Peavey JSX / flashing tubes and exploding traces

Discussion in 'Amp Tech Center' started by MrCoolGuy, Sep 23, 2021.

  1. MrCoolGuy

    MrCoolGuy Tele-Holic

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    Back in June I received a Peavey JSX head. The owner said the fuse had blown
    And that he saw a flash and expected a bad tube and screen grid resistor.
    I created a thread back then: https://www.tdpri.com/threads/peavy-jsx-head-r77.1073134/
    ...trying to understand this complex amp.
    Upon further investigation, I found that a few resistors had blown (R77, R121, R122)
    Which are elevated heater supply, and artificial center tap resistors respectively.
    Also, the trace going to the plate of V6 exploded likely shorting to pin 2.
    I replaced all these along with new screen grid resistors... and a couple diodes and caps that received collateral damage and the amp got new power tubes and I biased it. I made a jumper to replace the exploded trace. It sounded great and worked fine for about 3 months. Now, the guy brought it back to me, I opened it up and everything looks exactly the same as last time. R77, R121, R122 burned up.
    I replaced them all again as well as cleaned up everything around them, I installed a new tube socket (V6) due to it getting caked in suit twice. I powered it up with no power tubes through the lightbulb limiter, light dims properly. Voltages looked ok. I installed new tubes (all 8) and powered up through the lightbulb limiter. As soon as the tubes got warm, V6 started to flash...

    I haven't had a chance to do anything else since then, but I just learned that the owner had the speaker cab wired improperly. So who knows what impedance the OT saw... could that be the issue? I checked dc resistance and for continuity through the primaries and secondaries, which looked fine. Im going to recheck voltage going to the EL34's (without tubes) again when I get home.
    What else could I check? Any ideas.
    Since I havent been able to bias the new tubes, could there be too high voltage due to the bias being too low causing the tube to flash? 20210601_143014.jpg Screenshot_20210531-155625_Acrobat for Samsung.jpg received_892966474599713.jpeg.jpg
     

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  2. Preacher

    Preacher Friend of Leo's

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    Yeah, those burnt spots is where I would start. ;-)

    in reference to your question, if the replaced resistors are the same as what was in there originally, I would think you would not be drawing any additional voltage. But you might also look at those traces, on my old Valve Jr once a resistor cooked the board the traces seemed to come loose and connectivity was lost. I see you pulled the board, how is the chassis behind V6?
    Is it possibly arcing to the chassis under that component?

    It sounds like V6 is where everything is starting. I would really look that area over well.
     
  3. Jon Snell

    Jon Snell Tele-Holic

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    As an EL34 (either Sovtek or non Chinese valve) can run happily with 800vlts on their anodes, I don't think HT too high is your issue.
    If the amplifier is working hard and the loudspeaker is disconnected, that will do the sort of damage you are seeing here because the back EMF from the output transformer will be many kilovolts! Is CR103, the flyback blocking diode, intact and conducting as it should I wonder.
    Check/replace the speaker jack/lead/speaker.

    More info added
     
  4. MrCoolGuy

    MrCoolGuy Tele-Holic

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    The chassis behind v6 looks fine. No evidence of anything.
     
  5. MrCoolGuy

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    I checked many diodes... im not sure without being home to look, but I will check out cr103.
     
  6. Jon Snell

    Jon Snell Tele-Holic

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    Difficult to test with a DVM as the voltage drop is quite high.
    I teast them with my AVO 8.
     
  7. MrCoolGuy

    MrCoolGuy Tele-Holic

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    Are you talking about those old avometers that are like 100 years old?
     
  8. Jon Snell

    Jon Snell Tele-Holic

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    Yes, I still use mine. Great for checking transformers; shorted turns means low inductance so the meter moves to low resistance fast, good primary winding takes a second to read low resistance.
    They have a 15volt battery for the resistance ranges so selenium and other older type rectifier diodes read as do up to about a 12volt drop. I specialize in valve and older repairs so often come across this sort of item to test.
     
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  9. MrCoolGuy

    MrCoolGuy Tele-Holic

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    Wow,
    Thats awesome. Very interesting.
     
  10. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Can it be ruled out that the board is somehow conducting current between the traces at that particular point?
    It seems odd that it happens in the same spot/tube.
     
  11. MrCoolGuy

    MrCoolGuy Tele-Holic

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    Yeah, thats what it seems like. But im not sure how to rule that out besides just visually watching. But lastnight when I tried to do this... now the amp wont come on. When I switch it on, there is a slightly audible hum coming through the speakers, but the tubes aren't lighting up and the pilot light didnt come on. So I started checking power at the PT secondaries. The b+ looked ok, but something came up and I havent got to check the filament power yet. I'll keep ya posted.
     
  12. MrCoolGuy

    MrCoolGuy Tele-Holic

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    Also, the first time it happened, it blew out a chunk of the trace on the board. Ihad to make a jumper to fix it. I say this because that missing piece of the trace makes the traces farther apart making the chance of it arcing over less likely...
    But we will see. Ill post more pics later.
     
  13. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Has the socket been thoroughly checked or replaced?
     
  14. MrCoolGuy

    MrCoolGuy Tele-Holic

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    Yes, I replaced that socket.
     
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  15. MrCoolGuy

    MrCoolGuy Tele-Holic

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    Here are a couple views of V6.
    The jumper was added the first time I fixed the amp. 20210924_134322.jpg 20210924_134315.jpg 20210924_134300.jpg
     
  16. MrCoolGuy

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    That burnt spot looks like it was a short between traces. One going to pin 3 of v6... the other goes to pin 2 of v5.
    Which makes sense considering the other damage done. (Elevated heaters, and artificial ct resistors)... but how did that happen? Was it because a tube flahed over? What else could cause this?
     
  17. MrCoolGuy

    MrCoolGuy Tele-Holic

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    So... I got everything to power back up.
    My current dilemna (pun not intended) is this... when I take out the power tubes to check voltages and whatnot, something starts smoking. Is the edge of the right solder joint on cr103... the circuit board is turning red and burning right against the edge of that solder. See the picture from the first time I fixed this amp back in June. And see the pic from today. 20210601_143014.jpg 20210924_134300.jpg
     
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