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Thread: power tube preference in jcm 900 4500

  1. #1
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    power tube preference in jcm 900 4500

    Ok, so I picked up this Marshall head in a trade for a guitar and some work on the owners 6505. Date code on the transformers pegs it at 1996. There is a sticker on the chassis that says "this amp is fitted with 5881 tubes.....bla bla bla" But it has a newish looking set of TAD EL34's, and I looked at the board, all the mods to convert the original el34 circuts over to 6l6 seem to be in place.

    So, my options would seem to be either put a pair of 6l6's in, adjust the bias as needed and flip it from there, or go to the trouble of undoing all the factory mods, and set the bias correctly for el34's. As far as I know and can find info on, the least desirable option seems to be to leave the 6l6 mods in and run el34's anyway, which may be why the guy I got it from was so disappointed in the thing in the first place.

    So, question is, does anybody have an opinion on which is preferable in a 4500, 6l6 or el34, by how much and why?

    And yes, I know jcm900's aren't particularly well thought of, ect ect ect. I didn't choose it for a life companion, i'm just flipping it, so telling me to dump it and get a sl100, jcm800, whatever is not particularly helpful. :-)

  2. #2
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    Quote Originally Posted by jskeen View Post
    Ok, so I picked up this Marshall head in a trade for a guitar and some work on the owners 6505. Date code on the transformers pegs it at 1996. There is a sticker on the chassis that says "this amp is fitted with 5881 tubes.....bla bla bla" But it has a newish looking set of TAD EL34's, and I looked at the board, all the mods to convert the original el34 circuts over to 6l6 seem to be in place.

    So, my options would seem to be either put a pair of 6l6's in, adjust the bias as needed and flip it from there, or go to the trouble of undoing all the factory mods, and set the bias correctly for el34's. As far as I know and can find info on, the least desirable option seems to be to leave the 6l6 mods in and run el34's anyway, which may be why the guy I got it from was so disappointed in the thing in the first place.

    So, question is, does anybody have an opinion on which is preferable in a 4500, 6l6 or el34, by how much and why?

    And yes, I know jcm900's aren't particularly well thought of, ect ect ect. I didn't choose it for a life companion, i'm just flipping it, so telling me to dump it and get a sl100, jcm800, whatever is not particularly helpful. :-)
    Do you like ketchup or mayo to dip your french fries in? What is preferable to one may not be to another. Let your senses dictate your likes. Go against the grain figure out what You like. All we have is time.

    nosaj

  3. #3
    Supporting Member Jazz P Bass's Avatar
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    I would go with the sticker.

  4. #4
    Old Timer J M Fahey's Avatar
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    Measure what bias voltage you have today.
    Very different for 5881/6L6 or EL34, you canīt just plug one instead of the other and rip away.

    Label says 5881; it MIGHT have been adjusted for EL34 or some earlier user might simply have plugged them in doing nothing else, so please recheck.
    Juan Manuel Fahey

  5. #5
    Senior Member
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    if your going to sell it i vote for 6L6GC. those amps eat tubes so you do not want the new owner to come back crying on your doorstep. 6L6GC will probably not blow up like the new EL34's.

    check for grid stoppers on every tube, some 900's just have two instead of four.

    want to make it more bulletproof? install 1K screen R's and 5.6K grid stoppers. maybe up the 100 ohm screen dropper to 300 or 470 20 watt. unless it has a choke.

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