Post pot if the Lead channel is selected. But in both cases the signal still does not get to R28.

So given the above, if you had to make an educated guess, would you advise to pull that PCB again and tackle those two pots? Or something else I should look at?

Thanks,

- Bob

[dstrat]

Tube amps will generally survive those oops but silicon transistors and especially jfets not so much
check the J174 labeled Q1.

[bobloblaws]

Ok, is there a simple way to test it?

[dstrat]

check the gate for a voltage?
its a p-channel needs a positive voltage to open the connection to ground.

guessing here but you likely took out the switching control circuit.

[bobloblaws]

check the gate for a voltage?
its a p-channel needs a positive voltage to open the connection to ground.

guessing here but you likely took out the switching control circuit.

Do you mean to say it's likely more than just replacing that particular JFET?

Do you mean the channel switching? It seems to switch from Rhythm to Lead channel as per normal, as far as I can tell.

I'm also not familiar with "clamp". I gather it is some sort of common reference voltage but I can't find anywhere on the schematic indicating a power source or level for the "clamp" voltages.

[dstrat]

looks like there should be around +23 VDC at the gate of Q1, its getting voltage from a triac and a 6530 transistor, over close to the footswitch.
its likely a mute circuit to keep things from popping when switching channels.
I was just trying to point you to a possible cause of your problem. Transistors are not really my thing, more of a old tube amp guy.

[bobloblaws]

looks like there should be around +23 VDC at the gate of Q1, its getting voltage from a triac and a 6530 transistor, over close to the footswitch.
its likely a mute circuit to keep things from popping when switching channels.
I was just trying to point you to a possible cause of your problem. Transistors are not really my thing, more of a old tube amp guy.

OK, I'll look into Q1 as a possible source of the problem, thank you!

[dstrat]

yeah I was still looking at the schematic, you could try a test clip from the +23 volt rail thru a 1m resistor to the gate of Q1. ( its what I would try )
but as to testing the triac I am not sure how to go about that, the 6530 (Q12) is also a suspect.

[Enzo]

I would get out my clippers and slice the transistor off the board. If the amp wakes up, you found it. If that changes nothing, add the JFET to the list of whatever else you wind up needing.

How would we test, you look for gate voltage, but I suspect it will be there. I would measure resistance of R28 while the amp is running. I bet you don't see the 2 meg, I bet you see less than 100 ohms. Just my guess. In fact, compare this reading to your good amp. Remember, the relay connects one of the volume controls, which are in parallel. So max those controls, not whichever one is selected, you will have 500k there, which in parallel with R28 yields roughly 400k. I would expect about 400k reading for the 2.2meg R28 in circuit.

Do me this experiment, I'll explain in a moment. Watch for signal on the post controls with them up and the reverb up. Now shake the reverb pan to crash the springs. Does that reverb crash show up on the volume controls? I bet it doesn't.

I am guessing, but Q1 mutes the amp as channels switch, but Q4 does the same function at the reverb drive input. And both those JFETs are tied together at the gates. So if one shorts, they both are compromised. I am thinking the key here is you shorted the high voltage to the reverb pan. And I bet it frotzed Q4, which is shunting Q1 in the bargain.

Obviously there are many other scenarios, but mine is easily checked in a few seconds.

[bobloblaws]

I would get out my clippers and slice the transistor off the board. If the amp wakes up, you found it. If that changes nothing, add the JFET to the list of whatever else you wind up needing.

How would we test, you look for gate voltage, but I suspect it will be there. I would measure resistance of R28 while the amp is running. I bet you don't see the 2 meg, I bet you see less than 100 ohms. Just my guess. In fact, compare this reading to your good amp. Remember, the relay connects one of the volume controls, which are in parallel. So max those controls, not whichever one is selected, you will have 500k there, which in parallel with R28 yields roughly 400k. I would expect about 400k reading for the 2.2meg R28 in circuit.

Do me this experiment, I'll explain in a moment. Watch for signal on the post controls with them up and the reverb up. Now shake the reverb pan to crash the springs. Does that reverb crash show up on the volume controls? I bet it doesn't.

I am guessing, but Q1 mutes the amp as channels switch, but Q4 does the same function at the reverb drive input. And both those JFETs are tied together at the gates. So if one shorts, they both are compromised. I am thinking the key here is you shorted the high voltage to the reverb pan. And I bet it frotzed Q4, which is shunting Q1 in the bargain.

Obviously there are many other scenarios, but mine is easily checked in a few seconds.

Hey Enzo, I'm willing to try those experiments but I had already pulled the board again so I can't necessarily do everything at the moment (like the reverb test), but let me share what I did find. I put the preamp tubes back in the board, grounded it and connected the power ribbon cable. I'm looking in the area of Q1, Q4, Q12 and CR9. Those components are not getting the positive voltage that should be at the CR9 side of R55. The other side of R55 has 24V so I assumed R55 was my problem. But when I measured it out of circuit is bang on 1Meg.

By the way, I wrote down R28 resistance values with various permutations on both amps so I have them in case they still prove to be useful. I'm not sure what you mean by "not which one is selected". FWIW, on the working one I get 276K with the amp on and both "Post" volumes on 10, same with the Rhythm pot on 10 and the Lead pot at zero. That is with Rhythm channel selected. On the non-working amp the values are between 150 and 350 ohms using the same scenarios. So pretty close to what you deduced, no?

[Enzo]

Typo: should read NOW whichever one is selected...


Yes, darn close. I ccannot predict what the resistance would be for a blown transistor, so I just guessed.

Snip Q1 out and see if it wakes up.


I max both controls so whichever way the relay is set, I get the same readings.

[bobloblaws]

Typo: should read NOW whichever one is selected...


Yes, darn close. I ccannot predict what the resistance would be for a blown transistor, so I just guessed.

Snip Q1 out and see if it wakes up.


I max both controls so whichever way the relay is set, I get the same readings.

I can try that, but what is the most likely scenario, that all 4 of those components I mentioned (Q1, Q4, Q12, CR9) are blown? Does that jibe with the lack of voltage I see on the CR9 side of R55?
If the fix ends up being replacing one or more of those components I want to do it now while I have the board out, as I said it is a tricky one to get out and put back in.

[g1]

Q1 & Q4 should do it.

[bobloblaws]

Q1 & Q4 should do it.

Good call g1, replaced Q1 and Q4 and it's good to go. Thanks for the input everyone!

[g1]

That was from Enzo's post #9.

Q1 mutes the amp as channels switch, but Q4 does the same function at the reverb drive input. And both those JFETs are tied together at the gates. So if one shorts, they both are compromised. I am thinking the key here is you shorted the high voltage to the reverb pan. And I bet it frotzed Q4, which is shunting Q1 in the bargain.