Continuing saga of plexi clone build with harsh distortion

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

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    We are working with signal voltage just like in a pedal. 1/4W is fine.
     
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  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    These very low values are for power tube screen grid stoppers when used in ultralinear mode.

    He also uses 'only' 10k ones to limit noise at the input, but only with some additional capacitance to bring the cut off frequency to roughly 20kHz. A very common value is 34k (2x 68k in parallel), giving 25kHz. You could go up, but then you enter the audible range. You might do that intentionally as llc suggests.
     
  3. Mad Kiwi

    Mad Kiwi Friend of Leo's

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    EVERYONE in this thread knows a million times more than I BUT...sometimes the easy things get forgotten...

    Are you 100% sure you don't have a speaker and/or Cab problem...?
     
  4. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Double post
     
  5. itsGiusto

    itsGiusto Tele-Holic

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    He says this:
    So he is referring to preamp tubes as well.

    Also as mentioned above, I do have 68k/470k mix resistors, which I think are operating as grid stoppers to some degree (the two 68k in parallel make 34k, like he says), but they're not directly soldered to the pin of the tube.
     
  6. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Yes, he indeed talks about preamp tubes. I didn't mean to say he wasn't, but my post was not very clear.

    Here is a graph of the cut off frequency as a function of the grid stopper resistance:
    cof.png

    Grid stopper of around 600 Ohm, limits high frequency signals (also unintentional signals as PO) above 1MHz.
    You need something like 10k to start to influence 50kHz.
    The 68k/470k are grid stoppers. However, grid stoppers only act on signal that are present before them. Any signals that are coupling in afterwards are not affected. That is why they should be at the pins. Shielded cables limit the coupling in of signals.

    Every amp has the 68k on the inputs. If you look at my jcm800, you'll see I've put a 10k ( + extra capacitor) right at the pins and fed the signal through a shielded wire.

    Edit: inserted for convenience:
    Screenshot_20201022-184757.png

    And here are my JTM45 mixing resistors:
    Screenshot_20201022-183359.png

    Edit: and also here you can see the input grid stopper at the pins: the large square white wire wound resistor, fed by a shielded wire.
     
    Last edited: Oct 22, 2020
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  7. itsGiusto

    itsGiusto Tele-Holic

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    Cool, yeah, I thought about floating my 68k mix resistors on the pins, but wasn't sure about it.
     
  8. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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  9. itsGiusto

    itsGiusto Tele-Holic

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    Oh, well, I wasn't concerned about the weight. Just the "neatness" of having two resistors with floating wires attached to them, that somehow it'll cause me to do something like make a cold solder connection from wire to resistor, or whatever. And my board is already complete with those resistors on it, etc.
     
    Last edited: Oct 22, 2020
  10. _Steve

    _Steve TDPRI Member

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    @itsGiusto , I have pretty much the identical amp using mostly Valvestorm parts, and with MUCH worse lead dressing than yours. Would you like me to scope through the amp so you can compare?
     
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  11. itsGiusto

    itsGiusto Tele-Holic

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    Yes, that would be totally awesome!
     
  12. Nickfl

    Nickfl Friend of Leo's

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    As already discussed, they would be more effective directly on the socket, but I think more relevant potentially to your situation is that one of the 470k mixing resistors has a cap bypassing it, which means that you effectively don't have a grid stopper at whatever frequencies that cap is passing on that channel. Unless I've missed something in the discussion it seemed like that was the point in your circuit where you're picking up the onset of trouble so it makes me wonder if perhaps a grid stopper in that position would help. If I were you I would just stick a 100K grid stopper directly on that socket, 100K is probably Overkill but it's a large enough value that if it doesn't work you can say with some confidence that a grid stopper in that position is not going to solve your problems.
     
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  13. itsGiusto

    itsGiusto Tele-Holic

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    Cool, that makes sense, I can try that.
    Will doing something like that longer-term effectively kill the effect of the bright cap, though?
     
  14. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Pick 40k, then it is not in the audible range.

    You could also just lift one leg of the capacitor.
     
  15. _Steve

    _Steve TDPRI Member

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    Ok, ill try to do this tomorrow.
     
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  16. _Steve

    _Steve TDPRI Member

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    Here you go.

    Its a 1959 kit from Valvestorm. Everything is 1959 spec except I only have 2 power tubes and the 50W ClassicTone OT. PT is ClassicTone 100W and the entire power supply section is to 1959 spec (not 1987).

    Normal 10, Bright 10, TMB all 5, Presence 0

    Mains voltage was exactly 120VAC unloaded.

    DC Voltages:
    B+1 454V
    B+2 444.5
    B+3 318
    B+4 274
    B+5 256

    ---->82Hz signal at 500mv<----

    Normal Channel Out (just after coupling cap)
    NormalChannelOut.jpg

    Bright Channel Out (just after coupling cap)
    Bright Channel Out.jpg

    V2a Grid (DCCF A-triode)
    V2a_Grid.jpg

    LTPI Input (just after TMB coupling cap)
    LTPInput.jpg

    Power Tube Left Grid
    EL34L_Grid.jpg

    Power Tube Right Grid
    EL34R_Grid.jpg

    Speaker
    Speaker.jpg
     
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  17. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I modelled the preamp (single channel) and (roughly) used steve's settings:

    plexi.png

    I modelled it in ltspice. I can send you the model, if you want.

    I don't have the output tube models...
     
  18. itsGiusto

    itsGiusto Tele-Holic

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    Okay, I got around to trying this out. It's worth noting that my B+ is a lot lower than yours (B+1 is like 410), so consequently some of the oscilloscope outputs are lower as well. But overall, the shapes of the waves look pretty much the same. Sometimes, like in your speaker output pic, there were little transient peaks that mine didn't have.

    Were you jumpering the input? Which one was the primary, and how did you jumper it? And when you say 82hz at 500mv, is that 500mv peak to peak?

    I also got around to trying out larger grid stoppers. I tried 68k resistors for V1a, V1b, and V2a (I couldn't find my 100k resistors at first, but found them later). But unfortunately, they did not seem to make a difference in either the oscillation, or the harsh distortion sound I dislike.

    Should I keep trying to kill this oscillation? Should I increase the grid stopper values? If 68k didn't work, is it worth trying 100k? Or should I try something else?
     
  19. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    You say the scoop images look overall the same, yet in one of the first posts you show weird notches at the speaker.

    I think it is important where the scoop images start to differ. I hope it will limit the area where we have to search for bugs.
     
  20. itsGiusto

    itsGiusto Tele-Holic

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    Well, I was mimicking @_Steve's settings for this. He did an 82hz sine wave, and had his presence at 0, and TMB controls at 5. My first images were an 800hz sine wave with varying other tone stack settings, and the presence was not at 0. When I set it like he did, it looked like his images, no notches. I'd be curious to see if Steve used an 800hz signal, and messed with the treble and presence if he could get it to notch like mine.
     
    Last edited: Oct 27, 2020
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