Announcement

Collapse
No announcement yet.

Peavey Classic 30 signal bleed at zero volume

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Peavey Classic 30 signal bleed at zero volume

    I recently bought a Peavey Classic 30. I like the amp but it has a problem. On either channel, with the channel volume all the way down, a slightly muffled clean tone signal comes through. On the overdrive channel, the clean signal persists (staying at the same volume) as I turn up the channel volume, giving me a mix of clean and dirty, which is not good IMO. Once you get to about 3 or 4 you can’t discern the clean sound anymore, but the amp is then too loud for most home use.

    Last night I pulled the chassis and did some minor tests, stuff I could do without removing the PCB, using only my multimeter.

    here’s the schematic http://www.bustedgear.com/images/sch...classic-30.pdf

    All of the volume controls’ CCW tabs have continuity to ground; all of the cathode resistors and bypass caps have continuity to ground; the capacitors at the preamp power supply nodes seem fine, though I don’t think I have what I need to thoroughly test them.

    I tried grounding different signal nodes to see what would make the signal bleed disappear. Only grounding the output of the last preamp stage got rid of the bleed, along with any other signal. Strangely, if I ground out the grid of the last preamp stage, the bleed gets louder. That last stage is a mixer stage and has a 2.2M feedback resistor, and I wonder if that has something to do with the strange behavior.

    I’m thoroughly baffled. Anybody have any ideas about what might be causing this?

  • #2
    Bizarre.

    I suspected the source of my signal bleed was the power supply, so I tried jumpering the last voltage dropping resistor. I attached the jumper before I turned on the amp, and when I turned it on the signal bleed was gone.

    Then I removed the jumper while the amp was still running, and the signal bleed was still gone. I turned of the amp, turned it back on again, and the signal bleed returned.

    So I attached the jumper and the signal bleed went away again, removed it and it was still gone.

    Turned the amp off and back on without the jumper, and the signal bleed was back. So I attached the jumper, and it didn’t work! The bleed was still there.

    Turned the amp off, attached the jumper while off, then turned on the amp. The bleed was gone. Repeated without the jumper, the bleed was back, attaching it didn’t help.

    WTF?! My best guess is an issue with the filter caps, but I’m just guessing.

    Comment


    • #3
      Yes, possibly the filter cap for that preamp node. You can just tack in a good one in parallel to test.
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

      Comment


      • #4
        I clipped another cap in parallel to each of the existing filter caps at the preamp nodes, and it did nothing. I now think that my earlier tests didn’t do anything, that it was just a coincidence that the amp stopped acting up. Back to square one.

        Comment


        • #5
          Click image for larger version

Name:	Screenshot 2021-04-11 at 09.28.57.png
Views:	201
Size:	250.8 KB
ID:	928666 Check for bad connections;
          Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

          Comment


          • #6
            All of the volume controls’ CCW tabs have continuity to ground
            Continuity doesn’t have a precise definition, eg the bleeper on my Fluke responds up to about 200ohms.
            But also, as they’re wired as pots, set them to min and check the resistance between the wiper and 0V common.
            My band:- http://www.youtube.com/user/RedwingBand

            Comment


            • #7
              I’ll check as you all have suggested. The strangest thing is that the amp seems to randomly “decide” whether or not to bleed signal when I turn it on, then it stays in that state for as long as I keep the amp on. Thanks for the suggestions.

              Comment


              • #8
                Your described symptoms would be pretty much explained if there were an impedance that was common to the V1a and V1b stages like this. This would couple the two stages a bit like a long tail phase inverter.

                Click image for larger version  Name:	pv30ci.JPG Views:	0 Size:	95.1 KB ID:	928680

                This would seem to correspond to the thin ground strip on the PCB. Does it look okay?

                Click image for larger version  Name:	pv30tt.JPG Views:	0 Size:	158.9 KB ID:	928681

                I wonder about possible leakage on the PCB due to contamination or even the PC going conductive.

                Also worth swapping V1 with V2 to see if has any effect.

                I have had one of these once that had a broken wire that joined adjacent PCB in the tri-fold arrangement.
                Last edited by nickb; 04-11-2021, 03:28 PM.
                Experience is something you get, just after you really needed it.

                Comment


                • #9
                  So I pulled out the PCB. First of all, it looks pristine. All of the solder joints look perfect and the board itself is super clean. I tested the resistance to ground of all the suspicious parts of the circuit, and got nothing higher than 0.3 ohms.

                  I put it all back together and tried swapping V1 and V2. This made a difference. The tube that was originally in V1 worked fine in my Marshall SV20 so I figured it was a good tube. In fact, it was brand new; I only pulled it to try a different tube.

                  So, I tried out a bunch of different tubes from my 12AX7 stash in V1 and found that they allowed various amounts of signal bleed. The quietest so far is a JJ.

                  I was satisfied enough with this that I decided to go ahead with some minor mods to the amp, which is a subject for another thread.

                  Comment


                  • #10
                    I'm really not at all surprised you didn't find anything on the PCB. I am surprised that you had crosstalk in several tubes. This begs the question, which should have been asked in the first place, did you do any measurements to quantify the size of the effect?
                    Experience is something you get, just after you really needed it.

                    Comment


                    • #11
                      Originally posted by nickb View Post
                      I'm really not at all surprised you didn't find anything on the PCB. I am surprised that you had crosstalk in several tubes. This begs the question, which should have been asked in the first place, did you do any measurements to quantify the size of the effect?
                      No, I didn’t take any measurements. I figured any audible amount of crosstalk is wrong. I am still getting enough crosstalk that it’s irritating at nighttime bedroom levels.

                      Is it at all possible that the heaters could be carrying signal between tubes? I ask because all of the preamp tube heaters are wired in series, which I’ve never seen before. I feel like the only other possibility is that the board has become conductive.

                      Comment


                      • #12
                        Well it so happens that I had a Classic 30 here today and there is no bleed as you describe through at all. What you have is pretty gross by the sound of it. It did have a problem with a bad solder joint on the relay which I could only see under a microscope.

                        The big problem with these how hard they are to work on. You have to pull the assembly and then reconnect it carefully ensuring that nothing can move and short out. Even then it's hard to probe or examine. I think I'd have a really good look at the solder joints in the relevant area, perhaps even do shotgun resolder, not a technique I'd normally recommend but it might be justified in this case. If that doesn't do it, or you don't want to try that, then you'll have to scope it out.
                        Experience is something you get, just after you really needed it.

                        Comment


                        • #13
                          I would get under each little bare-wire jumper between boards with some small tool and gently tug. Want to see if any are cracked free And then I would check the solder closely on each.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #14
                            Originally posted by nickb View Post
                            Well it so happens that I had a Classic 30 here today and there is no bleed as you describe through at all. What you have is pretty gross by the sound of it. It did have a problem with a bad solder joint on the relay which I could only see under a microscope.

                            The big problem with these how hard they are to work on. You have to pull the assembly and then reconnect it carefully ensuring that nothing can move and short out. Even then it's hard to probe or examine. I think I'd have a really good look at the solder joints in the relevant area, perhaps even do shotgun resolder, not a technique I'd normally recommend but it might be justified in this case. If that doesn't do it, or you don't want to try that, then you'll have to scope it out.
                            I wasn’t at all sure how I was going to set up the PCB unfolded and plugged up, so I did a shotgun resolder. That mostly took care of the problem. There’s still the tiniest bit of bleed, but it’s quieter than the acoustic sound of my electric guitar. It’s a major improvement.

                            I’m satisfied. I just wish I knew what exactly was causing the issue. Thank you all for your input.

                            Comment


                            • #15
                              I'm glad you got it done. It's always nice to know the definitive reason but sometimes just have to go with the flow.
                              Experience is something you get, just after you really needed it.

                              Comment

                              Working...
                              X