O.K. Marshall experts, here's a strange one.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pfrischmann, Nov 6, 2005.


  1. pfrischmann

    pfrischmann Member

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    I recently blew the opt in my 100w Marshall. It seems the primary wires that go to the plates was grounding out to the common wire on the secondary.

    I replaced the opt with a known good Dagnall. The amp seemed to work and all the voltages are correcct. I started by using a Hot plate so I could pay attention to the amp as I played.

    Everything seemed fine.

    I then took the hot plate out of it and went directly into the cab. Hit one chord and I saw a spark come out of v5 (I think) and the HT fuse blew. (same symptoms as before). I had the amp open and upside down while playing so I saw the spark come from the bottome (wire side) of the socket.

    I'm guessing I have a bad socket but I'm not sure how to verify this, especially since it seems to only haped at full volume. I don't like this OPT much but I'd prefer not to blow it.....

    Any ideas?
     
  2. hasserl

    hasserl Member

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    There is probably a carbon trail laid down on that socket from the previous failure, which is conductive and will continue to cause arcing problems. You likely need to replace the socket.
     
  3. VintageJon

    VintageJon Member

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    What was said above, by hasserl
    - EXCEPT it only arced with Hot Plate.
    I suspect Hot Plate or additional cable.

    What is the additional cable??

    You probably have a "carbon trace" now as it worked with speaker but arced with Hot Plate.

    -Jon
     
  4. pfrischmann

    pfrischmann Member

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    Other way around Jon,

    It worked with Hot Plate but arced without it..
     
  5. Wakarusa

    Wakarusa Member

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    Regardless of the original failure (and whether the HP had anything to do with it), if you saw it arc, the socket probably needs replaced.

    edit: Might also check to make sure that when the speaker cab is connected that it gives the impedence it is supposed to. I.e. check for a bad speaker, bad cable, bad connection, bad impedence selector switch
     
  6. pfrischmann

    pfrischmann Member

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    Yeah I'm pulling the socket for sure...Good point about the cable and cab. I've been assuming they were O.K.

    The arching socket would explain how the amp suddenly went bad after months of use.

    Thanks guys,

    When I'm sure it's healthy, I going to install FREDS for fun!
     
  7. Wakarusa

    Wakarusa Member

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    There's a good chance that the socket first arced with the original OT failure -- not unusual for a smoked OT to take out other bits in the power section. Once the initial carbon trail is laid down then it's easier to have it arc again, though it won't always misbehave (i.e. it'll do exactly what you've seen).

    While you're in there replacing the socket I'd suggest also inspecting output tubes, screen resistors, grid stoppers, etc. Basically a once over to look for anything that might have been weakened or destroyed in the original failure. You'd be surprised at what can be bad in the power section and still have the amp "work".
     
  8. VintageJon

    VintageJon Member

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    What Todd said...

    + Never Assume Anything!

    -Jon
     
  9. pfrischmann

    pfrischmann Member

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    Yeah, got me on that one.

    I think it would have been easier to just build an amp from the ground up.
     

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