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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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    Which particular scope waves looked like blocking distortion to you?
     
  2. D'tar

    D'tar Friend of Leo's

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    Strictly for conversations sake and without a technical explanation.....

    Your PI is not symmetrical as photos show and the more you drive it the more you may notice. This could be the result of the tube itself or either the plate load or tail network resistor imbalance. Don't go out on a goose chase over it. It appears normal to my eyes. I'll be watching hoping you get this sorted out. I applaud your efforts and determination!
     
  3. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    I can't speak to the weird notch, but I'm tuned in to learn more on that. :)

    But I think what you're seeing from the phase inverter is normal. Textbooks usually show full out-of-phase sine waves in their pictures because that's simple, but when an output tube clips at the grid, that clamps the plate voltage in its phase inverter tube for the duration. Pretty sure that's why you're only seeing partial waveforms in your oscilloscope. Someone correct me if I'm off-base there!
     
  4. drew1d

    drew1d Tele-Meister Silver Supporter

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    I wouldn't know. I don't know anything about using a scope. And blocking distortion i'm still learning about. I was just trying to bring levity.
     
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  5. itsGiusto

    itsGiusto Tele-Holic

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    I don't really know what Spice is or how it works, but I assume that it exists for modelling amplifier stages. What is a start-up effect? Does the model work assuming that you have a sine wave as input?
     
  6. _Steve

    _Steve TDPRI Member

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    Hey do try increasing your grid stoppers to 50-100k. Its a technique for taming blocking distortion but also helps with oscillations. Its worth a try if not just to eliminate the possibility. You've tried just about everything else so you never know.

    It gonna be a couple of days before I can scope my amp again sorry. I was playing around with some ideas on the power section and a tube went kaboom boom so I need to diagnose and get to the bottom of that.
     
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  7. mrriggs

    mrriggs Tele-Meister

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    I have the same frustration with the FFT on my USB scope. Usually just fiddle around with ALL the settings until I get something that looks alright. One thing that helps is turning on "averaging" and setting it to maximum number of samples. It'll take several seconds for it to fill in the trace so you need to be using a repetitive signal (function generator).
     
  8. itsGiusto

    itsGiusto Tele-Holic

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    Cool, thanks, no worries!

    I do currently have 70k grid stoppers on V1a, V1b, and V2a. Are you saying I should increase my grid stoppers on my power tubes as well from 5.6k to ~100k? Or should I try adding more grid stoppers into other stages (idk if it's possible to put one between V2a and V2b since it's a cathode follower. Maybe I could put some on some of the PI tube's pins? Not sure which ones)?
     
    Last edited: Oct 28, 2020
  9. _Steve

    _Steve TDPRI Member

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    Yes increase the stoppers on the power tubes. Something between 50-100k. Its a bit of a long shot, but like i said you've tried everything else at this point.

    Definitely dont mess with the resistors on you CF and PI, that'll stop them working.

    Can you point me to a clip of the sound you're trying to fix again? I thought I saw it before but cant find it now.
     
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  10. itsGiusto

    itsGiusto Tele-Holic

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    I don't know if I have good footage of it. Definitely it doesn't seem to come through as well when recorded, but live in person it sounds so obvious.

    But listen to this video from 5:00 to the end, especially that last note I hit at 5:24:

    (Granted, this is from last year, and the amp has changed a lot since then. But the overtones are still present)

    You can hear this weird swirly, spitty sound layered on top of the notes I play, really high in frequency, especially when I hold the notes out.


    I'll also see if I can't get a better recording showcasing the sound in the next few days, and I'll post it here.
     
  11. itsGiusto

    itsGiusto Tele-Holic

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    I got some additional short clips trying to demonstrate the bad distortion sound.

    All clips are with volume on the respective channel at 100%, TMB and prescence on 50%, strat plugged into the hi-input.

    In this the channels are jumpered


    This is with the normal channel


    This is with the bright channel


    Listen for a little annoying modulating swirling buzzy sound, especially when I hold out notes.
    I think I hear it most in the jumpered and normal channel videos, but it's also there in the bright channel (and like I said, in person it's a lot more obvious on all channels).
     
  12. Nickfl

    Nickfl Friend of Leo's

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    This has been going on for so long I can't remember what you have and haven't tried, but what are you bias settings? I wonder if a warmer bias might help...
     
  13. itsGiusto

    itsGiusto Tele-Holic

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    Thanks, but I originally had the bias much warmer (because I formerly thought that the cathode resistors were 1.3ohms, not 1ohm, see https://www.tdpri.com/threads/1-tolerance-metal-film-resistors-measuring-different.1047740/ ).

