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Does my Marshall MB-150 need a new pre-amp tube, or is it worse?

Discussion in 'Amps [BG]' started by Flatpicker, Feb 16, 2014.

  1. Flatpicker


    Feb 26, 2011
    Likes Received:
    I have played through a Marshall MB-150 for about the last 3 years. I bought this used, so I don't know how old it is. I have loved this amp until now.

    The Classic Channel supposedly uses a 12AX7 pre-amp tube.

    When playing through Classic channel, as I am wont to do, if I turn the gain & volume up to live band-type levels (about 12 Noon on both volume and gain knobs), my signal will infrequently distort or break up (not in a good, fuzzy way, but in a loss-of-signal-sounds-like-thunder-in-the-distance kind of way). At first I thought I blew the speaker, but after further review that is not the problem at all.

    I suspect the tube because the Modern channel, which does not employ the tube, produces no such issues. I can turn it up loud, even with a Sansamp DI cranked up and no problems.

    There are no other signs of bigger issues like bad caps, no scratchy or crackly pots or anything.

    I'm a total amateur, and would appreciate any wisdom on this. Including how & where to change the preamp tube.

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