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Need help diagnosing a problem

Discussion in 'Amps and Cabs [BG]' started by rboyce, Jun 10, 2009.


  1. rboyce

    rboyce

    Oct 12, 2003
    Boston, MA
    Hey all.

    Looking for some help diagnosing a problem with my bass amp. The amp in question is an old Tube Works RT-3300. It has a 300 watt (@4 ohm) solid state (MOSFET) power section with a selectable preamp circuit (either tube or solid state).

    For the first few minutes after powering up, the amp produces a slightly distorted signal that occasionally breaks up (i.e. it fades in and out). After about 5 minutes or so, the problem goes away and it the amp sounds fine. This problem occurs using either the tube or the solid state preamp circuit.

    Any ideas before I bring it into a shop?

    Thanks in advance,

    Ray
     
  2. I don't, but probably it was caused by the bane of good and bad players alike...vibration. It's probably a crack developed in a solder joint that disappears as the solder warms up and expands. I learned this years ago from a tech, and ever since I started putting a folded towel under my head, or seperating the head and cabs, NO PROBLEMS! That is also why I don't like (trust) combos, even though I have had some nice ones. They all, under gigging conditions, will develop problems related to vibration. Just a question of when. I loved my L-6 combo, and liked my Ampeg, but eventually the cost me money to repair, from vibration fatigue. For what it's worth...
     
  3. rboyce

    rboyce

    Oct 12, 2003
    Boston, MA
    Bump.

    Look for some more ideas/suggestions. Thanks.
     
  4. AxtoOx

    AxtoOx

    Nov 12, 2005
    Duncan, Okla.
    Sounds like the preamp tube is going but not gone, except, you say it does it on SS. Hmmm.
    How old is the amp? Solid state usually goes out right away, or last many years.
     
  5. rboyce

    rboyce

    Oct 12, 2003
    Boston, MA
    Yeah, the problem happens using either preamp circuit. The amp is probably at least 15 years old, but I just had it serviced/checked out about a year ago.

    I like the idea that something is warming up which then causes the problem to go away.

    When/if an output transformer is starting to fail, what would be a symptom(s)?

    -ray
     
  6. Try this: with the amp on and volume up enough to produce sound, pick the amp up an inch or so and drop it. If you get the noise when you do this, you have a loose connection somewhere. You must then find it and re-solder the connection.
     
  7. rboyce

    rboyce

    Oct 12, 2003
    Boston, MA
    Thanks...I will definitely try that.
     
  8. Rick Auricchio

    Rick Auricchio Registered Bass Offender

    It's possible that the tube is still in the circuit no matter how you flip the tube/SS switch. It's possible they use half of a 12AX7 for the "tube" preamp, and the other half is in the circuit at all times, just not pushed as hard. (Could be a cathode-follower to drive the next stage.) Try swapping the tube.

    A transformer shouldn't behave the way you describe. And you don't have one with a FET power section.

    BTW, the friendly folks at GenzBenz took over the TubeWorks line, though they don't support it. They may, however, have schematics available for your tech.

    I still have three of the power amps: one MV982 1RU and two MV962 2RU units.
     
  9. I wouldn't!!!!

    Your amp has a problem. You do not have the skills needed to repair it. Do yourself a favour and tke the amp to a tech before your attempts to "diagnose" it results in an even higher repair cost.

    Paul
     
  10. Rick Auricchio

    Rick Auricchio Registered Bass Offender

    Yeah, skip the drop-test. Though it may confirm a connection problem, it could make things worse.
     
  11. I agree with Paul- if you have no repair skills, this won't do you any good. However I've done it with two different amps that had the sort of problem you describe. And both times this test led me to the problem, which I was able to fix. Good luck.
     
  12. rboyce

    rboyce

    Oct 12, 2003
    Boston, MA
    Rick-

    Good point about the preamp tube possibly still being in the circuit, regardless of whether "SS" or "Tube" is selected. I'll swap out the tube and see if resolves the problem. Thanks for the tip.
     
  13. AxtoOx

    AxtoOx

    Nov 12, 2005
    Duncan, Okla.
    My amp is 80 lbs w/ at least 9 tubes. I drop test it, and I'm doing a tube job.
    I've done worse to diagnose solid state though. I've beat the crap out of the item in question to get an answer.
     

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