Why will one set of tubes let me adjust to a higher bias than another set?

Discussion in 'Amps and Cabs' started by sbpark, Mar 1, 2015.

  1. sbpark

    sbpark Member

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    Had one se tot tubes in the amp, a set of SED Winged-C 6L6's. Biasing a Bassman RI LTD, that has an adjustable bias pot. With the Winged-C's I could get the bias to something like 22mA with the pot max'd out. With the TAD 6L6's I was able to get the bias to 37mA.

    Measured the plate voltage off of pin 3 at 440volts. Used a plate dissipation of 25 watts for the TAD 6L6GC tubes I'm using.

    (25w / 440v) x 0.7 + 0.397, or 39.7mA, but the highest I could get the bias before maxing out the bias pot was 37mA.

    So why can I bias the amp hotter with one set of tubes over another?
     
  2. slider313

    slider313 Silver Supporting Member

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    Each set of tubes will have a different current draw, even of the same brand, unless you buy the same numbered set. The TAD's you have draw more current. You will need to change the resistor on your bias pot for more range.
     
  3. Blue Strat

    Blue Strat Member

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    What slider said. This is why you order matched pairs because, otherwise, you could end up with 2 tubes that are 20mA apart.
     
  4. sbpark

    sbpark Member

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    I'm not talking about two tubes of the same brand. I'm talking about two different matched pairs from two different manufacturers.
     
  5. Blue Strat

    Blue Strat Member

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    The same applies. They COULD match, or they could be as much as 20mA apart....or maybe I don't understand your question.
     
  6. sbpark

    sbpark Member

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    I'm wondering why I could adjust the bias higher on the matched pair of TAD 6L6's compared to the Winged-C 6L6's. With the bias trim pot I could only get the bias to mid-upper 20mA, but with the TAD 6L6's, in the same amp, with the same trim pot, using the same method to calculate where to set the bias, I can get it much higher, to 37mA (can't go any higher because the trim pot is max'd out).
     
  7. RussB

    RussB low rent hobbyist Silver Supporting Member

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    worth repeating because this is the answer you're looking for
     
  8. Blue Strat

    Blue Strat Member

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    It's already been explained and is true regardless of brand.
     
  9. sbpark

    sbpark Member

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    So it's just a crap shoot on if I get a set of tubes that works and sets that bias hotter and sets that don't?
     
  10. Blue Strat

    Blue Strat Member

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    Yes, unless you're buying from the same seller and you order the same "rating number" for the pair, on subsequent buys.
     
  11. J M Fahey

    J M Fahey Member

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    You don't seem to understand the concept of matching. :(

    Tubes have wide manufacturing spread and are all over the map.

    Since in push pull amps you want the same current on both OT primary windings, for various magnetic reasons which affect performance strongly, you want to be able to get tubes which are close by, either as a pair,quad and even sextet .

    So a 20mA tube is matched by another 20mA tube (or within a few mA difference), a 37mA tube is matched by another 37mA tube, and so on.

    It does not mean that a pair matched at 20mA will also be matched to another pair, matched to, say, 37 mA (or any other value).

    Matched by itself does not indicate any specific, "universal" value, just that all tubes within that particular set are close.

    Some manufacturers who try to simplify your life, test and group tubes in categories, as few as 3, as many as 10 , and apply a proprietary indication label; problem is that nobody (who I know of) publishes the full (not even partial) spec of what they exactly mean by each grading.
     
  12. Diablo1

    Diablo1 Member

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    The bias adjustment pot changes the bias voltage - the difference in voltage between the tube grid and the tube cathode. Different tubes will have slightly different internal grid, screen, and plate geometries. So even if you apply the same bias voltage, the bias current will be different because of the different distances of the internal parts.
     
  13. sbpark

    sbpark Member

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    So the next time I'm in the market for tubes for the amp, how do I purchase the correct set of 6L6's that will be able to be biased properly for my amp?! What specs/ratings, etc. do I need to look for? Or is it a matter of just changing the sweep/range of the bias pot?
     
    Last edited: Mar 1, 2015
  14. icr

    icr Member

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    Once you have your amplifier biased to your liking, keep with tubes of similar "grade." These name brand tubes are more expensive but you are paying for the convenience of not having to rewire your bias circuit. Maybe your favorite 'discount' tube supplier will get you close to these known numbers.

    Mesa scale Groove Tubes scale
    Red........................ 4
    Yellow........................ 4
    Green........................ 5
    Gray........................ 5
    Blue........................ 6
    White........................ 6

    The Fender tube scale is
    Blue = GT 1-3 range
    White = GT 4-7 range
    Red = GT 8-10 range
     
  15. Blue Strat

    Blue Strat Member

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    You order the same current rating from the same dealer, regardless of which brand of 6L6s you're buying. The new set will bias properly in your amp IF you have previously biased the amp for a pair of tubes with the same current rating number.
     

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