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Plexi Clone Build - Cannot figure out why harsh distortion

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 27, 2020.

  1. itsGiusto

    itsGiusto Tele-Holic

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    I've posted many times about trying to debug this plexi clone amp I've built (see also https://www.tdpri.com/threads/how-t...-down-weird-overtones-in-plexi-build.1025094/ , https://www.tdpri.com/threads/not-sure-if-my-plexi-build-has-problems.984480/ and https://www.tdpri.com/threads/marsh...riations-get-breakup-at-lower-volume.1041560/). I've been trying to figure out over the past 11 months why the distortion sounds harsh and buzzy and I've made close to no progress. I'm losing my mind.

    At first I built it with a bunch of mods, but I've rolled it incrementally completely back to stock. Here are all of the things I've tried to figure this out. Throughout every change, the buzzy distortion persisted, and I heard no difference (in the harsh buzzyness).
    1. Take circuit completely back to stock, including removing tube rectifier
    2. Shorten leads as much as possible, make sure tube pin leads are not too close to each other
    3. Check all voltages in the amp
    4. Swap out all tubes for a brand new set
    5. Use my audio probe to listen to the signal at various points in the circuit. Built an attenuation circuit into the probe capable of attenuating the post power-tube signal to a listenable level, so I can listen to the signal post power tube and pre output transformer.
    6. Chop sticking
    7. Try using the 8 ohm impedance selector instead of 16
    8. Completely disconnect NFB from circuit
    9. Change the NFB wires to use shielded wire
    10. Change the input wires to use shielded wire
    11. Change the effects loop wires to use shielded wire
    12. Try upping the value of the post-phase inverter "stability" cap
    13. Try swapping in a different power transformer
    14. Try swapping out each filter capacitor for a larger value
    15. Try taking the filter capacitor for the preamp tubes completely out of the circuit, and connect it via alligator clips, in case it was too close to the signal caps.
    16. Take out most of the signal caps and figure out which end is the foil-end, mark it, and reorient appropriately
    17. Switch V1 from shared cathode to split cathode
    18. Take the effects loop out of the circuit and instead connect the treble pot directly to the PI section
    19. Try varying levels of NFB, from lots to completely disconnected.
    20. Buy an oscilloscope and poke around with it
    21. Take amp to a professional tech to debug

    I really don't know what else to try.. The amp sounds fine right up until it starts to break up. Then it gets this horrible buzzy noise, which I think might be parasitic oscillation. But I've tried everything I can think of to alleviate it, and I cannot figure it out.


    I've tried probing the circuit, and to be honest, I think I may be hearing the start of the oscillation way back in like the first or second tube stage. It's only once we get to the post-phase inverter stage that the oscillation starts to break up, which makes the buzzy sound really noticeable.

    I really built this amp so that I could play it overdriven, not clean, so I want to fix this so badly. Does anyone have any clue what else I could try to figure this out?
     
    Last edited: Aug 28, 2020
  2. itsGiusto

    itsGiusto Tele-Holic

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    Shot of amp:
    [​IMG]

    Close up of board:
    [​IMG]

    Close up of power section:
    [​IMG]

    Closeup of input section:
    [​IMG]

    Closeup of V1
    [​IMG]

    Closeup of V2 and V3
    [​IMG]

    Closeup of output jacks:
    [​IMG]

    Closeup of power tube sockets:
    [​IMG]
     
  3. itsGiusto

    itsGiusto Tele-Holic

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    Closeup of left power tube:
    [​IMG]

    Closeup of right power tube
    [​IMG]

    Closeup of filtering section
    [​IMG]

    Closeup of tone pots:
    [​IMG]

    Closeup of more tone pots:
    [​IMG]

    Closeup of more tone pots and input section
    [​IMG]

    closeup of input section:
    [​IMG]
     
  4. itsGiusto

    itsGiusto Tele-Holic

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    Back of chassis:
    [​IMG]

    More back of chassis:
    [​IMG]

    dent on PT (i originally thought this might be the problem, but swapping out the PT didn't seem to have any effect)
    [​IMG]
     
  5. itsGiusto

    itsGiusto Tele-Holic

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    Here are some short clips of the sound, probed at various points throughout the amp. To me, it sounds like the parasitic oscillation begins somewhere before the PI section, but is very faint. Then maybe it gets much louder afterward when it starts to distort and generate its own overtones. But I don't know if I'm reading into it and hearing what isn't there.

