'72 Super Reverb blowing fuses

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jzucker, Dec 17, 2009.

  1. jzucker

    jzucker Supporting Member

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    Amp is a '72 Super Reverb that's been rebuilt. Transformers have been replaced with Mercury Magnetics, the circuit card was replaced with a repro fender circuit card using a combination of carbon film and metal film resistors, sprague and illinois caps, etc. Pots and tube sockets are original though all wiring (including filaments) is new.

    OT is a 4/8 mercury magnetics super reverb / bassman style...

    The symptom is that I'll be playing the amp for 5 or 10 minutes and the fuse will just suddenly blow. Doesn't blow in standby or when switching the standby to the "on" position. Only blows after several minutes to several hours of playing. Mostly at low volumes too...

    Here's what I have tried so far:


    1. Replaced the rectifier tube - Still blows
    2. Replaced the power tubes - Still blows
    3. Noticed I was not getting any bias voltage
      1. Replaced 50uf bias cap. Seems like problem might be fixed.
      2. Fuse blew 2 days later, same circumstances as original.
      3. This time, i'm still getting -52v at pin 5 of the power tube
    4. Checked 470ohm resistor on bias board. Looks good.
    5. Checked 1.5k bias feed resistors across pin 5. Looks good.
    6. Checked for shorts in the heater wiring / jewel lamp assembly. Looks good.
    7. Rebuilt bias board with new 470 ohm 2W resistor, new cap, new diode
      1. Working so far but I have the feeling I haven't found the real problem
     
    Last edited: Dec 21, 2009
  2. mark norwine

    mark norwine Member

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    sounds like your current is "running away". have you lost bias?

    yank the output tubes and measure pin 5 (ref to ground). is there a neg voltage present? I suspect not.

    or....can you isolate this to just one power tube socket or the other? any carbon tracking on the socket(s)?
     
  3. jzucker

    jzucker Supporting Member

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    I'll check both of those. Thanks. One symptom I noticed before this issue is that earlier today I was hearing a lot of amp hum perhaps corroborating your theory about the bias. You think the bias cap could be bad? It's been replaced with a sprague atom fairly recently ...
     
  4. GearHeadFred

    GearHeadFred Member

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    Buzz is a side effect of a runaway situation.. If that is the case, the tubes are trying to fully conduct. Don't put any power tubes back in there until you verify that there is a negative bias voltage on pin 5 (grid) of them.. It should be around -45 or -50 volts DC, measured to ground (as Mark said).

    You will cook the output tubes if there is no bias. You can also do other damage.
     
  5. jzucker

    jzucker Supporting Member

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    raw bias voltage was -0.5. Turning the pot had no effect. Put in a new bias cap and it's working again. Thanks!

    Bias cap had been replaced but must have either gone bad or there was a cold solder joint thought it looked ok...
     
  6. jzucker

    jzucker Supporting Member

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    wow, amp sounds so amazing now. The cap must have been gradually going bad...
     
  7. Fuchsaudio

    Fuchsaudio Member

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    Make sure it's positive to ground Jack....else you'll have "smoking tone", in a bad way :)
     
  8. phsyconoodler

    phsyconoodler Member

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    I always change the bias diode too,especially in an old Fender amp.
     
  9. Fuchsaudio

    Fuchsaudio Member

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    Pennies well spent ! :agree
     
  10. zzmoore

    zzmoore Member

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    Andy, Mark, Phsyco.....
    What is the best modern day replacement for that, and also for the 6 rectifier diodes? This is for a 1969 Twin that has all new components except for the PT, OT and those diodes.
    Thank You
     
  11. mark norwine

    mark norwine Member

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    for me, good old 1N4007 diodes work just fine.
     
  12. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Yes, they sure do. I've been known to use N5408's as well, mainly because I bought a 100 of 'em :)


    [​IMG]
     
  13. plexi67

    plexi67 Member

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    was it a black sprague?
     
  14. jzucker

    jzucker Supporting Member

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    no, it was a 47uf/100v illinois. I was mistaken earlier when I said it was an atom.
     
  15. Fuchsaudio

    Fuchsaudio Member

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    I like the UF5408 or UF4007 (ultrafast diodes). They sound sweet !! lol.
    I can send you some Jack, if you like.
     
  16. Mike9

    Mike9 Supporting Member

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    Andy - do you think FREDS improve performance & tone? Inquiring mind wants to know - :beer
     
  17. jzucker

    jzucker Supporting Member

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    [UPDATE]

    The fuse blew again today. I'm starting to wonder if i'm getting more than 100v on the bias tap and it's blowing out the bias capacitor.
     
  18. slider313

    slider313 Silver Supporting Member

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    Did the fuse blow right away, as soon as you powered it up, or did you play it a while?
    Have you checked the 470 ohm bias resistor's value?
     
  19. jzucker

    jzucker Supporting Member

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    It blew after about 10 minutes. Same as last time. I did not check the 470 ohm bias resistor's value. If the bias resistor were out of whack wouldn't I be getting the wrong bias voltage constantly?
     
  20. RussB

    RussB low rent hobbyist Silver Supporting Member

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    You need to start troubleshooting by getting voltage readings and comparing them to the schematic
     

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