Peavey Classic Series 60 mono power amp bias issue

[Bubbastain]

Just picked up a Peavey Classic Series 60 mono tube power amp and I have a bias issue with it. I decided to check the bias on the two 6L6's(actually had short base Sovtek 5881's) before I played it. I have two of the Eurotubes bias probes so I can check both at the same time. The tube on the left went up to 69ma and the one on the right was about 16ma. I shut it down and put in a good pair of closely matched 6L6's. Again, the one on the left went up to about 65ma and the one on the right went to about 25ma. I then put the Left tube in the Right socket and the Right tube in the left socket with the same results. It's not the tubes but the tube socket.

I took the tubes out and measured -40v on pin 5 of the Left socket and -47v on pin 5 of the right socket.  Both sockets measured 30.5v at pin 2, 530v at pin 3 and 29v at pin 7. Pin #1 showed 527v with the standby switch in play mode but no tubes installed. I don't see any obvious burnt out anything. Any help would be appreciated.

Here is the schematic and layout.

[shooter]

on the left

tells me the problem is staying with the socket.  A schematic will help.

with power off, tubes out, caps discharged, ohm tube side left side, then discharge caps and measure right side.

[Bubbastain]

on the left

tells me the problem is staying with the socket.  A schematic will help.

with power off, tubes out, caps discharged, ohm tube side left side, then discharge caps and measure right side.

I was finally able to attach the schematic. Yes, the problem is staying with the left tube socket(looking at it from the front of the amp). I will admit up front that there are many things I don't know what they mean or how to do. I am comfortable working in the amp and always drain caps and verify with my MM. I don't know what it means to Ohm it.

[sluckey]

What happens if you swap the two bias probes around?

[Bubbastain]

What happens if you swap the two bias probes around?

Tried that too. Same result. Took the board out and didn’t see any loose solder joints.

[sluckey]

How does the amp sound? Any tubes red plating?

[Bubbastain]

How does the amp sound? Any tubes red plating?

I’ve only played it for a minute or so.  The tubes that I have tried luckily have not red plated yet. The tube does get physically much hotter quickly.

[sluckey]

I took the tubes out and measured -40v on pin 5 of the Left socket and -47v on pin 5 of the right socket.

That bias voltage should be the same for both pin 5. It would be helpful if you identify the tubes as V2 and V3 rather than left and right. Then we can use the schematic to suggest more specific things to try. Something is causing the bias voltage on pin 5 to be different with the tubes out. Try to correct that first.

Disconnect CR20 and CR21. Pin 5 voltage the same now?

Disconnect C1 and C7. Pin 5 voltage the same now?

Disconnect C2 and C5. Pin 5 voltage the same now?

[Bubbastain]

I took the tubes out and measured -40v on pin 5 of the Left socket and -47v on pin 5 of the right socket.

That bias voltage should be the same for both pin 5. It would be helpful if you identify the tubes as V2 and V3 rather than left and right. Then we can use the schematic to suggest more specific things to try. Something is causing the bias voltage on pin 5 to be different with the tubes out. Try to correct that first.

Disconnect CR20 and CR21. Pin 5 voltage the same now?

Disconnect C1 and C7. Pin 5 voltage the same now?

Disconnect C2 and C5. Pin 5 voltage the same now?

Okay. V2 is the socket with the issue. Do you mean to physically desolder and remove either one leg or both of those components? I'd probably like to order some replacements for each to have on hand before I do that.
Some observations I've made. CR20 and CR21 on the schematic are both 1N4003. That's what they are on the actual board and they are the red colored diodes. Zener Diodes?
CR1 and CR2 are listed as SR2873 diodes on the schematic. On the Circuit layout the Peavey provided along with the schematic, CR1 is listed as a 4003 diode. On the actual board, CR1 and CR2 are both  250-2 8929 diodes.

[sluckey]

A 1N4003 is a standard rectifier diode rated at 200V @ 1A.

On the Circuit layout the Peavey provided along with the schematic, CR1 is listed as a 4003 diode.

