Troubleshooting a 12AX7 tube preamp / sensitive to vibration...

Discussion in 'Amps and Cabs [BG]' started by Chad Michael, Mar 15, 2004.

  1. Ok, so a bass player friend of mine gets a used Hartke 2000 head and used Hartke 4.5xl cabinet (yes, I warned him that he is underpowering his cab ;) ). He gigs with the rig, and it sounds like bacon frying.

    He brought it to me for troubleshooting, I am able to recreate the bad sound. The Hartke 2000 head has a tube preamp with gain, and solid state preamp with gain. I have narrowed the problem to the preamp - if I run the SS gain alone (tube gain turned down) the amp doesn't misbehave and crackle. If I run the tube gain alone, the amp does misbehave and crackle. Nowthen, if I place the head on the cabinet (subjecting it to the vibration) it crackles when using the tube pre. If I remove the head from the top of the cabinet and set it on a bench (no longer subjecting it to the cabinet's vibration), the amp no longer crackles when using the tube pre.

    Is it possible for a preamp tube (marked 12AX7A made in Russia) to become sensitive to vibration? I'm planning on picking up a tube this afternoon to try it, but wanted to see if anyone had comments on preamp tubes and the effects of vibration.

  2. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    You bet. I went through two tubes in my Eden preamp in less than three months before I suspected the cabinet vibration, and now I use a one inch thick piece of dense foam between the rack and cab, and haven't had the problem since.
  3. Lockout


    Dec 24, 2002
    Yeah, sounds like the preamp tube is going microphonic. Replacing it should fix the problem.
  4. 12AX7's with shorter plate structures should be less prone to microphonics caused by vibration. Guitar players with combo amps really like the Electro Harmonix 12AX7 for this reason. I've actually had a similar problem with Mullard 12DW7/7247's in my vintage svt preamp, so even over priced boutique pre tubes are not immune from this.
  5. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Hi bimplizkit, here's a couple of thoughts to supplement what the others have said. One is, that often the tube socket itself can become dirty, and the contacts can become "loose" around the pins of the tube. The fix is to "retension" the pins in the socket, you can do this with a dental pick or a small awl (be careful, and be sure to discharge the filter caps in the power supply before doing this). Also, it's probably a good idea to inspect the components and wires connected to the tube socket, checking for charring, loose connections, and cold solder joints. The other thought is, there are "vibration dampers" you can put on preamp (and power) tubes, that will tend to lengthen the time it takes for a marginal tube to become fully microphonic. Basically they're just little plastic sleeves you slip over the tube. If the symptoms are still present with the new tube, you can try one of these options.
  6. Thanks for the tips :) .

    Replaced the tube, worked the gain control a few times, and it seems ok now. I'l recomend that the owner put a pc. of foam between the head and cab.

    The true test is when the amp goes to the next gig. :cool: