Changing Tubes In JCM 900

Discussion in 'Marshall Amps' started by steelhorse, May 4, 2008.

  1. steelhorse

    steelhorse New Member

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    I'm thinking about changing the tubes in the Marshall JCM 900 SLX-100 Model 2100 I recently acquired and wanted to ask the experienced Marshall players for recommendations and suggestions. It's pretty dusty in there and I was going to open the head up and clean it up anyway so I thought while I was in there I might as well do a tube swap.

    Nothing's wrong at all with it, she sounds great imo, but I'm not sure when the last time she had a good going over was.

    I have several EHX 12AX7's virtually new here that I have stored away and a matched quad of JJ EL-34's I took out of my JSX head when I installed TAD 6L6's instead.

    Any pointers would be greatly appreciated.

    I've had tube amps many years but never a Marshall until recently. Not sure if I would need to re-bias and if so, how. In my town the techs are pretty unreliable.

    Thanks!
     
  2. Brett Blackmore

    Brett Blackmore New Member

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    Hi have a JCM 900 100w amp - the preamp tubes should be the normal recommended ECC 83 - these give a 100% gain and no biasing needed.

    EL34s main tubes have a performance tolerance - I have some Russian Army tubes which are supposedly 'matching' tubes yet the tolerance varies a lot. I was told not to remove them and change their sequence. Still, normally you should have them biased to optimise performance, but in most cases you can get away without. So what can go wrong? If you have tubes which are all over with their tolerance and are pushing the limit of the biasing settings then the tubes will not work together and they will perfom badly, soon you will need new ones again - and the sound will suffer.

    If you are happy with the way it sounds and performs then ok, you have saved money! Go buy a case of beer instead!

    Cheers Brett
     
  3. coldsteal2

    coldsteal2 New Member

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    yea you should get the power tubes rebiased,
    but to tell the truth i never do and its fine
    but dont listen to me hahah


    (probably why i used to blow fuses)
     
    Last edited: May 6, 2008
  4. steelhorse

    steelhorse New Member

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    Thanks for the info folks!

    Not sure about how the whole re-bias thing goes as Boogie tubes are already biased and with my JSX head I can get in the back of the head and adjust it myself.

    I'll search the net about info on that and I plan on calling a tech in another town who's reputable, as the one's here are flakey.
     
  5. coldsteal2

    coldsteal2 New Member

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    The biasing happens to the Amp not the tubes
     
  6. steelhorse

    steelhorse New Member

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    I understand that coldsteal but is that something a layperson can do, and if so where can I get info to go about doing it?

    Getting an amp biased if it must be done by a tech isn't the easiest thing to do in this town as I stated above.

    Thanks
     
  7. Adwex

    Adwex New Member

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    I'm not sure.
    Yes, you can do it yourself, but you'll need some equipment.

    Check this site for good info:
    Eurotubes
     
  8. steelhorse

    steelhorse New Member

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    Thanks for the info. When I'm ready to do this I'll probably just haul it to the city and have a tech give it a good going over and install the tubes and set it up properly.

    I really wanted to avoid this, not so much because of the cost but because I'd have to leave the amp behind and do another 3-4 hour drive (1 way) to pick it up, besides I'm not afraid of doing it so long as I have the correct bias settings and know where the trim pots are etc. info which has eluded me thus far
     
  9. Adwex

    Adwex New Member

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    Didya check the video:
    euro-q

    Dude, for the cost of gas, and your time, driving 3-4 hours.....twice....you should equip yourself to do it. You CAN do it.
     
  10. steelhorse

    steelhorse New Member

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    Yes I checked the video and absolutely I can do it, I've done it on other amps before however I have the settings for those amps and know where the adjustments need to be done whereas, with the JCM 900 I don't!

    The video isn't for the JCM 900 but you knew that.

    p.s. those drive times are 1-way and I agree for the cost and time for TWO 6-8 hour trips, I'd MUCH rather do it myself!
     
  11. 601 Blues

    601 Blues New Member

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    on the videos select jcm 800, and it sez it works the same way for 900s as well
     
  12. steelhorse

    steelhorse New Member

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    Thanks for pointing that out.
     

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