new tube day yesterday

Discussion in 'Amps and Cabs' started by CADdrafter, Feb 23, 2012.

  1. CADdrafter

    CADdrafter Senior Member

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    Amp: Dabeck brand clone of a Marshall 50 watt model 1987 (built by Dave Smith in Richardson, Texas)



    the matched pair of winged c SED EL34s came in yesterday.

    i started by measuring the bias and plate voltage of the existing JJ E 34 L power tubes and here is how it worked out:

    Tube 1: bias 33ma plate voltage: 447v

    Tube 2: bias 37ma plate voltage: 447v


    after installing and checking the new winged c SED EL34s:


    Tube 1: bias 34ma plate voltage: 451v

    Tube 2: bias 40ma plate voltage: 451v


    sounds like the winged c SEDs have a little more headroom and thickness.
     
  2. big mike

    big mike Administrator Staff Member

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    Don't seem to be very well matched.
    Did you rebias?
     
  3. CADdrafter

    CADdrafter Senior Member

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    still tinkering with it this evening
     
  4. big mike

    big mike Administrator Staff Member

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    Honestly if I was getting more than 3ma apart in a 'matched pair' from a dealer, I'd send them back.

    Make surer it's the tubes and not the amp though.
     
  5. Blue Strat

    Blue Strat Member

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    Notice the pattern of the first tube being lower?

    If it's the amp, switching the tubes into the opposite socket will get them more closely matched.
     
  6. waggclan

    waggclan Par 4 Holer Gold Supporting Member

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    But did you rebias the new tubes or just check the readings without rebiasing. From what you wrote it sounds like you put the new tubes in and then took readings based on the bias of the JJ's. You definitely need to rebias those SED's.
     
  7. CADdrafter

    CADdrafter Senior Member

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    i did.

    i'll try swapping the SEDs and take another reading to see if that will get them more closely matched.
     
  8. big mike

    big mike Administrator Staff Member

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    Interesting. i've never had that happen. I would swap and see if it followed the tube though.But with the amps I've been in (far less than you!!) never experienced that.

    thanks for the info as always!
     
  9. big mike

    big mike Administrator Staff Member

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    BTW Mike, sorta on topic still...how are you finding the current SED's?

    Still as good as the older ones? I love the Siemens but man even those are getting pricey and harder to get. And my cathode bias amps eat them up.
     
  10. CADdrafter

    CADdrafter Senior Member

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    why would the plate voltage increase when switching to a different brand of tube?
     
  11. Blue Strat

    Blue Strat Member

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    How new? I haven't bought any in over a year and may not. Wholesale prices, and consequently retail prices, seem to exceed market willingness.
     
  12. Blue Strat

    Blue Strat Member

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    It wouldn't. Plate CURRENT can change even if you switch to a different pair of the same brand. That's why we do matching.
     
  13. CADdrafter

    CADdrafter Senior Member

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    gotcha
     
  14. big mike

    big mike Administrator Staff Member

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    I wondered about that. Not too much of a stretch to upgrade to Siemens from there eh? Especially when the new Tungsol and VA are almost 1/2 the SEDs.
     
  15. Blue Strat

    Blue Strat Member

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    Siemens cost about 2X as much as SED's and last 6 times as long, on average. Unless you know some newer math than I do, the Siemens tubes are the way to go. ;)
     
  16. big mike

    big mike Administrator Staff Member

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    Not me man, anything over 10 I gotta take off my shoes.
     
  17. CADdrafter

    CADdrafter Senior Member

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    apparently one tube socket is "hotter" than the other. i tried switching the tubes around and it didn't change the fact that one socket biases higher than the other. i confirmed this buy using two different sets of power tubes.

    what would cause this and is it a concern?
     
  18. Rambergwest

    Rambergwest Member

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    Different values in the circuit feeding the tubes would be the simple answer. Most likely one side has a component/s that has drifted. This is never good but how dangerous is hard to tell. I would want it fixed before something smokes.
     

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