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El34 winged C's running at 45 ma. Issue?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by kingsxman, Jan 28, 2005.

  1. kingsxman

    kingsxman Silver Supporting Member

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    I just checked the bias on some Winged C EL34's that a tech installed and supposedly biased in my 1987 JCM 800. Both were running at 45 ma current draw and had a plate voltage of 450 v. (I was using a weber bias kit probe and my multimeter). I had previously installed a set of Groove Tube EL34 (6CA7EGE.. or whatever that GE reissue is) in the amp and biased them at about 38 ma. I had the bias pot all the way down to get there..

    I called the tech and said "what up!". He said he usually biases them at 35 ma...so they must have drifted. wha??? 10 ma drift?

    Anyway, a little background that my previous tubes blew up after 3 gigs. I've played 3 gigs now with the Svets and figured I was safe. Is this high reading a cause for concern? Could the amp be slowly going and causing the current draw to increase? I'd be more apt to think that if 1 tube was high...but they both are.

    comments?
     
  2. Greggy

    Greggy Member

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    Given your plate voltage and maximum wattage dissipation for el34s at 25 watts, you should bias within the range of 33-39 ma.
     
  3. kingsxman

    kingsxman Silver Supporting Member

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    Greggy, I'm guessing that my bias pot is adjusted all the way down. I'm going to check today.
     
  4. Greggy

    Greggy Member

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    If that's the case, on some amps you can change out the resistior immediately after the bias pot to a different value to increae the range of the pot (forgot if you have to go up or down in value to increase range). Someone here should know.
     
  5. kingsxman

    kingsxman Silver Supporting Member

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    Turns out I DID have some wiggle room in my adjustments. I put it at 35 ma draw with no load or signal. Think thats a good number?
     
  6. Greggy

    Greggy Member

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    See above.
     

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