Diagnosis needed

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by erplander, May 28, 2008.

  1. erplander

    erplander Member

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    I need help figuring out what's wrong with my amp. It's a SF Bassman that I recently had the Allen mods done to. Just got it back last week and played it at rehearsal--no problems and it sounded great.

    Then last night at rehearsal, intermittently, I was getting a static sound, kind of like the ocean. It occurred on both channels, then would go away and later return. Also, when I cranked the either channel past about 3-1/2 I would get lots of loud bassy crackling (almost more like a rumble) whenever I played with hard attack, especially low notes. Sometimes the crackling would continue for awhile after I stopped playing. I was able to play at low volume and it sounded fine, but whenever I tried cranking it just a little, the problems would return.

    I'll try swapping out the tubes later tonight (didn't have a spare set with me last night). Does this sound like tube problems? Could something have jarred loose during shipping? Could it be a speaker problem? (I didn't have a spare cab to compare)
     
  2. jawjatek

    jawjatek Member

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    Probably a tube, or a bad connection (recent mods = suspect solder joints). Good luck.
     
  3. mbratch

    mbratch Member

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    I agree with jawjatek. Check the tubes. Also check the tube sockets. See Blue Strat's FAQ page for cleaning the sockets. And check the solder joints for a bad connection.
     
  4. erplander

    erplander Member

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    Thanks for the help. I was guessing it was problem with the tubes. Will try switching them out and cleaning the pins.

    I contacted the tech who did the mods, and he thought it could be a problem of the two channels being out of phase and said I'd need to keep the volume at 0 on the channel I wasn't using. The Allen mod results in the channels being out of phase, but I thought I'd be able to switch between them. Last night I was using a Lehle Dual, which in theory should be able to handle any phase issues (or so I thought). I'm sure I've read here and other forums of guys using an AB switch with an Allen modded Bassman. Any thoughts?
     
  5. Blue Strat

    Blue Strat Member

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    Could be tubes, tube sockets, dirty tube pins are any one of 150 connections that's intermittent.

    Is the head on the cabinet when you're playing? If so, the vibrations are likely what's aggrivating the intermittent connection.

    Does banging on the head with your hand bring on or kill the problem?
     
  6. erplander

    erplander Member

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    Yes, the amp sits on top of the cabinet, with a towel between them. I didn't try banging on the head but will do that later. My feeling is that will bring on the problem and make it worse. Will check it to see.
     
  7. Blue Strat

    Blue Strat Member

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    Also try wiggling the tubes in their sockets with the amp on. See if that brings the problem on or reduces it.
     
  8. erplander

    erplander Member

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    Update: Last night I tried cleaning the pins and sockets, and that seems to have solved the problem I was having. I played the amp cranked a good deal louder than I usually do for a long while after cleaning the sockets and noticed no sign of the symptoms I was having before. I'll keep an eye on things and have a spare set of tubes handy, but for now all is good. Thanks for the help!
     
  9. drgonzoguitar

    drgonzoguitar Member

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    Thanks for posting an update!
     

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