Gain Differences in Halves of Dual Triode

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by SLG, Mar 15, 2005.


  1. SLG

    SLG Member

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    I was retubing some amplifiers this past weekend. I was installing some NEW JJ and EH 12AX7 tubes. I was running all the tubes through my Jackson 658 tube testor. I always sort tubes to find those with closely matched triodes for the phase inverter positions. All the tubes were good, but I was noticing a fair discrepancy between both halves of the same tube. Some tubes were spot on matches (most of the EH), but some of the JJ's varied as much as 105-87. I realize that's within a 20% tolerance, but I rather expected them to be a bit closer. I have noticed that the GT12AX7M tubes tend to be within 10 from one side of the tube to the other. Good NOS tubes vary even less. Have others had similar experiences?

    I was retubing a tweed Bassman clone. The amp uses each half of the V1 tube for a separate channel(normal and bright). The 20% difference does make a noticeable difference in gain between the normal and bright channels.
     
  2. Blue Strat

    Blue Strat Member

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    I've had very similar results in testing 1000s of 12AX7s.

    The JJs were so far out (up to 25% in some cases, which disqualifies them as being called a 12AX7 and makes me wonder how they were ever shipped from the factory) that I had to reject 25+% from the last batch I bought. This was some time ago and I don't buy them any more.

    It's difficult to justify spending so much time doing JJs testing for them. I have much more productive ways to spend my time.
     
  3. Reeek

    Reeek Member

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    Amen and cheers!
     
  4. Blue Strat

    Blue Strat Member

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    Yeah, I agree with you. It's funny, though, that other manufacturers of new 12AX7s seem to produce a more consistent product. Or maybe you have to be an expatriot of their homeland to get the "good" ones.;)
     
  5. SLG

    SLG Member

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    I like the sound of the JJ's in some of my amps, but the mismatched tube halves are a problem. Hell, some of the 12AX7's I've tested have had half of the tube testing at normal 12AX7 levels and the other half testing at a lower gain level than a 5751 or 12AT7.

    I'm not a big fan of the Sovtek preamp tubes (except the 12AX7LPS), but every one I tested had the triodes matched within 2. Not one tube , not a couple of tubes, but every single one. The Sovteks rarely go microphonic. I'm not a big fan of the Sovtek sound, but they certainly know how to build good preamp tubes.

    I realize I'm probably living in a dream world, but if we could just get the quality control guys from Sovtek to train the quality control guys at EI we would see some great preamp tubes that rivaled most of the NOS. I loved the EI 12AX7's until the quality control and microphonics went down the toilet. I even preferred them over my NOS Mullards.

    It's like George Carlin said, "If I find something I like or use, sure enough, they stop making it."
     
  6. Reeek

    Reeek Member

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    It makes one wonder if they are manufacturing their preamp tubes for pennies on the dollar if they are willing to accept a 25% failure rate without improving upon that number.

    I myself have seen at LEAST a 10-12% microphonic or short rate on JJ preamp tubes.

    Because of this, and because I have never had a NP tube that sounded better than a good vintage tube, I don't even go there, whatsoever . . . Vintage is my choice!

    Call me what you want for having that attitude but I call it smart regardless of the flaming I may get from some :D :cool:
     

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