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Continuing saga of plexi clone build with harsh distortion

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Oct 20, 2020.

  1. itsGiusto

    itsGiusto Tele-Holic

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    Hmm, I don't really know. Whatever would have been stock in a mid-90s MIJ strat.
     
  2. itsGiusto

    itsGiusto Tele-Holic

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    @_Steve here's the measurements:

    Mains voltage was 119.9VAC unloaded.

    DC Voltages:
    B+1 413 V
    B+2 411 V
    B+3 304 V
    B+4 263 V
    B+5 248 V

    V1a cathode 1.24 V
    V1b cathode 1.24 V
    V2a cathode 0.94 V
    V2b cathode 149 V
    V3a cathode 33.2 V
    V3b cathode 33.2 V
    V4 cathode 42.7 mv
    V5 cathode 49 mv

    My signal generator (app) was not able to generate a 1V peak-to-peak signal, so I instead ran the output of my phone into my Boss GE-7, which I used as a booster to get it to the 1V signal you asked for. Hope that's okay!

    I added stuff from the "measure" function on my oscilloscope. But adding all of it made it impossible to see the bottom part of the wave, so I just added frequency and peak to peak voltage.

    This is the signal at the very input:
    [​IMG]

    This is the signal after the V1a normal channel coupling cap:
    [​IMG]

    This is the signal after the V1b bright channel coupling cap:
    [​IMG]


    This is the signal at V2a's input grid:
    [​IMG]


    This is the signal after the TMB coupling cap, at the input to the LTPI:
    [​IMG]


    This is the signal after the left PI coupling cap:
    [​IMG]


    This is the signal after the right PI coupling cap:
    [​IMG]


    This is at the speaker output (16 ohm tap)
    [​IMG]

    Why is my speaker out such higher voltage than yours? From your picture, it seems to be on the order of millivolts. Mine is 90 volts!
     
    Last edited: Nov 1, 2020
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  3. _Steve

    _Steve TDPRI Member

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    Thanks for posting these. On the surface it looks fine(!?) but ill go through more closely when i get some time today.

    My output was on the 8 ohm tap which would explain some difference in voltage although that is quite a big one. 90V p-p seems quite high, and mine seems quite low. I'll do the the math later on too. I know mine is biased a bit low too.
     
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  4. elpico

    elpico Tele-Afflicted

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    What load did you have connected to the speaker jack during this test?
     
  5. itsGiusto

    itsGiusto Tele-Holic

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    I had it connected to a Weber Mass 200 attenuator, which was turned all the way down. Then I had a 16 ohm speaker plugged into the attenuator.
     
  6. andrewRneumann

    andrewRneumann Tele-Holic

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    I cover my speaker cabinet with heavy duty moving blankets. Obviously you lose a lot of high-end definition unless you’ve got a mic in there to monitor it, but you can still hear quite a lot. Pillows, couch cushions, blankets, comforters, whatever you’ve got.
     
  7. elpico

    elpico Tele-Afflicted

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    I see. You wouldn't expect the output to be the same as _Steve's unless you're both using the same load.
     
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  8. dan40

    dan40 Friend of Leo's

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    If these are lower output singlecoils, it could explain why you need to turn up the amp so much. Do you have any humbucker loaded guitars that you could try?
     
  9. itsGiusto

    itsGiusto Tele-Holic

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    Sure, I'll try.
     
  10. itsGiusto

    itsGiusto Tele-Holic

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    I tried with earplugs and without the attenuator, and the ghost notes still exist like that as well.

    But, since @dan40 said this is normal for this circuit when the volume is maxed, and given that I've tried almost everything under the sun to fix it, I tried really playing the amp again. I've just been playing it for a few days to see if I like it. And I think it's actually sounding pretty good!

    It's possible that my first iteration of the amp had the ghost notes worse, before I overhauled the whole thing, so maybe I was totally fixated on getting rid of them entirely, which wasn't realistic. The way it sounds now is nice, even at 8 on the volume knob. If I'm not listening to it under a microscope, I don't really notice the ghosting/overtones.

    So, what I'm trying to say is that, at long-last. I'm DONE with building this amp! Thanks all for all the help and support over the past year! I'm on to build another one, maybe a Dumble-style amp, and also maybe make some switchable mods to this one!
     
    Last edited: Nov 4, 2020
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I would still be concerned with the ultrasonic hash displayed on the o-scope.
    Just to ease your mind...
    Every so often put your hand on the transformers to make sure they are not working too hard.
     
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  12. itsGiusto

    itsGiusto Tele-Holic

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    Cool, yes, I am still concerned with the 50,000 hz sine wave appearing in V2, and I am concerned with the fact that I've blown a bunch of fuses. So far, though, with moderate usage (typically running around 8, not 10) that hasn't yet happened to me. If it continues to blow fuses, I think my next plan of attack will be to try to more aggressively quell the ultrasonic oscillation, in the hopes that that was causing the fuses to blow.
     
  13. dan40

    dan40 Friend of Leo's

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    Glad to hear you are enjoying the amp. The builders on the old Metroamp forum always suggested burning the amp in by playing for 10-20hrs. They claimed that the sound would change after this burn in time and the amp would smooth out a bit. I'm not sure if this phenomenom really occurs but it may explain why the amp is starting to sound better after a few hours of playing it.

    I forgot to mention earlier that your amp came out very nice after the rebuild. Hope it gives you many years of trouble free enjoyment!
     
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