HELP!..can't bias JMP high enough

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by shooto, Jan 28, 2012.

  1. shooto

    shooto Supporting Member

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    wtf?...so, I'm trying out some JJ 6ca7s instead of the EL34- hook up my AMP-HEAD bias probe- get specs from the EL34s and with a plate voltage of 385, they're biased at 31 & 32...50%...I usually bias at 60%...whatever-

    check out the 6ca7s...plate voltage same but the mA reading is in the 20s...so I turn it up (shooting for 38-40mA...60%) - bias adjustustment tops out at 33 on my multimeter...

    what's going on?...any ideas?

    thanks!!!

    ps- got these from Doug, plate current spec on the box for the 6ca7s is 29, is that the same as watts?...if I do the math with that, they should be biased at 45mA (60%)...is that right?...or, since they are an american EL34, should I just go with the 25w dissipation of EL34s?...in that case, it's the 38-40mA I'm shooting for but can't get-
     
    Last edited: Jan 28, 2012
  2. gldtp99

    gldtp99 Member

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    Plate current spec on box is for supplier matching only--- it doesn't apply to any other spec, only a comparison for matching tubes on their test equipment--- so ignore it unless selecting higher/lower drawing tubes from the same supplier.
    Check the bias probe--- if it has a 1 ohm resistor, make sure it's still in spec---- make sure all contacts are clean and tight.
    If bias probe is ok it sounds like you need to make a resistor change in the bias circuit of your amp to get the bias voltage in range for the output tubes you wish to use--- has the bias circuit been previously modded ? Such as in a 6550 to EL34 conversion ? Sometimes the quality of work done by some people doing output tube conversions in JMP Marshalls (i'm thinking you have a PCB as in most JMP's) leaves a lot to be desired----- they often "cheat" by soldering a resistor in parallel to the existing resistor and call it good----- this is a lazy, half-assed way to do the mod but you see it a lot because you don't have to pull the board to do it this way--- pulling the board and installing proper value resistor is a better way to do it.
    What is the proper value for your amp ? Can't tell because we don't know what model, what wattage your amp is and we have no idea what the values are in the rest of the bias circuit--- or if the bias circuit has been modded, and if so: modded properly/modded poorly .....................gldtp99
     
  3. diagrammatiks

    diagrammatiks Member

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    30-32 should be good on that amp.
     
  4. LPMark

    LPMark Member

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    Isn't that plate voltage Low for that amp?
     
  5. dazco

    dazco Member

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    All you have to do is replace the bias set resistor with something a bit lower. For example, if the most you can get is 32 Ma and you need more, if the resistor 68k replace it with a 56k. If it's a 56k, replace it with a 47k. Just dropping it to the next common value will give you the extra range you need.
     
  6. shooto

    shooto Supporting Member

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    ^ thanks- it is modded for EL34s, I'll have to check if I see a cheap job as you suggested...makes sense- thanks
     
  7. Ronsonic

    Ronsonic Member

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    Couple of things here ....

    What amp is it????? JMP only tells us what era it's from.
    The plate voltage isn't unreasonable for a Marshall 50W amp so we'll guess it is.

    Mods for EL34 are all over the place. Sometimes with a hotttish set of tubes a guy can get them in range without changing anything and .... well .... they don't. Tubes are all over the place a particular set might not bias up in a particular amp.

    The piggybacked resistor is a bit of a cheat, it is also a good thing if you want to change back to 6550, ever. Makes it easy.

    I do recommend ignoring the whole deal of setting bias to a percentage or precalculated value. It is well to check those things, but better to listen and look at a scope and know what you are trying to achieve.

    The classic Marshall sound came from amps biased according to factory instructions which left them a bit cold compared to what you get from modern tube seller recommendations. Nobody cares if it's biased to 60%, only if it sounds good.
     
  8. Alvis

    Alvis Member

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    Actually quite common in mid 70s 50 watters
    I have a pair of Slyvania 5881's in mine ....killer
     
  9. CDA

    CDA Indigenous Donkey

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    Lower voltage amps are warmer...

    Most "Marshall" tones people hear and die for are amps running 70% idle..

    Just my experience. Have been in allot of them.
     
  10. shooto

    shooto Supporting Member

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    ^ '78 JMP 2204 MkII

    I don't have a scope and will probably never get one for biasing-

    I did throw the EL34s in there an brought it up to 38-40 mA and it sounds MUCH better...it was raspy biased cold...sounds much more "together" now...still want to fix the bias resistor so I can check out the 6ca7s though
     

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