To Bias, or Not to Bias... Thanks Mesa!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by gnappi, Jan 28, 2006.

  1. gnappi

    gnappi Member

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    I've been around a long time, long enough to have owned original production "Classic" amps from Fender, Marshall, Vox, Ampeg, and Hiwatt. Back then when we didn't know any more than "you replace tubes when the tester at the drug store told you to, or your amp farted". You replaced them with whatever Radio Shack, "Lafayette Radio" had in stock, or what you could dig out of another amp. Today, we have designer caps, matched power tubes, selected pre-amp tubes, wiring fetishists, Blackface / Silverface snobs, and bias mania. It made me wonder... What's really in it all? I went looking for a link that made sense of it and found: http://www.mesaboogie.com/US/Smith/biasadjust.html While you're there (if you go visit), there's a couple of other cool articles. Thanks Mesa for taking the time to research some of these issues! Regards, Gary
     
  2. John Phillips

    John Phillips Member

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    FWIW, I agree with a lot of what Randall Smith says. I know that it isn't very popular round here.

    The bottom line is, I have never rebiased any Mesa amp or any other well-designed non-adjustable amp when fitting it with good tubes at least similar to those it was designed for. (You don't have to use Mesa's tubes if you know what you're doing.)

    It's certainly true that if you want to experiment with different tubes or bias settings, a non-adjustable fixed bias amp can be a pain.

    But I do find it odd/funny/ironic (you choose ;)) that there are a large number of cathode-biased amps which don't come in for the same criticism... and not only are they just as hard to rebias, you can't even be sure that using different tubes will change the operating conditions - they're even more fixed in many ways, and not always where you want them either. These amps usually get praised for being 'self biasing' and not needing to be worried about...
     
  3. Matt H

    Matt H Guest

    john- i hear ya on the cathode biased bias (as it were). i've been grumpling about it in a few posts over in the amps and cabs section...
     
  4. bob-i

    bob-i Member

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    Actually I've found that bias can make a big difference in the amps sound. Randall kinda over simplify's the bias process. You really need to measure more than the bias voltage.

    I'm guilty of the "Lafayette Radio" tester syndrome in the 60's too, but I've learned since then. I remember blowing 4-5 tubes in a row because the bias voltage wasn't getting to the tube socket. Now I make sure I measure the current, just to be sure that everything is healthy. I agree that once the bias is set it seldom needs adjustment.
     
  5. amper

    amper Member

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    I don't think Randall Smith "oversimplifies" the issue. I think he just doesn't get into the esoteric details of why it is that it's probably always better to use "in spec", matched tubes rather than trying to compensate with bias adjustments.

    That said, I've been using Groove Tubes KT66HP's in my F-50 for a long time now, with absolutely *no* ill effects; however, I do use tubes in the range recommended by GT for Mesa amps, and always matched pairs.

    It would be nice if Randall would let us amp geeks play around with things, but he's a fairly opinionated guy, and he builds the amps he wants to build. He does what he does because he feels it the "best" way to do it. There are things about Mesa I love, and things I don't love so much, but they build a good product and give good customer service. If you don't like what Mesa offers, you don't have to buy it. I have a lot of Mesa gear, but I have a lot of other brands, as well.

    I could bias my own amps if I had to. In fact, I think I may just pick up an adjustable bias amp just to play around, but it wouldn't replace my Mesa gear, just add to it.
     

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