Using 12AT7 as phase inverter in Blues Jr.?

Discussion in 'Amp Central Station' started by BuckStone, Sep 30, 2016.

  1. BuckStone

    BuckStone NEW MEMBER!

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    I'm looking at ways to get a cleaner, higher-headroom sound out of a tweed Blues Jr.. Have replaced V1 with a 5751, speaker with Cannabis Rex (was Jensen C12N) and amp has BillM tone stack mod (just the one involving a jumper on the mid pot to get full off on the mids...no tone caps replaced as yet). Also have a replacement output tranny that is waiting in the wings as next mod. I had some 12AT7's lying around, so I plugged one into V3 (actually a NOS Sylvania 6201...high-class 12AT7), and to my ears the amp sounded better/cleaner. A day or so later I turned the amp on and got the dreaded loud hum followed by fuse blow as one of the JJ EL84s failed. My question is could the 12AT7 in the driver position have caused the EL84 to fail, or was it just ready to go and this just a coincidence? I know that 12AT7's have less gain than a 12AX7 but are capable of producing 10 times the current. Are they just not a good match for driving push-pull EL84s (or is further modification to the circuit necessary to make them a good match)? Should I stick with a tube that has more similar specs but lower gain (e.g. 12AY7 or 5751). Liked the sound with the 12AT7 but don't want to be constantly replacing power tubes.
     
  2. theprofessor

    theprofessor Friend of Leo's

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    I'm no expert, but I can't imagine why a 12AT7 would have been connected to the failure of an EL84. I think they're completely unrelated and that it's just a coincidence. You should be able to run a 12AT7 in that PI slot without any problems.
     
  3. Jeru

    Jeru Tele-Holic

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    I know basically nothing compared to some here, but I know how these circuits work fundamentally, and agree with the poster immediately above.
     
  4. BuckStone

    BuckStone NEW MEMBER!

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    Thanks for the response. I found an old BillM post that said basically the same thing (also said that EL84s don''t draw very much current so 12AT7 being able to produce 10X the current not a factor). Most of my experience being with 6L6 amps, I'm not used to having power tubes fail that way. I bought the amp used on Ebay, but it looked brand new and had replacement JJ power tubes, so hard to imagine that it had hundreds of hours of use on the power tubes (but possible, I guess). I think that I'll try putting in a mod to make the bias adjustable while I have it apart to replace the output transformer. Increasing the negative bias a bit should cool it down and make the output tubes last longer.
     
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