Brownface Pro Amp blowing fuses..

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by steveb, Mar 1, 2012.

  1. steveb

    steveb Member

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    This amp belonged to a co-worker's dad who is now deceased. Unfortunately, the son doesn't have the electronic knowledge his dad had. He told me about this amp, & that it doesn't power up. I suggested checking the fuse, & he replaced it, but it blew after 3 seconds. I suggested that he bring it to work, & I would take a look at it. When he brought it, it had no power tubes in it. I pointed that out, & said I would take it home to look at it. I put good 6L6's in it, replaced the fuse, & it also blew within 3 seconds. I took the chassis out, & everything looks to be original except the filter caps & one of the power supply diodes. Looks like a germanium? It looks clean, especially considering its age & the damp climate here in Louisiana. I can't tell if the transformers have been changed, but both are also very clean with no rust. None of the caps are blistered.

    When powering it up, I hear a slight hum, a bit higher pitched than 60hz, then it blows the fuse.

    So, I don't know what else to try other than suggesting that he take it to a local amp tech.
     
  2. SatelliteAmps

    SatelliteAmps Member

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    The next step would be to pull the rest of the tubes, and see if it blows a fuse (use a smaller fuse than needed, just to be safe). If it pops again, pull the secondary's from the PT and fire it up again. If it pops fuses again, then you most likely have a blown PT. If not, then you will need to take it to a tech to trace the circuit and find the short.
     
  3. zenas

    zenas Member

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    If you have a light bulb limiter this is the time to use it. Insted of blowing fuses the bulb takes the juice and glows full bright. Then you can pull tubes or if it comes to it wires off the secondary side of the PT untill the light goes out or dims.
    If you end up with all the wires off the secondarys and the lights still full bright then you need a PT it's shorted. (or a rewind) (or the power cords shorted inside)

    OH unplugg it BEFORE you unsolder wires and so forth !

    Don't have a limiter build one ! Very handy and cheap.
     
  4. steveb

    steveb Member

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    Thanks for the suggestions. I'll give those a try.
     
  5. TimmyP

    TimmyP Member

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    Pull all the tubes, disconnect the hum balance (if it has one) and disconnect the PT's HV secondary. If the fuse blows, the PT is bad. If not, connect the hum balance. If good, re-connect the primary and disconnect the line to the first filter cap (which connects to all the rest). If the fuse blows, bad diode(s). If not, it's likely the first cap.
     
  6. steveb

    steveb Member

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    It WAS the first filter cap. I thought the discoloration on the outer covering was from the foam strip on the inside the tank. Looked closer at the cap & there was a small split in the side of it. I also changed one of the diodes because I was getting no reading from it while in the circuit.

    It now works fine. Thanks to all who offered suggestions.
     
  7. Blues_N_Rock

    Blues_N_Rock Member

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    Did you replace all of the PS filter caps? If not it might be worthwhile.
     
  8. steveb

    steveb Member

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    No, but I know I should have. But, the owner of the amp only uses it play a radio through it in his workshop. Sorta' sad, but what can you say? I am having a hard enough time convincing him to replace all of the tubes instead of just replacing the missing power tubes. Oh well.... I think it's more of a sentimental thing to have a reminder of his dad instead of using the amp for it's intended purpose.
     

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