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Rectifier tube flash, blowing fuses Dynaco MKII

Discussion in 'Tube Audio' started by Adam N., Oct 12, 2018.

  1. Adam N.

    Adam N. Trying to keep it simple. Subscriber

    Messages:
    317
    Location:
    Northampton, MA
    A few days ago I picked up a subwoofer and was hooking it up. I accidentally connected live line level outputs to the dynacos and it made a loud thump. At the same time that this was happening the smart switch that use to turn the Dynacos on and off had a server failure and the amps power cycled a lot quickly while I tried to figure out what was going on.

    Anyhow, one MKIII blew a fuse but I don't know if it was due to the power cycling, the clumsy sub hookup or something else.

    When I tried a new fuse it was OK for a few seconds then there was an arc in the rectifier tube and it blew its fuse. I am assuming that the rectifier tube was damaged either due to the power cycling issue (hot restarts) or the line level thump. I ordered a new JJ GZ34 / 5AR4 Vacuum Tube rectifier and some fuses 3a at 320 volts.

    Am I following the right course? Did I get the right fuse? Is the damage probably limited?

    I know pretty much noting about electronics. The board is original but was recapped and was running fine.

    Your help is appreciated.
     
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  2. peterh

    peterh AK Subscriber Subscriber

    Messages:
    1,892
    Location:
    Gothenburg,Sweden
    Yes, your plan of action is a good start. Assuming you have 120VAC ( and not 320 )
    A tube that once have arc'ed has high risk of a new arc, in a rectifier it means that it shorts the internals and draws
    a lot of current, if you are lucky it will blow the fuse.
     
  3. Adam N.

    Adam N. Trying to keep it simple. Subscriber

    Messages:
    317
    Location:
    Northampton, MA
    Yep, 120. Fingers crossed. I ran it on a variac to 115. Thank you.
     
  4. primosounds

    primosounds Powered with pure tube sounds. Subscriber

    Messages:
    2,818
    Location:
    Terra, 21st century CE
    It is very bad to have a short on/off cycle with a tube rectifier, unless you make a circuit to circumvent that. I notice that the currently made 5ar4, are also not as forgiving of a heavy load and will arc if there is a lot of initial capacitance or high voltage, like in Dynaco MK3, on the rectifier.
     
  5. Adam N.

    Adam N. Trying to keep it simple. Subscriber

    Messages:
    317
    Location:
    Northampton, MA
    Could the short have been caused by the frequent restarting? I looked underneath and nothing looks amiss. Not in the quick look I took anyhow.

    Is there a chance that with the new tube it's going to be OK?

    Is the 3 amp 250v fuse correct for that amp?
     
  6. UncleBingo

    UncleBingo AK Subscriber Subscriber

    Messages:
    1,341
    Some of the newer rectifier tubes are not as durable as the older tubes were. Remember, many of the old tubes had to be able to cut the mustard under heavy industrial/military use.

    Put the new rectifier in and power it up. Monitor the amp closely for the 1st 10-15 minutes. If there is no glow in the output tubes after the 1st minute or so, shut it down. If there is a cherry red glow in either of the output tubes, shut it down right away. If there's no audio after a 2 minute warm-up, shut it down.
     
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  7. Adam N.

    Adam N. Trying to keep it simple. Subscriber

    Messages:
    317
    Location:
    Northampton, MA
    Thanks for your help.
     
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  8. kward

    kward AK Subscriber Subscriber

    Messages:
    3,512
    Location:
    Utah
    The issue with fixed bias amps in general is that when you hot cycle them, the bias voltage can bleed down quicker than the plate voltage and cathode temperature. So a hot cycle could put the output stage into a momentary situation where the tubes will full-on conduct until the bias voltage comes back up to nominal value. That can easily blow a correctly rated fuse and/or if using tube rectification, especially with a new production rectifier, and even more especially with JJ rectifiers, it will arc the rectifier, which in turn, might in rare cases damage some other circuitry in the amp. I've personally never had anything else damaged when accidentally hot cycling as it always either blows the fuse or arcs the rectifier or both, but nothing else. I've had this specific arcing issues show itself more so with JJ 5AR4 rectifiers than with other new production rectifiers, so much so that I won't use JJ rectifiers anymore. But...if you wait the requisite time between cycles of off/on, a new JJ rectifier should be fine. I have found that the generic Chinese 5AR4s are quite rugged, relatively speaking to the JJ anyway, and specifically the Tube Amp Doctor (TAD) 5AR4 (Chinese made to TADs spec) is quite robust.

