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Tube Troubleshooting (help!)

Discussion in 'Amps and Cabs [BG]' started by chaosMK, Aug 20, 2005.

  1. chaosMK


    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp
    I had a show a few days ago and plugged my rig back in (Mesa Bass 400, pair of 1x15's) today to practice. I noticed that at higher (stage/rehearsal level) volumes, there is some distortion/baffle on the lower notes, like a crappy unwanted overdrive. The higher notes come out clear and sound great. I've tried all manner of lowering my bass volume, lowering the volumes on my pedals, testing out high/low channel/master volumes and it seems like the problem comes at a certain volume.

    I switched cabs, switched speaker cables. Tested my bass on another amp to see if my battery was dying

    I thought that my speakers might have been blown from the show because I had to drive them pretty hard to fill that venue, and that might be part of the problem. But on inspection they both looked fine and dont make a flapping noise or anything.

    Any thoughts? What does it mean when a tube goes "microphonic" ?

    I dont have a super fancy shock mounted rack for my head and am just using a reinforced regular one. I am ultra careful trasnporting it, but maybe it suffered a tiny impact or two.
  2. Rumple


    May 12, 2005
    I just played a show last night with my mesa 400+ and had it cranked because I had no monitor. I noticed that on the E string if I was attacking to hard I would get a simular problem. The only cure for me was not attacking to hard.... on the heavy songs it sounded cool :) . But definatly not something that I always want. I figured Its just preamp clipping from such a huge signal coming from the pickups, but I'm not that much of a amp pro so...... yeah. If this is happening on light attack to I would mabey check your eq and make sure your not over boosted. mabey roll back the bass a little. Sometimes when I eq I will try to stand far back from my rig so I can hear how it sounds when the sound really opens up. Anyway probly not much help Good luck. :bassist:
  3. as dar as I know tubes dont clip. if I am wrong someone please tell me. but I think that is why you can crank a tube amp as compared to a SS amp it that their is no cliping only an overdriving of the tubes.
  4. popinfresh


    Dec 23, 2004
    Melbourne, Aus
    The overdriving is clipping, just, different.

    How old is the amp? How long since it was last re-tubed? What were the master and channel gains at? Roll back a little bass on the EQ and see how you find that.
  5. Your amp needs some tech attention. It could be a bad output tube(s) or a cold solder joint or any number of a few hundred other things. The only thing for certain is that when your amp is asked to put out lots of current, it doesn't like it. If your amp is under warranty, find a GOOD Mesa warranty repair tech (I've never met one, BTW) or send it back to them. If not, find someone who know tubes and let them work on it.

    It means that vibration causes the tube to emit voltage- that's bad, BUT all tubes (and transistors, too) are microphonic to some extent. I doubt that is your problem.

    When the amp isn't running (tubes not hot), it doesn't matter.
  6. chaosMK


    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp
    If I play real soft, the problem is minimized somewhat but I think regardless of volume (well, if master/gain are in the 5-7 range), I dont get clear/profound low notes below G on the E string.

    It'll be a mission to find a tech in these boondocks. :bawl:

    This head was made in 1982 (almost as old as I am!) and according to the previous owner, has the original tubes still in it. From what I gather it was used very little and it sat in basement for most this time.

    Thanks for the thoughts, I am real new to tubes and amp maintenance and this stuff is putting me on the right track.