Pulling my hair out over here...any ideas??!!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by fretless, Jul 29, 2008.

  1. fretless

    fretless Silver Supporting Member

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    Last month I bought a used Ampeg Superjet RI (model SJ-12R). It had a little exterior wear, tarnished control plate, but heck it was under $200, so I figured I'd give it a go. When I demo'd it the tone was thin and the tone controls had little effect on shaping the sound. Figured "probably beat tubes, and I'll probably swap speakers anyways.."

    Now, it had EL-34's (Ampeg eventually went to 6L6's in these). Well, I prefer 6l6's, so I figured I'd swap them with all fresh tubes. Called Eurotubes....they said no problem, but I should bias them. New tubes come in, I set the bias, all appears good. After a few minutes of playing....pops, static, occasional hissing. Present even if there's nothing plugged in. Again, tone controls did very little, if anything to alter the tone. Bass and treble? Fuggitaboutit!

    Dropped it off at the tech. It's a reliable shop, plus their an authorzed Ampeg repair facility. Two weeks later....tech calls..."well, the plate voltage on these really isn't meant for 6L6's". He swears he tried numerous sets of EL-34's and couldn't make the amp make any unusual noises. Only with the 6L6's was noise an issue. For good measure he re-traced the solders on the boards and replaced both preamp tube sockets. In;ut sockets checked ok with the exception of one bad solder, which was touched up. $75 later............

    Took it home, popped in the old 34's...mucho popping, static. Tried the 6L6's...ditto. Now, the preamp tubes were also new JJ's. Tried at least 6 different 12AX7's I have laying around (that all have been fine in other amps)...some more static than others (?). Only a couple of the preamp tubes allowed for any adjustment in the eq, with the exception of the mid control which goes from cut to HONK. I cleaned all of the pots, even swapped 2 different speakers...nada....same ol' scratchy, static filled noise. Oddly, with one of the speakers the noise was much more pronounced if that matters..

    Now I'm at the WTF stage. What's the source of the noise??? Lastly, what's up with the tone stack??? I cannot believe the 2 bands of eq simply have knobs for decoration. There's gotta be something somebody is missing here. Granted, I'm no electronics tech by any means, but I suspect there's another possible cause.

    Any clues? I'd really love some advice..
     
  2. trey85stang

    trey85stang Member

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    im confused, it was working for the tech.. you take it home and swap in another set of tubes and it no longer works? Does it work with the tubes the tech had in it?
     
  3. phsyconoodler

    phsyconoodler Member

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    First off,6L6's and EL34's usually run near the same plate voltages,so what is this tech talking about?
    There is obviously something causing the noises,and it hasn't been located yet.It may require some more investigation.If it is a circuit board amp,the most likely suspects are any large wattage resistors on the board.The board needs to be removed and the resistors should have the old solder removed and new solder put on.There is likely a cold solder joint somewhere.The tone controls should do something,and there may be a connection between those two symptoms.A sharper tech may be the answer here.
     
  4. fretless

    fretless Silver Supporting Member

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    I don't have the tubes the tech used to make it "quiet". He swears it's tubes...I disagree.

    Physyconoodler...thanks for the insight on the resistors...I was thinking along those lines. I'm thinking about looking into another tech as you suggested. The trouble is there's only a couple in my area, which is a shame because I live in a HUGE area of Virginia with more musicians than you can shake a stick at.

    Crazy question....any chance it could be a bad speaker?? I'm only asking because years ago I had a bass amp do something very similar to this and a tech said the speaker was done for. I changed the speaker...end of problem. Worth asking I suppose.
     
  5. slider313

    slider313 Silver Supporting Member

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    It could be the speaker. A cone that's out of alignment with the spider won't produce much bass,it will work because the voice coil is not shorted but it will sound like ****. Try crossing it over to a known good speaker.
     

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