    The way to get around this is to wait about 30 seconds before powering on again. Or with more time and expense, you can build a more sophisticated circuit to delay application of voltages even if power switch is quickly cycled. I prefer just to exercise care and not hot cycle the amp. Invariably though, it does happen from time to time, for me usually around the holidays or family gatherings when curious fingers are running amok in my listening room.

    3 Amp fuse is what the design calls for, yes.
     
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  9. arts

    arts Super Member

    Messages:
    3,851
    Location:
    Qc, Canada
    I strongly recommend one of these for anyone using tube gear,especially if it also uses tube rectifiers. While sold as a plug-in GFCI device ,these units also quickly trip during short power interruptions. Perfect for preventing power company induced hot cycling,but not much use in preventing your own ''oopsies''. Can't have everything;)

    https://www.amazon.com/TRC-90033-Shockshield-Portable-Protection/dp/B000XVG72G

    If you are looking for something more substantial (I use one for my system and another for my power tools) or just require more current capacity,this can't be beat:

    https://www.amazon.com/Leviton-GFM20-3C-20-Amp-120-volt-Manual/dp/B00HRF2KSG/ref=sr_1_1?ie=UTF8&qid=1539374802&sr=8-
    1&keywords=leviton+gfm20-3c&dpID=416V75JRzAL&preST=_SY300_QL70_&dpSrc=srch


    I have stopped using JJ octal-based tubes due to reliability issues.
     
    Last edited: Oct 12, 2018
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  10. Adam N.

    Adam N. Trying to keep it simple. Subscriber

    Messages:
    317
    Location:
    Northampton, MA
    Thanks, I picked one of these up. Good idea. I may do a GFI outlet eventually.
     
  11. Adam N.

    Adam N. Trying to keep it simple. Subscriber

    Messages:
    317
    Location:
    Northampton, MA
    New tube and fuses arrived. System powered up and seems fine.

    All looks good! Thanks to everyone for the input. It's great to have a community of experts to lean on.
     

     

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  12. RS Steve

    RS Steve Tube Junkie Subscriber

    Messages:
    7,543
    Location:
    Jupiter, Florida
    This diode mod was what I followed when I built my MK III's, allows the use of modern cheaper built rectifier tubes.

    TubeRectifier-DiodeMod.jpg
     
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  13. triode17

    triode17 Super Member

    Messages:
    1,365
    NO ! Do NOT use a GFCI wall outlet. These will reset once the power comes back on and will allow the same faults to occur. The portable one has to manually reset after the power returns. This way, when the power drops out , the GFCI outlet will NOT return power until you are sure everything is OK. Be careful about this.
     
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  14. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
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  15. Adam N.

    Adam N. Trying to keep it simple. Subscriber

    Messages:
    317
    Location:
    Northampton, MA
    The little GFCI that I got does require a manual reset. Just does the one outlet though. Now it will go outlet>power strip>smart plug>GFCI>Variac>amps. My 1890s house wasn't made for modern electronics. It could use 4x the number of outlets.

    If I understand this now the power comes in to the amp, the rectifier tube crudely chops it down to DC then it passes to the big can capacitor to smooth it out. When I turn off the amp and it slowly gets quieter thats the can capacitor draining down its power. Also: there are scary dangerous voltages lurking in there. Is this more or less correct?
     
    Last edited: Oct 15, 2018
  16. cademan

    cademan Addicted Member

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    9,052
    :thumbsup:
     
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  17. Adam N.

    Adam N. Trying to keep it simple. Subscriber

    Messages:
    317
    Location:
    Northampton, MA
    The replacement JJ tube just failed.

    Is there something more reliable than these or is this pointing to some deeper underlying issue?
     
  18. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
    931
    The current production Gold Lion GZ34 is supposed to be better. And, yes, there might possibly be another problem.

    How are you setting your variac? Are you going by the scale on the face of the variac or are you setting it with a handheld meter?
     
  19. glue_ru

    glue_ru New Member

    Messages:
    12
    Location:
    Columbus Ohio
    check the associated resistors and electrolytic capacitors, #1 choice is resistors fried and now runs too high
     
  20. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    10,065
    Location:
    New Jersey
    If its all original,
    It's not unlikely the big can cap is failing.
    It's certainly well past its expiration date.
    Also did you notice audible hum from the power transformer when it was working ?
    That would indicate that cap is leaky.
    I had a few mkIIIs (close enough to a mkII for sake of conversation) come through here
    a,while back.
    I found before replacing said can , PT had excessive hum.
    I'm thinking your cap is probably failing a little more.
     

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