Using Mesa Boogie tubes in a Marshall: need to rebias?

Discussion in 'Amps and Cabs' started by TubeStack, Feb 19, 2015.

  1. TubeStack

    TubeStack Supporting Member

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    My Marshall JCM800 4010 came with Mesa Boogie power tubes already installed. I've read that Mesas amps don't need rebiasing and their (relabelled) tubes are kept within a tight range.

    Does this mean that if my Marshall was biased with Mesa tubes, I can buy Mesa tubes and not need to rebias?

    Thanks.
     
  2. SteveO

    SteveO Member

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    Assuming it was biased for the Mesa tubes when they were installed, things "should" be fine. However, it would probably not be a bad idea to have the bias checked anyway, just to be sure. You never know what happened to that amp before it became yours.
     
  3. TubeStack

    TubeStack Supporting Member

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    Thanks.

    I did buy it from a store, and was told it had recently been retubed and biased, but who knows. It sounds incredible and seems like the bias is set correctly.
     
  4. TubeStack

    TubeStack Supporting Member

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    So, I bought a new pair of Mesa tubes, but they're green coded and the ones I'm replacing are grey. From what I'm reading, this doesn't matter?
     
  5. Primakurtz

    Primakurtz Member

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    Mesa's color codes don't really mean much, to my knowledge. They just identify different batches.
     
  6. The Funk

    The Funk Member

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    The fact that mesa amps don't need biasing is due to the design of the amp rather than the tubes. If the amp you have has mesa tubes (which are rebranded tubes from other companies, often JJ's) and the bias is fine with them, that it doesn't matter. You should check the bias just to make sure the amp is running well though if you just got it.
     
  7. wyatt

    wyatt Member

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    They are similar to GT's "hardness rating", the color indicates how cold the tube should bias relative to other Mesa coded tubes.

    From lowest dissipation to highest dissipation... RED < YELLOW < GREEN < GRAY < BLUE < WHITE

    Notice I didn't say "hot"...in a stock Mesa, the "hottest" color WHITE, biases up at ~50-60% dissipation...the rest just get colder from there. Since 60% is what many people shoot for when biasing their amps, many won't buy anything but WHITE and BLUE for Mesa's, because the rest run really cold.

    BUT... an adjustable fixed bias amp (like the OP's Marshall), this is all moot, because an amp tech can bias any color code to dissipate 60% (or whatever is desired). For the OP, the GREEN may run colder than the GRAY, the color coding tolerances are pretty wide, so YMMV.

    There is nothing to the design of that amp except the per-determined bias resistor Mesa uses in the amp instead of a pot. The tubes are all loosely coded to how well they'll bias up.

    A replacement GREEN should bias up roughly the same in ANY amp that has been previously biased for GREEN tubes. Probably better than it'll run in a Mesa since a GREEN will bias pretty cold (<50% dissipation?) in stock Mesa amps.
     
    Last edited: Feb 20, 2015
  8. The Funk

    The Funk Member

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    Right. But what I was pointing out is that you don't automatically have to rebias your marshall because the tubes say Mesa on them, and that the fact that mesas don't have adjustable bias isn't due to the tubes, its due to the design of the amp. I know people like to put in bias pots sometimes in mesa's.
     
  9. wyatt

    wyatt Member

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    Gotcha, I think I misread your earlier post.

    I agree the OP should probably have his bias checked, at his convenience, not just because of the new tubes but because he has no idea how well the store set it up.

    But, assuming the amp was biased for the Mesa GRAY tubes, it should definitely be in the ball park for the Mesa GREEN tubes he just bought. As a matter of fact, they should run better (IMHO) in that amp than in a real Mesa.

    Now, I have to ask the OP...why the new tubes if the amp "sounds incredible?" If nothing has gone wrong you had plenty of time to shop around for tubes...even if just for more Mesa GRAYs.
     
  10. TubeStack

    TubeStack Supporting Member

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    Thanks again, everybody.

    The amp was blowing fuses at practice last week and I had an upcoming gig, that was tonight. (I have another thread about this, seems to have been because of a different power cord and/or extension cord, weirdly.)

    Worked fine tonight with same tubes (not the new ones), though.
     

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