Matched quads - how far out should they be?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Randy, Feb 12, 2008.


  1. Randy

    Randy Member

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    I just bought a 'matched' quad of winged =C= 6L6GC's. When biasing according to the Weber tables, average is 38ma, while hot is 44ma for my 475 volt amp.

    With this quad, I can set 2 of the tubes at 40, while 1 reads 36 and the other 44. Will this amount of variation be harmful to my amp? Harmful to my tone?

    I tried switching heads around on my bias rite with no significant difference.

    Also, right now I have the outer pair with the big discrepancy. Will I get any benefit from swapping an inner and outer tube, making the 2 working pairs more closely matched?
     
  2. Blue Strat

    Blue Strat Member

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    Bias Calculators not withstanding, I'd set them no higher than 40mA. 60% is good, 70% is a waste of good tubes.

    Go for the best balance between the left and right pair of sockets. That's what matters most. Something like: 40+40 = 80 on the left and 36 +44 =80 on the right.

    Anything within 10mA is usually fine and won't harm anything.
     
  3. Randy

    Randy Member

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    Excellent - thanks Mike!
     
  4. jspax7

    jspax7 Member

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    Did the place where you bought them say they would be matched within 5mA, 3mA, or simply "matched?" Also, do they burn them in? Some places do, as it's supposed to weed out tubes that drift out of balance.

    Curious to know where you got them.
     
  5. Randy

    Randy Member

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    I bought them from a forumite here with supposedly only a few hours use on them.

    However they were originally purchased from the Tubestore. The card that comes with them says they are matched within 5% of measured plate current, and all 4 tubes are labeled 30 (is that milliamps?).
     
  6. Guinness Lad

    Guinness Lad Silver Supporting Member

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    If it starts to hum most likely the bias is too far off. When I owned a Guytron GT-20 it had a bias pot for each power tube, the farther one tube drifted from the other the louder the hum got. When they read exactly the same or within 1ma there was no hum at all. It would be cool if all manufacturers had this system, the only other one I know of was the Tone King Meteor.
     
  7. Blue Strat

    Blue Strat Member

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    They must use a different matching "method" north of the border:D
     
  8. booj

    booj Member

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    Try them in the balanced mode, or approximately equal current draw for both sides and then try them in a mismatched mode on both sides. You might be surprised at the results. I've done that on a couple of repair jobs where the customer supplied the tubes (mismatched) and I found the best sound to be mismatched. Don't take that as advice. Just experience. There are more factors to a good sound than strict matching.
     

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