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Broke Preamp Tube?

Discussion in 'Amps and Cabs [BG]' started by iplaybasstexas, Sep 28, 2008.

  1. iplaybasstexas


    Apr 5, 2008
    I turned on my Mesa today and it was really "electricy-synthy" sounding. i de-EQ'd it and turned it off.
    and its really wierd.

    and when i turn the "active-mid" knob, it shrieks.
    same for the gain.

    I have a Mesa BB750.

    Do i have broken tubes?
  2. craig.p


    Sep 28, 2008
    New Hampshire
    Frankly, it could be anything. Was the amp bumped hard or did it fall since you last had it on? Especially while the tubes were hot or had not yet cooled down after the amp was shut off? Are the fans working properly, both speeds? If the tubes are identical, and I think they are (all 12AX7, correct?), you could try swapping them around and see if the problem changes or goes away. Could even be something as simple as a corroded or loose tube socket; check every little socket that each little pin goes into for corrosion or gap too wide. Each pin should get a good tight grip. Could be a tube working its way out of place (unless the amp has tube straps, and I don't know whether this one does). You could try *lightly* tapping each tube with something very light, like a penny or paper clip (go easy!) while the amp is running, but keep the volume down a bit because if a preamp tube is coming apart it won't be shy about letting you know when you ask. Not much help, I know, but at least you have a couple of things you can check now.

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