    This is my test loop, playing through a different amp:


    This is my test loop playing through the plexi amp, everything on 10, through attenuator and into a greenback speaker:


    This is my test loop playing through the plexi, probed at the output of the V1 stage


    This is my test loop playing through the plexi, probed at the output of the V2 stage, but prior to tone stack:


    This is my test loop playing through the plexi, probed at the output of the tone stack:
     
  6. itsGiusto

    itsGiusto Tele-Holic

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    This is my test loop playing through the plexi, probed at the left PI section:


    This is my test loop playing through the plexi, probed at the right PI section:


    This is my test loop playing through the plexi, probed at the right tube output:


    This is my test loop playing through the plexi, probed at the left tube output:
     
    Last edited: Aug 28, 2020
  7. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I like the perseverance and systematic approach!

    As far as I know, the output of the first stage should be clean (in any amp). The 100-200mV guitar signal is not enough to make it distort. Yours clearly is not clean. It might be the result of the warm stage after the probe point though. Just to be sure, could you check at the volume pots?

    In simulations (I don't have a lot of practical experience) I've seen weird behaviour if the B+'s of the various stages are not decoupled (I forget to add the capacitors from time to time).
    Probe at the input of the volume, with the volume at 0. Or even better if all components can stand the higher B+: remove all (signal) tubes but V1 to make sure some weird stuff is not 'feeding back' to early stages.

    Scope images tell me more than audio does. I read you've purchased a scope but I don't remember images. Could you make some? Especially early (earlier) in the signal path.
     
    Last edited: Aug 28, 2020
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  8. itsGiusto

    itsGiusto Tele-Holic

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    Thanks! What should I do when probing? Play a sine wave through the input? What frequency should I use?

    I have a digital oscilloscope, so the signal always looks a little noisy. Should I use the "averaging" function, so it displays the average of the last few trigger points, to make it display more like an analog scope?
     
    Last edited: Aug 28, 2020
  9. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Yeah, a 1kHz 100-300mV pp sine would be fine. It's not that critical.

    I've no experience with digital scopes. Signals that have a fixed phase relation with the main signal will become clearer, those that don't might be averaged out. So... both for now?
     
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  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You have tried most everything I can think of. I doubt these observations will help, but maybe a hail Mary is worth a try.
    InkedInkedripple_LI.jpg
    Your HT CT is allowing ripple current to flow through the chassis. Ideally it should be soldered to the negative of the first filter cap. Second best would be to bolt it to the same bolt as the filter caps.

    I know you have tried a disconnected NFB which should solve a positive feedback issue. Since you have tried everything else, try swapping the OT primaries. Do not twist the OT primary wires.

    Consider a faulty OT.
     
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  11. nathan5782

    nathan5782 Tele-Meister Silver Supporter

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    I noticed on all your audio samples that you are jumping the channels even on the other threads. This may sound overly basic but you’re using 3 input jacks every time, try each channel separately and each jack one at a time and other jumper cables. Is the distortion worse on either channel or about the same?
     
  12. dan40

    dan40 Friend of Leo's

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    You have quite a few wires all bunched up together over the power tube sockets, some other wires crossing over the v1 socket and many of the components on the board are spliced together. I know a lot of this has come from your rework and troubleshooting but it may save you a bunch of headaches if you pull the board and start with a fresh one with new components. Try to match the wiring and layout shown in these Metroamp instructions and once you have a basic circuit working properly you can begin to implement the mods that you want.

    http://valvestorm.com/sites/default/files/50_WATT_KIT_INSTRUCTIONS.pdf
     
  13. itsGiusto

    itsGiusto Tele-Holic

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    Thanks, but yeah, it's the same either channel, even un-jumpered
     
  14. itsGiusto

    itsGiusto Tele-Holic

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    Do you have a recommendation for multi-stranded wires (preferably not crazy expensive) that actually stay put where you put them? Posing the wires has proven to be an issue, because I prefer multi core to single core.