Well, on the schematic you provided CR1 is a SR2873, just like CR2. But those diodes have nothing to do with the difference in bias voltage on pin 5 with the tubes out. The components I mentioned above do. So, do what I asked above, in that order, and report back. You only need to unsolder one end of each of those components. I would not solder anything back until you have tested all 6 components. If none of those components is causing the voltage on V2-5 to be -40v, then reconnect all of them. No need to replace if they are not the issue.

We may have to address CR1 and CR2 later, but for now just concentrate on fixing the bias voltage. One thing at a time.

[Bubbastain]

Got it. I really appreciate the help.

[Bubbastain]

I disconnected one leg of both CR20 and CR21 and then measured pin 5 of V2 with no power tubes, power on and amp in standby. I get a reading that’s kind of all over the place with readings of as high as -65 at initial contact and then the number will lowering. Initially it was reading between-40 to -44 when the amp was first turned on.  I’ll move on to removing C1 and C7.

I just realized after removing C1 and C7 that I forgot to hook up J9 when I did the last test. That probably explains the weird numbers. So after removing C1 and C7 I ended up with -47.1 at pin 5 on both V2 and V3. I'll move onto C2 and C5 and test.

After removing C2 and C5 it seems there is no change from the previous test. Both V2 and V3 are right at -47v at pin 5.

[sluckey]

OK, let's wade through the weeds. Since we now know that V2 is the tube with the low bias voltage on pin 5, we can forget about V3. So reconnect C5, C7, and CR20. After removing CR21 ***AND*** C1, the bias voltage went up to -47.1 which is what we want. So, either CR21 or C1 is the culprit. Can't say for sure which  because of your weird readings after removing CR21.

Reconnect C2 since it's not the problem. This leaves C1 and CR21. Reconnect CR21 (leaving C1 disconnected) and check the voltage at V2 pin 5. If it is still -47.1V, then C1 is the culprit. If the voltage dropped back down, then CR21 is the culprit. So, which is it?

If it turns out that CR21 is the culprit and you don't have a 1N4003, you can use a 1N4007, or 1N5408, or any other rectifier diode that is suitable for the B+ rectifier found in a tube amp.

Now totally remove and replace the bad component, reconnect the other component. Recheck the voltage on V2 pin 5. It should be -47.1 just like the voltage on V3 pin 5. If so, reconnect your bias probes and plug in the tubes. What is your bias current now?

[Bubbastain]

It turns out CR21 must be the culprit. I reconnected C2 like you said since it made no difference in the last test. I then connected CR21 and tested the bias on pin 5 and it dropped back down to -40. So I will try to order some 1N4003 diodes either from Peavey on monday or from somewhere else tomorrow. If they are not available I will order the other ones you mentioned. CR21 and CR20 are both the red clearish looking diodes. All the ones I've seen so far with the 1N4003 are the black diodes with the silver ring at one end. Are they the same for all intents and purposes? I will have to order another .047 400v cap to replace C1 since one of the legs broke in the removal process. Is there anything else I should go ahead and replace while I'm at it? There is no bulging or leaking that I can see on any of the caps.

Once again thanks for your help. You have come through for me once again. I probably won't get the parts delivered to me until later in the week.

[sluckey]

CR21 is not really necessary. Leave it disconnected while you are waiting on a replacement. Use your bias probes to check the tube current. What do you have?

[Bubbastain]

CR21 is not really necessary. Leave it disconnected while you are waiting on a replacement. Use your bias probes to check the tube current. What do you have?

Well I already completely removed C1 and C7 since I broke one of the legs of one of them when I was disconnection one leg of each. So I'll have to be a bit patient.

[sluckey]

You can use your bias probes to check the tube current without those caps.

[Bubbastain]

Success!! Got my parts in the mail today. Replaced both CR21(the bad one) and CR20 with new 1N4003 diodes. Also replaced C1 and C7 with new caps since I broke the legs off one of them during the troubleshooting process. Bias on V2 now reads 29.9ma and V3 reads 25.5ma with the pair of short base Sovtek 5881 tubes that are of unknown age. I will get a proper pair of matched 6L6GC's very soon. Thanks for the help once again Sluckey.