    So without knowing it, I think I consequently originally had the bias at something like 90%! But it still sounded the same to me.

    Right now, I'm basically right at 70% dissipation for one of the tubes, and the other one is a little less at like 65%. Are there other bias settings you think I should try?

    For some of the other things I've tried, see https://www.tdpri.com/threads/plexi-clone-build-cannot-figure-out-why-harsh-distortion.1044918/
     
  14. Nickfl

    Nickfl Friend of Leo's

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    I doubt it's going to end up being what solves your issue but just for the sake of trying everything if I were you I would try a much colder bias setting, around 50% let's say, just to see if it makes any difference.

    There was a thread recently about a 6g3 experiencing blocking distortion with the tremolo on because of a very large negative bias voltage. That was an extreme situation that was only manifesting because of the bias vary tremolo swinging the bias voltage even more negative when it was on but it's a thought and since you've eliminated all the obvious culprits on this one a long time ago we might as well take a swing at the distant possibilities.
     
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  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    My mind keeps taking me back to the o-scope with the 50k ultra high frequency hash. The grid stoppers and snubber caps should have had an effect if large enough values were used. So far no luck though.

    Listening to the clips again makes me wonder if a filter cap could be allowing a ghost note situation happen. It does have that trailing double note thing going on. Several items were changed when this was rebuilt but some of the filter caps are the same. As Nickfl said swinging at distant possibilities.
     
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  16. itsGiusto

    itsGiusto Tele-Holic

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    One of the filter caps is original, but I have tried switching it out at one point in the past, but the sound remained. I can try switching it out again.

    I could also try putting on higher and higher snubber caps and grid stoppers, too, until I get the oscillation to go away.
     
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  17. dan40

    dan40 Friend of Leo's

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    Are you hearing this noise at a more reasonable volume setting? With both channel volumes on 10, I would expect some unpleasant distortion and maybe some ghosting from this circuit which is perfectly normal. Mine is rip roaring loud by 5 on the volume dial. Are you running it through an attenuator to be able to play it this loudly?
     
  18. itsGiusto

    itsGiusto Tele-Holic

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    Yes, I always run it through an attenuator. I've generally had good luck with running amps through attenuators to get distortion at bedroom volumes, I do it with my AC15HW1X and Marshall 18 watt. Unfortunately, I really have no means of running this without an attenuator. It's so loud I honestly would probably get the police called on me, and permanently damage my relationship with my neighbors, as well as my ears.

    If I run the volume knob-setting lower, there's definitely less ghosting, for sure. But there's also a lot less distortion. Distortion doesn't seem to kick in until maybe 8 on the volume knob. I could certainly run it at 5, and it'd sound MOSTLY okay (there's still some of the bad overtones going on). But then, even at 5, if I kick on a fuzz face in front of it, it's bad, spitty-overtone city.

    If it'd be helpful, I could try to make some recordings at other volume settings.
     
  19. dan40

    dan40 Friend of Leo's

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    That's kinda the nature of these old four hole style circuits. They were meant to be run at arena volume levels to get that great overdriven tone. Unfortunately that is too loud for most of us at home. Running a 5000pf bright cap on the bright channel's volume pot does give the amp a bit more hair but it also gets loud very early on the volume knob. A bypass cap on the v2a cathode will also make the amp a bit gainier at lower volumes. You could try subbing in a pair of 6v6's and running a 16 ohm cab with the amp set on 8 ohm. It will give you slightly less volume but it's still gonna be loud. A ppimv will also help but it does change the amps sound and many folks don't like it.

    It's strange that a fuzz pedal doesn't sound good at lower volumes though. Did you build it to the split cathode lead spec or the shared cathode bass spec?
     
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  20. itsGiusto

    itsGiusto Tele-Holic

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    Cool, is the 6v6 really just a drop in replacement?? I could definitely try that, if so.

    I was also thinking of lowering B+ to get more distortion, a'la the later 70s Marshalls (see https://www.tdpri.com/threads/marsh...riations-get-breakup-at-lower-volume.1041560/). Would that be a good idea?

    It's built to the shared-cathode bass spec. It's basically just a JTM50 with a diode rectifier.


    But I also wanted to clarify what you said to see if I understand correctly:
    Are you saying that you think if I were to actually run it loud, without the attenuator that it'd have the "great overdriven tone" without the ghost notes? Or maybe the ghost notes would be there, but you wouldn't be able to hear them cause the normal notes would be way too loud and overpowering?

    Or are you saying that you think there should be a sweet spot somewhere on my volume knob before 10, where I should be able to get some type of overdrive, without the ghost notes?
     
    Last edited: Oct 30, 2020
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