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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    This just in: I've also found that if I do the same thing, (jack in hi-treble channel, volume all the way up) except I turn the presence knob all the way up, and the treble between 8 and 10, I got another sine wave superimposed on it. This seems to be 10,000hz, and I can actually hear it coming through the speaker:
     
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  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Either put in some 470k mixing resistors and get rid of the 68k mixing resistors or, put a 330k to 470k grid stopper on that V2a pin.
    Repeat scope.
     
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  3. itsGiusto

    itsGiusto Tele-Holic

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    Sorry, I know I said 68k mixing resistors, but they're actually 470k, I misspoke. But I can try the grid-stopper.
     
  4. itsGiusto

    itsGiusto Tele-Holic

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  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    http://www.valvewizard.co.uk/Grounding.pdf

    Jcm800
    IMG_20200726_190507866~2.jpg

    You can ignore most of the numbers, but 6 indicates the only ground.

    Here is a layout. It is an early one; it might be slightly different than the actual amp.
    image%3A38890.png
     
    Last edited: Oct 21, 2020
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Pins 1 and 6 are the pins for the plates of the dual triode. I think you mis-typed "the resistor that goes from pin 1 to pin 6".

    The *plate load bypass cap* performs a similar function as the *plate to cathode cap*. They both reduce very high frequency signal. Marshall used both methods. The 800 used the *plate to cathode cap* so I might favor it for this build but you can easily try both.
    Just clip in a cap across the plate resistor for the *plate load bypass cap*.
    Just clip in a cap from pin1 to pin3 for the *plate to cathode cap*.
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Hey, it seems @Nickfl and @Tom Kamphuys are on board with an attempt to change the ground scheme. Maybe between us we can come up with and draw a layout.

    You have the layout in post #2

    If you are up for changing the grounding please post the schematic and a couple of good pictures of the whole chassis from the input side to the filter caps.
     
  8. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I don't know if a single point grounding scheme would solve his issues. I just provided an example as @itsGiusto asked for it and am willing to support him if that's what he wants.
     
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  9. itsGiusto

    itsGiusto Tele-Holic

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    Yeah, I could try putting a cap across pin 1 to pin 3 for sure. But I did mean the resistor that goes from pin 1 to pin 6, I think. Look at the layout I posted at the beginning of this thread. V2a's plate resistor is soldered from the plate for V2b.

    Also, I was wondering if I'm able to see this oscillation after V2a, but not before, where is it coming from? Is it coming from parasitic coupling between the input and output of V2a, or the input and output of V2b? This case seems kind of confusing to me especially because the output of v2a goes directly into the input of v2b, because this tube is in cathode follower configuration.
     
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Ah, I get it. You were looking at V2. I was looking at V1.

    I am thinking of the gain. We are hypothesizing we have an ultrasonic oscillation. V1 provides a lot of gain which is increased even more with V2. If we can keep ultra high frequencies from being amplified early, hopefully that will kill the oscillation later.
     
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  11. Nickfl

    Nickfl Friend of Leo's

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    I'd start putting snubber caps on the v1 and v2 plate resistors. Try 47pf or so on each of those and see if it helps. I'd also see if removing the 500pf bypass cap on the 470k mixing resistor had any effect. Those are easy to tack in place and I think the most likely to kill an oscillation if that's what's happening.

    Try grid stoppers second and grounding changes last.
     
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  12. itsGiusto

    itsGiusto Tele-Holic

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    It's difficult for me to clip in these caps to test this out. I think that the long alligator clips I'm using are actually picking up more noise than the cap is removing. Do you have a suggestion for an alternate way to try clipping these in?

    So far I've tried 100pf across V1b's plate resistor, which didn't seem to have much of an effect. I also tried it across V2a's plate resistor, and from V2a's plate to its cap. Then I tried 4.7nf across V1b's plate resistor which seemed to attenuate, but not completely block out the oscillation.
     
  13. itsGiusto

    itsGiusto Tele-Holic

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    Maybe I've had some success with clipping a 100pf across V1b's plate resistor simultaneously with clipping a 100pf across V2a's plate resistor. But it's hard to tell, for sure, because the oscilloscope signal starts to get a little crazy when the alligator clip leads are on. And the oscillation never seems to fully go away, I'm still seeing the sine wave there at 50,000hz, just attenuated a little bit.
     
  14. Nickfl

    Nickfl Friend of Leo's

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    Just tack them in place with a little bit of solder, the test leads are going to act like antennas.
     
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  15. itsGiusto

    itsGiusto Tele-Holic

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    Okay, I can try that.

    Also, I tried putting in 600ohm grid stopper resistors directly on the pins of V1a, V1b, and V2a. None of them stopped the oscillation, or the bad sounding distortion.
     
    Last edited: Oct 21, 2020
  16. Nickfl

    Nickfl Friend of Leo's

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    Try a higher value for the grid stoppers, 600ohm isn't enough, try 100k or even 470k.
     
  17. itsGiusto

    itsGiusto Tele-Holic

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    Really? I thought that for stopping PO, lower values are better, and that higher values are better for stopping RF from the input jack getting in (for the first amplification stage). Won't putting in an additional 100k resistor, in addition to the 68k/470k mix resistors I already have in each stage, create a large low-pass filter?
     
    Last edited: Oct 22, 2020
  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Low pass filter: 100k with a miller capacitance of 192pF has a cutoff frequency of ~8300Hz.

    Job one right now... You want to stop the oscillation. If you can get it to stop then you can go back and try smaller resistors to give more highs.
    Same with caps. Try a little larger value and adjust once the amp has calmed down.
     
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  19. Nickfl

    Nickfl Friend of Leo's

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    I forgot you already had some sort of grid stopper on all these stages anyway. I'd put a 100k directly on the pin of the socket just for proof of concept and then go from there if it seems like that eliminates your problem.

    I've never seen anyone suggest that a lower value is better for preventing oscillation, but theoretically a lower value could work if that's all you're trying to do because it will have a higher cutoff frequency I suppose.

    I think if grid stoppers are indeed the solution to your problem it's not so much the value but the fact that they need to be directly on the socket to really work well and you have them located on the board with relatively long leads to the actual socket itself.
     
  20. itsGiusto

    itsGiusto Tele-Holic

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    Yeah, that's kinda what I was thinking, I just want to add something on the pin, which is why I used a low value.

    I didn't mean to suggest that lower value is better for preventing oscillation, but that it's better for not having other adverse effects, and a low value is all that's necessary for stopping oscillation. FWIW, here they recommend only using a few hundred ohms for stopping PO:
    http://www.valvewizard.co.uk/gridstopper.html

    Also, I don't have a good supply of different values of 1/2 watt resistors, only 1/4 watt. So I happened to have some 1/2w 620 ohm resistors lying around so I used those. But could 1/4 watt resistors suffice?
     
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