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Changing the tubes of a Marshall JCM900 (EL34)

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  • #16
    Re: Changing the tubes of a Marshall JCM900 (EL34)

    Originally posted by Gtrjunior View Post
    As far as the bias question, you’ll get different answers from different people but I ALWAYS bias a new set of tubes. It’s just good business.
    ALWAYS...
    To be clear, I always bias mine with new tubes too. Just letting him know it's not an absolute requirement if you use a rated tube system like Groove Tubes.
    “For me, when everything goes wrong – that’s when adventure starts.” Yvonne Chouinard

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    • #17
      Re: Changing the tubes of a Marshall JCM900 (EL34)

      Originally posted by NegativeEase View Post
      To be clear, I always bias mine with new tubes too. Just letting him know it's not an absolute requirement if you use a rated tube system like Groove Tubes.
      Understood.
      I just like the peace of mind in knowing I have the correct voltage flowing.
      I buy tubes from tube depot and they use a rating system to match tubes. I always ask for tubes rated the same or close to what I am taking out of the amp. 9/10 a minor adjustment is all that’s needed.

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      • #18
        Re: Changing the tubes of a Marshall JCM900 (EL34)

        Tubes are very easy to change yourself.

        Be sure to get a balanced set of power tubes. I love Mullard EL34 tubes. For the preamp tubes, the V1 tube is the most important. Spend more money on that one tube. If you want most gain go for 12AX7/ECC83. If you want less the 12AT7 is an option. Biasing tubes for your amp should be done by a pro.

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        • #19
          Re: Changing the tubes of a Marshall JCM900 (EL34)

          I ordered one Tung-Sol 12AX7/ECC83 and one TAD JJ ECC83S/12AX7 to experiment V1 swaps. Thanks everyone !

          I have a last dummy question : What is the main risk for not biased new power tubes (two balanced sets of EL34) : It will sound bad ? It will wear faster ? Or both ?

          Someone mentioned a adding clipping on the Lead channel of the JCM900 perform by a solid state circuit. Do get this only if you crank the gain past a certain value (like 12 for example) ? Or always ? (I mean with guitar vol. not rolled down)
          Smartphone Zombies won't shred

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          • #20
            Re: Changing the tubes of a Marshall JCM900 (EL34)

            Originally posted by Francois View Post
            I have a last dummy question : What is the main risk for not biased new power tubes (two balanced sets of EL34) : It will sound bad ? It will wear faster ? Or both ?
            Both. In the worst case scenario the new tubes could red plate.

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            • #21
              Re: Changing the tubes of a Marshall JCM900 (EL34)

              Originally posted by Gtrjunior View Post
              Tubes are easy to change but you absolutely need to bias them once installed (power tubes).
              That means you’ll need to take measurements of your plate voltage with either a multi-meter or buy a bias probe.
              Then there is an easy math formula to compute which will tell you what your bias setting should be.
              I’d you have never done this before I highly recommend that you take it to a qualified tech.... there are very dangerous voltages present in an amplifier even if it is not turned on....you absolutely can get hurt or killed by these voltages. A Marshall will carry about 500V coming off the plates....get bit by that and you’re gonna have a bad day.

              Preamp tubes can simply be plugged in and that’s all you need to do.



              Preamp tubes usually last a very long time....decades sometimes.
              Power tubes last much shorter. With heavy use maybe as little as a year or 2....or they may last longer. There is no set rule.

              The amplifier uses some tubes for the clean channel stages and some for gain stages.

              If you were happy with the JJ tubes, no reason to switch. They are well made, sound good and are reasonably priced.
              The Marshall 900 has a SS rectifier if I am not mistaken
              "So you will never have to listen to Surf music again" James Marshall Hendrix
              "When the power of love overcomes the love of power, then the world will know peace."-Jimi Hendrix

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