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Need help with my '65 Deluxe Reverb Reissue

Discussion in 'Amp Tech Center' started by strangledangle, Sep 21, 2020.

  1. strangledangle

    strangledangle TDPRI Member

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    Hey all, this is my first post on this forum so I apologize if my formatting is a little off.

    A few years back I made the ultimate mistake of using my DRRI without the speaker connected. I was using a Radial JDX DI to record straight from the amp to my interface but neglected to connect the speaker throughput. It took all of ten minutes before the amp made a loud noise and subsequently started smoking. I quickly turned it off and put it on the bench for inspection.

    Immediately I saw that resistors R70 and R71 were absolutely shot and the PCB around them was burned badly right through the traces. The schematics say that these are 100ohm .5W carbon film resistors. My grandfather bought me a replacement loaded PCB and this week I FINALLY got around to installing it. After installing I turned on the power and let it cook (under supervision) for about ten minutes. Everything was fine until I took the amp off standby (speaker connected this time) and the same thing happened instantaneously. Loud noise and then smoking at R70 and R71, which leads me to believe that the true problem lies elsewhere. Fortunately I caught the issue before it burned through the PCB so my hopes are that this board is still healthy.

    I'm beginning to think that this could be the result of a bad tube or something in the power circuit that failed due to overcurrent, but I'm not entirely sure. I don't have a tube tester or bias probe but I've got a solid multimeter and *enough* electronics knowledge and time to get to testing and to the bottom of this issue. At this time I don't have any replacement tubes but I can get some easily enough.

    Where do I begin with the troubleshooting process? What do you suppose is the root of this problem? Any and all recommendations are welcome as I'm left directionless and scratching my head. Pictures and descriptions are posted at the following imgur link. Thanks in advance!

    https://imgur.com/gallery/7IFl2fP
     
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  2. Jon Snell

    Jon Snell Tele-Holic

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    I would replace the output valves as they may be the cause of the issue.
    With no loudspeaker connected, there may have been high voltage spikes making one of the valves flash over and damage itself.
    When you fit new valves, check the bias and adjust as necessary.
     
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  3. dogmeat

    dogmeat Friend of Leo's

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    70 and 71 are the artificial center tap in the heater circuit. the standby switch puts B+ on the power section, so thats where the problem should be... something is bleeding across. test the tubes if you can, if not take the 6V6s out and see if it blows with no tubes. check all the wiring & sockets first. I don't think a shorted output transformer could do that but I would ohm the primary side just to to see. easy to do with the tubes out
     
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  4. strangledangle

    strangledangle TDPRI Member

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    I'll have to wait until I get a handful of replacement resistors to fully test this out but in the meantime I've pulled the 6V6s and tested for continuity across the pins. From what I understand pins 7 and 2 should be the heater circuit and should be the only pins with continuity. A quick test confirmed this, no other combinations have continuity. Are these tubes good as far as continuity goes?
     
  5. strangledangle

    strangledangle TDPRI Member

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    Thanks for the reply, I just got another matched set of 6V6s and another 5AR4 just incase. I'll test once I can swap out the blown resistors and report back.
     
  6. dogmeat

    dogmeat Friend of Leo's

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    measure ohms blue to brown on the primary side of the transformer. I think you can use pin 3 to pin 3... socket to socket with the tubes out, on standby, no power. the classic tone replacement is 6.6k, so apox 3.3k from either one to red

    pin 8 on the power tubes should go to ground.

    the old tubes are probably roached but I would check those things just because
     
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  7. Jon Snell

    Jon Snell Tele-Holic

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    Your values are impedance readings. Not many people can read impedances easily. The transformer, if short circuit primary, will not cause catastrophic failure, it will sound tinny and squeaky.
    The DC resistance can be as low as 80 Ohms one side and 100 Ohms the other and be within specification as the inner winding has less wire for the same amount of turns but the impedance at say 1kHZ is 6k6 etc.
     
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  8. zook

    zook Friend of Leo's

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    If there's a short in one of the output tubes it can put DC voltage into the heater circuit. That will cause the artificial center tap to burn up.
     
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  9. schmee

    schmee Doctor of Teleocity

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    Without a speaker connected you likely blew the Output Transformer the first go around.
     
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  10. Jon Snell

    Jon Snell Tele-Holic

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    That would not take out the virtual ground resistors and is most unlikely.
    A short in a 6V6 can be the only cause.
     
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  11. strangledangle

    strangledangle TDPRI Member

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    I'm hoping you've got the right idea here, my resistors are coming in this afternoon. Will swap those and the output tubes and retest.
     
  12. strangledangle

    strangledangle TDPRI Member

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    UPDATE:

    I replaced the R70 and R71 resistors and turned it on without the output tubes connected. Once I flipped it off standby I noticed some some arcing across pins 2 and 3 of the second output tube socket (V9) so it seems I might have located the issue. Now I'm off to order a new tube socket and will try again!
     
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  13. JDRNoPro

    JDRNoPro Friend of Leo's

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    Need to figure out what is causing the arcing too, I would think - i.e., is it just a problem with the socket itself or some other underlying issue producing it or that caused damage to that socket?
     
  14. Jon Snell

    Jon Snell Tele-Holic

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    That was caused by flashover from the output transformer.
    What can happen when an output valve gets an intermittent Anode to Cathode/Heater fault is the valve becomes a switch and all of the HT energy is absorbed by the output transformer in one go, the valve relaxes as the voltage is greatly reduced causing a spark from the back EMF from the transformer of many thousands of volts through the other half of the transformer winding. (Petrol engine ignition coils are a good example). This damages the insulation between high voltage pin and the pin next to it, in this case the heater pin which is the shortest point to ground.
    I would replace the virtual ground resistors, (already done but check them), replace the valve base and a new set of 6L6s.
     
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  15. JDRNoPro

    JDRNoPro Friend of Leo's

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    Thanks for that excellent explanation!
     
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