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AO29 - unwanted distortion in vibrato channel

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  • AO29 - unwanted distortion in vibrato channel

    Hello everyone,
    ever since I've owned it, my M3 slightly distorts in the vibrato channel. From what I can hear on Youtube, it doesn't seem to be a motorboating scanner. The more notes are played together, the worse it gets. I've recently installed the e-caps kit by TGH, which included a few resistors, but the problem persists. I've also tried swapping the tubes. Is the next step to replace all the caps and resistors in the vibrato channel circuit?
    Thanks in advance for any suggestion!
    Martin
    A100, X77, M3, M100, E100
    Leslie 147, 145, homemade road Leslie
    My youtube channel

  • #2
    Have you replaced V3 - 12AU7?

    Try listening to the vib channel with headphones. The signal at the driver term C will be rather hot. If there is distortion - amp problem. If clean - something in the delay line or scanner. You can also listen with phones at the vib line box taps.

    Jim

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    • #3
      Problem solved!
      V3 was the source of the problem. There was a 12AX7 instead of 12AU7. I put in a 12AU7 and bingo, nice clean sound. Around the same time that you posted this, Jim, I was looking at the schematic and was discovering that the vibrato channel uses this tube. It was the kind of distortion that made me think "yeah, it's distortion, but I'm not sure it's the kind I want." Thanks again.
      A100, X77, M3, M100, E100
      Leslie 147, 145, homemade road Leslie
      My youtube channel

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      • #4
        You recapped the amp without checking the tubes? That's not to say that the amp didn't need recapping at this point, but it's an unusual order of diagnostic priorities.
        I'm David. 'Dave' is someone else's name.

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        • #5
          The main reason I recapped is because the old caps are 60 years old, I would have done it anyways even if I didn't have this distortion problem. I did the same on my A100 and M100 as preventive maintenance even though they sounded fine before. Also, in my previous readings about diagnosing the vibrato/non-vibrato channels, I had mostly seen posts about swapping the 6AU6's to see if the problem follows the tube. I always assumed the tubes were good. In this case, the 12AX7 is actually probably good, but it was the wrong tube for this application. It might have been intuitive for someone with lots of experience with tube amps and electronics, which I don't have.
          A100, X77, M3, M100, E100
          Leslie 147, 145, homemade road Leslie
          My youtube channel

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          • #6
            Because tubes can easily be removed and replaced, in the event of difficulties, it's good to check them to make sure someone hasn't been messing about with them and making inappropriate substitutions. It's not common, but it happens.
            I'm David. 'Dave' is someone else's name.

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