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tube matching question

Discussion in 'Electronic Basics' started by tempus fugit, Aug 29, 2005.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I just put 4 new tubes in my Carvin guitar amp and was wondering about
    matching. The current through the cathode of the tubes is:

    30mA
    32mA
    25mA and
    36mA.

    The tubes are supposed to be matched, but evidently tolerances weren't too
    tight. Are these tubes "matched" enough? I guess what I want to know is
    whether I should take the tubes back and get an actual matched quad, or if
    it doesn't really matter a whole lot. I know about 10 years ago I replaced
    the tubes and blew 3 of them in the first 6 months. I definitely don't want
    that to happen again. Could the mismatch (particularly the high and the low
    tube) cause that kind of problem? The power being dissipated ranges from 10w
    to 16 watts, depending on the tube, so I think I'm safe as far as that goes.
    Is there any reason I should be concerned about the mismatch, or should I
    just leave it? I tried swapping the tubes around, and the currents were
    pretty much the same, so I think I can also rule out problems with the amp's
    circuitry (can't I?).

    Thanks
     
  2. BobG

    BobG Guest

    If they are matched, all the symmetrical even harmonics are cancelled,
    leaving the harse sounding odd harmonics. If you just mismatch them on
    purpose by pullin one out, you get the nice rich bluesy sound you've
    been seeking... lots of odd and even harmonics.
     
  3. tempus fugit

    tempus fugit Guest

    Thanks for your reply Bob.

    I've read that argument also - that unmatched tubes will give a better sound
    than matched ones. To be honest, the sound is fine to my ears, but I just
    want to make sure that no damage will result (to amp or tubes) with the
    mismatched state they're in now.
     
  4. BobG

    BobG Guest

    When a tube shorts out and draws big amps, it smokes the output
    transformer primary. Big bux.
     
  5. This isn't actually true in any typical guitar tube amp as they operate
    in class AB (i.e strongly class B). Only if the output is class A will
    the even harmonics be cancelled.

    You need both halves to be active all the time to effect a v^2 - (-v)^2
    cancellation.

    Oh.. the question...the accuracy of matching noted in the original post
    is reasonably ok.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  6. BobG

    BobG Guest

    Hey Kevin..... You didn't reply to my inquiry about your mic preamp
    design progress.... wondering if its to the component selection and
    layout stage yet....
     
  7. Unfortunately, I am rather busy at the moment.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
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