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Matching Power Tubes Question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by DAB, Jul 25, 2006.

  1. DAB

    DAB Member

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    Is the idea that when you set the bias you want it to be the same for both tubes and to do that they need to be closely matched?

    If so, can't you set up where each tube has its own bias adjustment thus eliminating the need for matched sets?

    Seems to be simple so there must be a reason why this won't work else everyone would be selling boutique amps with individual bias adjustment.

    I just saw a matched quad of old Mullard XF1 EL34s go for $1,600+ dollars on eBay. Individual Mullard EL34 can be had for as little as $30 used. So this seems like a big deal to me.

    Thanks in advance......

    Dave
     
  2. slider313

    slider313 Silver Supporting Member

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    Some high end tube stereo's have individual bias adjustments to fine tune each tube,so I would guess it can be done with guitar amps also. Perhaps it would be an option if you asked some of the boutique builders.
     
  3. hasserl

    hasserl Member

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    Sure it's doable on guitar amps. Kevin Oconor does this with the amps he sells at London Power.
     
  4. Hacksaw

    Hacksaw Time Warped Gold Supporting Member

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    I like the effects of running two different matched sets in my El34, 4 tube amp. one set early beakup and one late. gets the overtones of the breakup with the defined clean mix. I agree, just like different speakers. I like not being in the " box" with the rule of 100% matched.. be creative! try things! and see what happens!

    So how about loading a amp with ( may have been mentioned before) a pair EL34's and a pair 6L6 family tubes? individual bias adjustments called for here? or just 2 bias pots?
     
  5. Blue Strat

    Blue Strat Member

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    I'd love to agree with you on this one, but it just ain't so. I see idle currents of 20 to 40 mA in some power tubes from the same manufacturer and of the same type (6L6s, EL34s, etc). But again, this is only my limited experience in dealing with over 80,000 tubes.
     
  6. PaulC

    PaulC Member

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    IMHO it's always best to start off with tubes that are matched as close as you can get them. There's alot more to that then just getting your idle current balanced to reduce hum.

    Say you had one tube that took -37vdc to give you a bias current of 35ma, and another tube that took -49vdc to give you the same current. Having two bias pots to get them balanced for current is cute, but those tubes are anything but balanced for other specs. The -37vdc will clamp way before the other tube. You might find that you're driving the snot out of one tube while not working the other much at all.

    If you like the way it sounds then more watts to you, but I do feel it's best to have a set thats matched for the most part. If I wanted to get some asym sounding stuff going on I'd do it with the driver myself.

    PaulC
     
  7. DAB

    DAB Member

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    So when someone (anyone) says "matched pair" or "matched quad" then the odds are they really have no idea whether they are truly matched or not? Or do most people run all of these tests?

    Dave
     
  8. Blue Strat

    Blue Strat Member

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    On Ebay, many sellers claim that tubes with the same brand and part number are matched.

    Some dealers match for idle current and transconductance. I've never heard of anyone matching for plate resistance. I defy any listener to "hear" unmatched plate resistance in multiple blind tests. I also defy a listener to hear the difference between tubes that are, say, 6mA out as compared with 1 mA out.
     
  9. rockon1

    rockon1 Member

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    Ive(I know everyone will say Ive been lucky or something) been swapping out tubes unmatched for over 25 years with zero failures due to it.These days they come (supposedly) balanced so its not as much an issue but I have never worried about it.
     
  10. Blue Strat

    Blue Strat Member

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    Does this mean that you don't set the bias either?
     
  11. rockon1

    rockon1 Member

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    Guilty.Im not much of a boutique guy. FWIW-Mostly been using Peaveys for the last 15 + years.They claim its a plug and play affair. I guess its true cause I havent had problems and they sound good to me.Ive swapped tubes in my Fender HRD and Crate VC's too- no problems. My JSX has adjustable bis and I cant hear a lot of difference from cold to hot-just an increase in volume when hot. Yeah, I know Ive been lucky.
     
  12. Blue Strat

    Blue Strat Member

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    Worst case, there's approximately a 1 in 6 chance that the tubes will run SO hot that any damage will occur. There's about a 1 in 6 chance that they'll run so cold that they'll sound horrible. So, the 4 in 6 chances that you're in the "OK", if not "optimal" region are pretty good.

    Those mysterious cases "back in the day" when things didn't go so well with "plug and play" may have been attributable to those 2 in 6 cases.
     
  13. Blue Strat

    Blue Strat Member

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    Maybe this is what Groove Tubes uses in their dynamic matching process.
     
  14. rockon1

    rockon1 Member

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    Like I said,probably lucky. I plug em in and if they sound good and dont red plate Im happy.
     
  15. donnyjaguar

    donnyjaguar Member

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    Well, if the tubes are unmatched you'll get more even-order (aka musical) distortion products. You won't net maximum power though. FWIW, I run with matched sets or damn close to it. I don't reckon I'm getting my money's worth if I can't get the volume.
     
  16. Blue Strat

    Blue Strat Member

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    Just a note that tubes don't "red plate" until you hit about 200% of maximum power disipation. If anyone is wondering, you're not supposed to operate tubes beyond 70% of max disipation.
     
  17. Jammin'John

    Jammin'John Member

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    I don't like my tubes to be perfectly matched.
    I like 'em off a little.
    They seem to "crunch" better & have a little "hair" on the notes !
    Nuthin' wrong with a little "hair".
    I play tweeds,YMMV !

    JJ
     

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