How much EL34 bias mismatch is acceptable?

Discussion in 'Amps and Cabs' started by devbro, Feb 15, 2006.


  1. devbro

    devbro Member

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    I'm attempting my first bias on my EL34 Shiva. I purchased a dual bias tester, $20 DMM and a pair of SED winged "C" EL34's.

    I put the new tubes in, let them get real hot and the closest I can get them adjusting the bias pot is within 5mv of each other. One tube measures 30mv and the other 35mv.

    Is this much mismatch acceptable or should I tell the guy I bought the tubes from they're not a matched set? :confused:
     
  2. poutserman

    poutserman Member

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    Most people agree that a 5% mismatch is acceptable but just barely.Others think that it's ok for the tubes to be as far apart as 10%.I prefer a close match,somewhere in the 2% area.
     
  3. phsyconoodler

    phsyconoodler Member

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    What does it sound like?Within 5ma is considered a good match.If you waste too much time trying to get them right on,you will miss the tone.Sometimes a mis-matched set sounds better than a closely matched set.Bias for tone,not for an exact match.Total waste of time.
    Some boutique builders use dual bias pots just to get the mis-match where it sounds the best.It's often a waste of time to look at what's happening on a scope.Unless the scope is a high end unit that is mega-accurate,it is often a false and mis-leading reading.
    Why do we bias tubes in the first place?Two reasons:eek:ne,to make sure they don't exceed their design limits.Two,to get some good tone out of the amp. Besides,after the amp is played for a few days,go back and check the bias again.Chances are it has changed,even with the most prefectly matched set of tubes.
     
  4. Blue Strat

    Blue Strat Member

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    5mA is no big deal. If the amp isn't humming, and it shouldn't if you're within 10mA, you're ok.

    Matching, while worthwhile, is grossly over rated.
     
  5. joseph

    joseph Member

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    If it sounds good I'd say you're 'done'.

    Also, unless your a tech I wouldn't go any further than the bias tester....there's probably around 450 volts (or more) hovering around those tube sockets.....BBZZZTT...
     
  6. Roccaforte Amps

    Roccaforte Amps Member

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    As tubes wear, they drift. I prefer a closer match to start,
    but to answer your question, 5ma apart is not a problem.
     
  7. hipfan

    hipfan Member

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    Coincidentally, I've been experimenting with intentionally mis-matching the pair of EL34's in my DST U34-CL. It has a dual bias control, so you can get each tube exactly where you want. I've always tried to set the bias for both tubes as close as possible to identical.

    However, after remembering how great an old Traynor I had sounded with horribly mis-matched 6CA7's (one tube was at death's door), I decided to try mis-matching the power tubes in the DST. I moved the two tubes 10 ma apart, and it sounds fantastic. More interesting and lively sounding than when I had the tubes dialed in identically. I'm going to keep playing with the bias settings to see if there are some defined sweet spots I can record and try to duplicate in the future.
     
  8. BozoTone

    BozoTone Member

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    Does this "5 mA" rule of thumb apply to other power tubes likes EL84's for example???...I do notice a bit more "swirl"/harmonic content with lightly mismatched tubes.....
    BT
     
  9. ngtubeamper

    ngtubeamper Member

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    Call Myles at Groove Tubes !
     
  10. sundaypunch

    sundaypunch Member

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    I have also found that a mismatched set (8mA, say) sometimes sounds fantastic. Put me in with the "matching is overrated" crowd.
     
  11. devbro

    devbro Member

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    Thanks for all the feedback. Just so I understand, lower bias (colder) equals lessbreak-up and hotter makes it break-up sooner right?
    I haven't tried it yet but I set them at 28mv & 33mv
     
  12. Blue Strat

    Blue Strat Member

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    I had a customer who found that a 15mA mismatch did it for him. Go figure.;)
     
  13. DAB

    DAB Member

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    All of this bias matching has me wondering.. How come you boutique amp guys are not building all of your multi-power tube amps with individual bias controls?

    With the shortage of matched pairs and quads of NOS and other vintage tubes its seems like a logical step. Is it the extra cost? Seems like a good feature.

    Dave
     
  14. trisonic

    trisonic Member

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    You mean like Maven Peal does (or did)?????
    Ah, there's the rub.


    Best, Pete.
     
  15. fyler

    fyler Member

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    THD Flexi 50
     
  16. hipfan

    hipfan Member

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  17. mmorse

    mmorse Member

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    I thought it was the other way around. :confused:
     
  18. DAB

    DAB Member

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    OK so my idea might not work. I was just thinking that since it is so hard to find old tubes (vintage) that are matched out of the box that you could buy singles and match them up by adjusting bias. If there is a way to do that it would be a great help since there are still a reasonable amount of EL34s, 6l6s, etc., just not very easy to match up a quad for your super lead.

    Dave
     
  19. devbro

    devbro Member

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    Anyone know how tone is affected by raising or lowering the bias. Will 35mv distort earlier than 30mv?
     
  20. rockstah

    rockstah Member

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    if 35ma is called for - then yes it will before 30ma - how much of difference is up to your ears and your amp on 10. my 2 cents
     

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