100W PT in a 50W head

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by adamsapple, Jan 15, 2008.

  1. adamsapple

    adamsapple Member

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    Folks, are they any issues with swapping out an 50W PT with a 100W PT. The reason is i like running my 50W jtm45 clone full tilt and I keep popping the HT fuse intermittantly. The current PT is from a '65 Bassmen head and Im thinking its arcing over from overcurrent in the secondary B+ and thus popping the fuse. I'm running the filaments on a separate tranny so its not the filment winding overheating.
     
  2. AdmiralB

    AdmiralB Silver Supporting Member

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    The PT wouldn't be on my top five list of things to look at.

    If it's tube-rectified, my first thought would be that you're arcing the rectifier. My next thought would be power tubes.
     
  3. adamsapple

    adamsapple Member

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    I'm using a weber copper cap for the rectifier, and I've used different sets of output tubes with the same result. The reason I suspect the PT is that its old, and the PT was in a previous amp where I blew the mains fuse by running w/o a load (it was hooked up to a talk box and i forgot provide a load). I've checked the bias ckt, filter caps etc and everything looks good. I also removed the end bell of the PT and found a lot of shellac or whatever sitting in the bell.
     
  4. AdmiralB

    AdmiralB Silver Supporting Member

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    If it's the PT, you wouldn't be blowing the HT fuse...you'd be blowing the mains. I still think it's tube-related.
     
  5. adamsapple

    adamsapple Member

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    hmmmmmmm, I've run this same set of tubes in my 100W plexi clone full tilt with no probs at all. Maybe its that copper cap. So ur saying that PT ('65 bassman) should be able to handle running 2 el34's full tilt with no probs.?
     
  6. AdmiralB

    AdmiralB Silver Supporting Member

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    If it were the PT, the HT fuse wouldn't be affected. Whatever is drawing excessive current is on the other side of the HT fuse - which might rule out the rectifier, depending upon how the amp is configured.

    Tubes seem most likely, but not certain.

    What value fuse are you using? Could simply be a little undersized.
     
  7. adamsapple

    adamsapple Member

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    I've been using 500ma slo blo and fast. Thats right for 50W's right? Maybe its old tube sockets. They're pretty old although i've scrapped 'em clean btwn pins.
     
  8. jay42

    jay42 Member

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    Do you have 1 ohm sense resistors? If so, it might be a good idea to monitor them and see what you're getting current-wise.
     
  9. adamsapple

    adamsapple Member

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    the HT fuse is in the PT center tap to ground. I've monitored the current in each tube with the 1 ohm resistors and at idle I get 34ma per tube, then while plugged in and playing hard open E chords each tube conducts from a low of about 110ma per tube up to about 210ma per tube with amp cranked into matched resistive load. While monitoring I've never yet reached up to 500ma total current thru the HT fuse. So there must be an intermittant short somewhere.
     
  10. AdmiralB

    AdmiralB Silver Supporting Member

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    Then in that case the rectifier is in the circuit downstream of the HT fuse. I'd try subbing an SS module.
     

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