    If I'm going to have to start moving around things like the OT wires, I'll have to splice them. They're out of slack cause I cut them short, trying to reduce the length of wire runs. And FWIW, the bad distortion sound preceded many of the board-splices.

    Which wires are you referring to running across V1? Do you mean the heater filament wires? Is that bad to do that?
    You may not be able to tell, due to the lack of 3D in the pictures I took, but I did try to generally keep wires apart from each other in the 3rd dimension, especially signal wires coming and going from the tubes, and power wires, except for wires that I though I'm supposed to keep twisted, like the post PI signal wires. Ground wires, I give myself some leeway on. And FWIW, chop-sticking all the wires around V1 and the power tube did not make any sonic difference.

    But, yeah, I could rebuild it from scratch... just not sure how I'll feel about having to buy basically the whole thing new again, and then what if it still doesn't work?
     
    Last edited: Aug 28, 2020
  15. dan40

    dan40 Friend of Leo's

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    If you are handy with a solder sucker you can remove all of the components, and then remove all of the old solder from the turret board. This will save you a few dollars on a new board and a fresh set of coupling caps and resistors shouldn't set you back too far. The rest of the components (sockets, switches, pots and transformers) can be reused after cleaning up the old solder.
    For the short transformer leads, personally I would cut them back and carefully splice some new 22awg wire onto them with a couple pieces of heat shrink over each connection. This way you will have fresh new pieces of wire inside of the chassis to work with. My choice of wire for all of my builds these days is the 22awg pretinned Topcoat wire sold by Valvestorm. It is pretinned for easy soldering, the insulation stands up very well to the soldering heat and the wire holds it shape almost as well as solid core.

    http://valvestorm.com/Products/Wire
     
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  16. dan40

    dan40 Friend of Leo's

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    I was mistaken. I noticed the red wire flying over the v1 socket but I now see that it is the plate lead connecting to pin 6. Ideally these should be laying down on the chassis with the grid and cathode wires raised up a bit on their way to the socket. The Metroamp instructions I linked to will give you an idea of the way that Marshall ran the wires to each socket for quietest operation.
     
  17. itsGiusto

    itsGiusto Tele-Holic

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    Are you certain that I should not twist the OT primaries? The Metroamp instructions @dan40 sent indicate that the OT primaries should be twisted.

    Upon playing through my amp again, I actually think that while the bad sound is definitely still there, it's at least POSSIBLE that the recent changes I made caused it to be a little less obvious than it used to be. Maybe the reason I haven't been able to track it down is that the PO is not caused by one single smoking gun, but that it creeps in lots of locations all across the amp. Maybe some of my recent cap reorintations, etc made it slightly better.

    I just ordered a bunch of stuff from the Valvestorm site. Maybe I'll try to redo most of it and do everything like in those instructions you sent, in the hopes that it really is just lead dress, etc, that is causing the problems.
     
  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    When you twist lines that have AC signal there can be capacitance coupling which may reduce highs. It is usually a fairly short run but this phenomenon does happen when wires are run together. Rob Robinette mentioned he no long twists OT primaries in post #29 of
    Twisting Output Transformer Wire Question
    Discussion in 'Shock Brother's DIY Amps' started by TobyZ28, Jan 28, 2020.
     
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  19. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Some argued that inside the transformer the wires are in near proximity of each other for meters, so a few cm extra would not make a difference.

    It seems logical to me.

    unnamed.jpg
     
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  20. elpico

    elpico Tele-Afflicted

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    Twist a couple foot long pieces of 600v insulated wire together sometime and measure it.

    Spoiler alert, you're gonna need a hell of a good meter to get a real reading because you're talking about <10 puff.

    Bonus question, how much inductance does this twisted pair have?

    Second spoiler alert, none of this has any effect in a guitar amp's passband.
     
    Last edited: Sep 1, 2020
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