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SVT preamp distortion on pick attack

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  • SVT preamp distortion on pick attack

    I have an 80s Japanese SVT on the bench that is giving me the runaround. There is a nasty crackling distortion that occurs most reliably when a loud open E is struck with a pick and appears to be originating from the last stage of the preamp (6C4 tube). Sounds like a biasing problem to me but the voltages all check out, aside from the first stage of V3 which reads about 10% low on the plate, however the signal is still OK after that stage. I've tried subbing tubes, changing coupling caps, changing all the plate resistors in the preamp, testing all the coupling caps for DC leakage and checked the connection on the umbilical cable, the amp was re-capped in the last five years, and the power amp is functioning perfectly when driven by an external preamp. I am clearly missing something, anyone encounter a problem like this, SVT or otherwise?

  • #2
    SO is one stage clipping? What does the distortion look like?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Originally posted by Enzo View Post
      SO is one stage clipping? What does the distortion look like?
      It appears to just be the 6C4 that is distorting, but I wouldn't describe it as clipping because it is asymmetrical, it only does it with a percussive input from a bass, can't get it to reliably appear with a sine wave input. Looks like a squiggly distorted section on the top half of the waveform, but it just appears for a second or two when each note is struck, otherwise the amp functions as normal. Sorry, not the best description here.
      Last edited by emulator; 11-12-2019, 03:34 PM. Reason: Clarity

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      • #4
        This is happening with a dummy load or with a speaker?
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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        • #5
          Clipping can be assymmetrical, all it means is the signal exceeds some limit of the circuitry. Often it is easier to clip positive than negative or v.v.

          Now your description sounds more like parasitic oscillation though.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Originally posted by g1 View Post
            This is happening with a dummy load or with a speaker?
            Either/Both

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            • #7
              Originally posted by Enzo View Post
              Clipping can be assymmetrical, all it means is the signal exceeds some limit of the circuitry. Often it is easier to clip positive than negative or v.v.

              Now your description sounds more like parasitic oscillation though.
              The thought has crossed my mind, any suggestions for that?

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              • #8
                Spent some more time on the amp and reading what I can find, as Enzo suggests, it seems more like parasitic oscillation causing the distortion sound. On a scope it resembles what I've seen described as 'boxcars' on the positive peak of the wave at the output of the 6C4. Given that the power amp functions flawlessly with an external preamp, I'm wondering if there might be some interaction between the long coax cable from the preamp and the phase inverter input. I'm considering increasing the value of R51 from 1K to maybe 10K as a grid snubber to see if it makes any difference. I've also seen suggestions on other threads to increase the value of the cap between the plates of the PI. This is a touring amp that I've worked on a number of times over the years, but I hadn't seen it for quite a while. Since the last time I sent it off working another tech has done a lot of work to attempt to suppress noise and oscillation, foil tape on the bottom of the chassis, additional ground wires to each input jack grounded to the preamp chassis, filter cap job, additional terminal strips to support caps and totem poles, etc. I don't remember it being a fussy SVT in the past, just perplexed as to what could have changed with it. Any suggestions are appreciated!

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