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tube /valve Biasing

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  • #1

    tube /valve Biasing

    Hi have a Marshall JCM 2000 DSL ,one of the tubes is a faulty ,the wires inside the prongs at the base of the tube seam to lose contact when at full heat ,i have run some solder on them and run it up to temp and its ok ,but im a bit worried it may let me down.Now the thing is its got them expensive svetlana tubes in it .Question is will it hurt to put a cheaper one in its place until i can afford a new set,and will it need re biasing ???
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  • #2
    It's a good idea to check for the power tubes in fixed bias tube amps to be checked, when fitted then after every few hundred hours of operation.
    Otherwise they may be running anywhere in the spectrum of way cold (poor tone) to unsustainably hot (red plate, fail, blow fuse).
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      what i mean is if i just get the one valve it obviously wont be matched will i find it hard to bias or is all this matched pair thing a bit of a myth ?

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      • #4
        A big problem with randomly selected tubes is that their operating points can be way apart, and the vast majority of amps only offer a shared bias adjustment.
        So one may be running cold at the setting where the other is running as hot as is sensible.
        If buying unmatched tubes, it's a good idea to get a few eg 5, to get a fair chance of identifying a decent pair amongst them.
        Anyway, the point is if you put a random EL34 in there without verifying it's happy, it may pass 10mA and sound rough, 100mA and fail shortly, or anywhere in between.
        My band:- http://www.youtube.com/user/RedwingBand

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        • #5
          It may be that the socket in question has poor tension in the contacts and that can cause heating which I have seen melt the solder in the tube pins in an extreme case which could cause your faulty tube problem.

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          • #6
            Hi folks thanks for the advice ,took it on board and got a matched quad set , i will get the tube sockets checked out when i take it to be biased, Now next question who does amp repairs around north staffs ???? Im also willing to travel about 30 miles cheers

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            • #7
              Consider sending Mick a message http://music-electronics-forum.com/users/19611/
              My band:- http://www.youtube.com/user/RedwingBand

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