I put a 12AT7 in the PI spot of my amp and...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by midnightlaundry, Jan 27, 2012.

  1. midnightlaundry

    midnightlaundry Member

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    It sounded like a 50's flying saucer spaceship movie. :D

    Wats' doin' with that?
     
  2. Blue Strat

    Blue Strat Member

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    What amp? Maybe the 12AT7 is bad. Try cleaning the pins.
     
  3. midnightlaundry

    midnightlaundry Member

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    Tw Express build. The sockets are new Belden's; they're bloody tight!
     
  4. Blue Strat

    Blue Strat Member

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    Was it working with another tube? Maybe the 12AT7 is shot.
     
  5. midnightlaundry

    midnightlaundry Member

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    Yeah, the amp is good. Maybe I should to try another one.

    Btw, said tube was a Nos GE 6201 from 1986. The pins look a little dull and blued. Do tube pins tarnish over time?

    I was afraid to rock the tube in the socket because they're so tite. I killed a Telefunken by messing around with another amp that had tight tube sockets; it ripped the pin right off' the tube. :(
     
  6. Blue Strat

    Blue Strat Member

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    That can happen if the sockets are very tight.

    Yes, tube pins oxidize over time making for bad connection. Always have some contact cleaner on hand if you own a tube amp. A bad tube pin to socket connection is the most likely problem to occur.

    The blue on the pins is normal and they came out of the factory like that. It's probably just a bad tube but always try cleaning pins first.
     
  7. guitarcapo

    guitarcapo Senior Member

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    Sounds like a tube issue. lol

    spray contact cleaner in the socket.

    try a new tube.

    maybe check that something isn't crossing on the other side like two wires touching as they leave the socket.
     
  8. midnightlaundry

    midnightlaundry Member

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    I tried a few others know to work, and they did the same thing. It must be the way they react with the design of the amp, although I can't explain it technically.
     
  9. soundchaser59

    soundchaser59 Supporting Member

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    What tube was in the PI slot originally (from the factory)? Is it supposed to have an AT7 in it?
     
  10. midnightlaundry

    midnightlaundry Member

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    No. I think it's extreme blocking distortion in the power tubes with all' the current from the AT7. It will barely amplify a sound without cutting out and making loud poping sounds like I'm touching the plate pins with a test lead.
     
  11. Rockledge

    Rockledge Member

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    If the tube that was originally in it was suppose to be a 12AX7, and you replaced it with an AT, the AT will be a bit weaker than the AX.
    I have replaced AX preamp tubes with ATs in the past in order to cause the preamp to saturate at a lower volume.
     
  12. midnightlaundry

    midnightlaundry Member

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    It's not a regular amp. It's a Trainwreck Express.
     
  13. GearHeadFred

    GearHeadFred Member

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    I'm not an expert on the TW circuit, but I believe the PI is a very 'touchy' area of that circuit.

    My buddy had a TW express build. He noticed that the PI circuit was very unbalanced (like 90/10). He mucked around with values to balance it 50/50, but it changed the character of the amp (not in a good way).

    So he put it back to 'stock'.. and now he "rotates" his output tubes every 5000 miles!
     
  14. conundrum

    conundrum Member

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    Are all the 12AT7's you're using long plates?
    A Trainwreck is very prone to oscillation, and even in the PI slot, a long plate could be making things worse. I very much doubt it's blocking distortion.
     
  15. vibrostrat43

    vibrostrat43 Supporting Member

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    If it has those .1 uf coupling caps to the phase inverter's grids it has a better chance of being blocking distortion than most other ltp designs...especially if the phase inverter doesn't have enough filtering.
     

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