Super Reverb quits after 2 hours

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by swiveltung, Apr 16, 2016.

  1. swiveltung

    swiveltung Member

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    So I have this basket case SR I've been working on for months, just off and on. For a while it crackled loudly after maybe an hours use. I seem to have gotten rid of that problem with a tube socket. A while back I played it at rehearsal and after 1.5 hours it blew the fuse. Replaced the fuse and tried to find any issues/voltages out of wack etc but see none. I decided to try it for rehearsal last night after it sat for a month or two. Worked great for 2 hours. Then while sitting idling, I noticed the pilot light was out. Yep blew the fuse again. Go figure. At this point I'm thinking the only answer is replacing the PT. Any thoughts from you amp gurus? :>) (correct slo blo fuse is in it)
     
  2. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Have you replaced the rectifier and the power tubes to eliminate them? I'm assuming all the 'lytic caps have been done?
     
  3. VaughnC

    VaughnC Supporting Member

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    Power transformers tend to be fairly robust and usually don't become intermittent...so that would be the last thing I'd suspect.

    However, there are at least a dozen other less expensive things that could cause a fuse to blow and intermittents can be buggers to find. Tubes can be intermittent, so I'd start there with at least new power & rectifier (if so equipped) tubes. And, if the electrolytics are near 10 years old, I'd replace those as well for routine maintenance.

    Beyond that, if that doesn't help, I think I'd replace the power supply diodes (if so equipped) & bias supply diode as I've seen diodes become intermittent and they're inexpensive. Beyond that, if the fuse still intermittently blows, it could make for interesting troubleshooting ;).
     
    Last edited: Apr 16, 2016
  4. zenas

    zenas Member

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    Keep using it whilst it blows fuses and pretty soon it'll get a 20 amp fuse or the tinfoil wrap. Then you'll smell that sickening sweet smell of a power transformer burning.

    Definitely replace the electrolytics if they're old or originals.
     
  5. swiveltung

    swiveltung Member

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    Filters are new. Most everything is actually new at this point. Complete cap job other than the Blue Ajax. New TAD power tubes. New Mojo Output transformer. New diode/bias supply. However, I do have an older rectifier in it. Maybe I'll put in my SS Rectifier module and play it again for 2 hours! It would be easier if it didn't play for a couple hours then fail! I was thinking maybe the PT is heating up and arcing in there somewhere.. grasping...
     
  6. slider313

    slider313 Silver Supporting Member

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    Try another set of power tubes...................
     
  7. Dan40

    Dan40 Supporting Member

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    Since it has only blown fuses after being used for a couple of hours, it does seem like something is getting hot and failing, which in turn blows the fuse. May be tricky to track the problem down.
     
  8. zenas

    zenas Member

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    Could be Royal bitch to track down. The trick is getting it to do it with the chassis out. Intermittent problems are the worst.
     
  9. Bob Pollock

    Bob Pollock Supporting Member

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    Check the resistors across the power tube sockets.
     
  10. swiveltung

    swiveltung Member

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    I will they are new 2 watt metal oxide. But the 1500 ohm are just carbon comp.
     
  11. pdf64

    pdf64 Member

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    Original CC?
    I suggest that the resistors mounted on the power tube sockets are replaced periodically, eg along with the ecaps, as they're subject to thermal and physical stress. Allow some 'give' in the resistor leads, eg put a bit of a kink in them somehow.

    Maybe replace everything in the bias circuit (other than the PT) eg diode, current limiter resistor, reservoir etc cap, pot, grid referencing resistors, grid stopper resistors, tube sockets, wiring!
    And the coupling caps between phase splitter and power tube control grids, using a 630V rated type (they may be exposed to >400V under some conditions).

    Consider attempting to lift up the top circuit board, use a dental mirror etc to see if there's a solder blob or something between the upper and lower boards, that may be causing an intermittent short between eyelets.
    Applies to the main, bias supply and doghouse boards.

    Consider fitting a series pair of 1N4007 diodes in series with each tube rectifier plate, eg mounted on the spare tube socket terminals.
     
    Last edited: Apr 17, 2016
  12. The Dirty Tube

    The Dirty Tube Member

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    This.

    How hot is the power tranny at the end of 2 hours? I know this is a tricky and subjective question, as PT's naturally get hot, and tubes hanging upside down in a cabinet will make it even hotter.

    A popping fuse is indicative of too much current. A PT, a rectifier, or a short will pop a fuse. A circuit short cropping up like clockwork after hours of play isn't likely.

    You are using a GZ34 / 5AR4 in there, right?

    Definitely pop in a new rectifier tube and report back.
     
  13. swiveltung

    swiveltung Member

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    Yeah I thought I should have felt the PT for heat... but didnt think until after rehearsal was over... was distracted. I think I have a 5V4 in there right now, worried about blowing a Vintage GZ34 until I have all sorted out. I need to get the SS plug in in there and see what happens I guess.
     
  14. VaughnC

    VaughnC Supporting Member

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    Resistors don't short, and open wouldn't cause a fuse to blow...so I'd consider those last. Capacitors, diodes, and transformers are most likely. Good luck....intermittents can drive you crazy :eek:.
     

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