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Mesa 400+ Problem

Discussion in 'Amps and Cabs [BG]' started by Padams4, Mar 14, 2008.

  1. Padams4


    Mar 13, 2006
    Just last night during rehersal my 400+ power just dropped out by like 3/4 then went up and back down about 4-5 times. Happened again when I plugged in this morning. Any thoughts? All the tubes look lit up.


  2. amos


    Oct 23, 2003
    SE Portland Oregon
    get the tubes tested...this same sort of thing is going on w/ my tube guitar amp.
  3. K-Funk


    Sep 24, 2007
    Auburn Hills, MI
    could be anything. My 800RB was just doing that a few days ago, and it turned out to be the input jack. and that's a solid-state unit.
  4. mikeswals

    mikeswals Supporting Member

    Nov 18, 2002
    Seattle / Tacoma
    Pay attention to the preamp tubes when this happens. The power tubes may stay lit the whole time, but see if the preamp tubes go dim when the volume cuts out (may need to remove the top lid to watch them clearly).
  5. Padams4


    Mar 13, 2006
    thanks - I'll check tonight.

  6. anderbass


    Dec 20, 2005
    Phoenix. Az.
    Are you using any other pedals, devices or rack gear with this setup?
    (if so eliminate them one by one to positively isolate the problem)

    Did you get the same exact symptoms using both 400+ input channels?
    If so, the more likely defective tube candidates would be one of the two small tubes opposite the input jacks. (the inverter and driver tubes)
    If only channel one cuts out, you could logically suspect the preamp tube closest to the input jacks.
    If only channel two cuts out, be thinking the next one over.

    If your using an active bass, make sure its battery isn't nearly dead.
    Also make sure your speaker cable isn't actually a melted guitar cord. (unscrew the ends and look)

    Something easy you could try next time it acts up is insert a standard guitar cord between your effects send and return jacks. If this immediately corrects the malfunction, your receive jacks switching contacts may just need cleaning by repeatedly inserting a cord, or with the preferable addition of a short spray of electrical cleaner first. (with the amp unplugged)

    Also make sure your effects blend knob is set to zero unless your actually using the effects loop.

    Good luck.
  7. Visible tube "glow" has NOTHING to do with their operation. It is only an indicator of heater filament visibility.
  8. Padams4


    Mar 13, 2006
    Did not have a chance to pull the cover yet but still having the power drop, no effects and both channels. I can actually hear the amp hum quiet down, it is as if it is just powering down by a big margin, then it comes up again.

    Pre tube per a earlier reply??

  9. I've had similar problems like this myself. Sometimes it's tube related other times it was simply a matter of plugging in to a different wall jack. Crappy wiring can screw up everything. Do you use a power conditioner? They help.

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