1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Continuing saga of plexi clone build with harsh distortion

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

  1. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    668
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    Ok. I see. Your first set of scope images were done at different settings as what Steve used. If you use Steve's settings, all's fine.

    I was hoping to give you your own reference with the spice model. I have the output tubes now, but the OT modeling is new to me.

    I'm pretty sure the notches should not be present.
     
  2. itsGiusto

    itsGiusto Tele-Holic

    Posts:
    652
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    I did a little bit of experimentation to see what triggers it:


    So as the treble goes up from 50% on the knob, the notch gets more defined. As mid goes down from 50% the notch gets more defined. As presence goes down the notch gets more defined, and as bass goes up the notch gets more defined.

    It's also worth noting, the reason I wasn't seeing that notch when mimicking @_Steve's settings is because the notch does not appear at all if I input a sine wave under ~300hz. Only at around 300hz does the notch start to appear for any settings on the amp.

    Here's some pictures showing the trace along the amp. This is with volume all the way up, treble a little bit up, and mid a little bit down, and bass and presence at 50%.

    Here's the signal immediately after the first amplification stage, the plate of V1b:
    [​IMG]

    Here's the signal right before the second amplification stage, the grid of V2a:
    [​IMG]

    Here's the signal immediately after the second amplification stage, the cathode of V2b:
    [​IMG]
     
    Last edited: Oct 28, 2020
  3. itsGiusto

    itsGiusto Tele-Holic

    Posts:
    652
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    Here's the signal after the 250pf treble cap:
    [​IMG]

    Here's the signal after one of the 22nf mid or bass caps:
    [​IMG]

    Here's the signal coming out of the tone stack, right before going into the PI:
    [​IMG]

    Here's the signal after the PI, non-inverted side:
    [​IMG]

    Here's the signal post PI, inverted side:
    [​IMG]

    Here's the signal at the speaker output:
    [​IMG]

    Here's the signal in the NFB loop, after the 27k resistor:
    [​IMG]

    Not a great picture, but here's the signal in the pre PI stage, where the NFB signal meets the normal signal, at the junction of the two 1M resistors, the 470 ohm resistor, and the 10k resistor:
    [​IMG]

    So in short, my take on these photos is that the notch seems to first appear after the treble cap in the tone stack, then it gets heavily amplified in the PI section, and possibly reinforced by the NFB signal?


    Also, another thing I'm wondering - are the post-PI signal and inverted post-PI signal supposed to be the exact same signal, but just flipped? According to Robrob here, I think they are. If so, that seems to not be the case in my photos. The inverted signal is different and more pronounced than the non-inverted signal.

    In fact it seems like maybe they each just show one half of the wave, the non-inverted side showing the top half, and the inverted side showing the lower half. Perhaps the other half is getting clipped out of existence for each of these. Is that normal?
     
    Last edited: Oct 28, 2020
  4. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    668
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    This is (one of) the output of the model of the LTPI. It looks very familiar:

    plexi_notches.png

    It takes some time to build up, but I see notches. What is going on here?

    This was with the volume at full, treble almost full and a 500mV signal (which is extremely large)
     
    itsGiusto likes this.
  5. itsGiusto

    itsGiusto Tele-Holic

    Posts:
    652
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    Does that mean they the notch is normal? Also, why does it change over time in your images?
     
  6. _Steve

    _Steve TDPRI Member

    Age:
    41
    Posts:
    83
    Joined:
    Jul 14, 2020
    Location:
    Los Angeles
    OK - I ran it at 82Hz because i suspected your problem was blocking distortion related. I can run it at 800Hz and post the scope trace again. I suspect it will look similar to your previous scope images. Its normal to to diagnose with presence at 0 as it opens a can of worms and its hard to know what you are actually looking at. Better to diagnose that separately.

    My clone is internally jumpered. So each channel gets the input signal equally with individual 34K stoppers and 1Meg shared leak. I wouldnt get hung up on this aspect.
     
    itsGiusto likes this.
  7. _Steve

    _Steve TDPRI Member

    Age:
    41
    Posts:
    83
    Joined:
    Jul 14, 2020
    Location:
    Los Angeles
    Try the grid stoppers on the power tubes. Also do you have the snubber cap (usually 49pF) across the plates of the phase inverter?
     
  8. _Steve

    _Steve TDPRI Member

    Age:
    41
    Posts:
    83
    Joined:
    Jul 14, 2020
    Location:
    Los Angeles
    I have the whole amp modelled in LTSpice if you want it to save you some time? It take a long time to run though
     
  9. elpico

    elpico Tele-Afflicted

    Posts:
    1,182
    Joined:
    Sep 14, 2011
    Location:
    Vancouver BC
    Spice is useful when you know what it's not useful for
     
    SerpentRuss and ThermionicScott like this.
  10. itsGiusto

    itsGiusto Tele-Holic

    Posts:
    652
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    I do have grid stoppers on the power tubes, 5.6k. And I also have a 47pf snubber cap on the plates of the power tube. I've also tried (much) higher values for the snubber cap, but it had no effect on the weird sounding distortion.
     
  11. mrriggs

    mrriggs Tele-Meister

    Age:
    41
    Posts:
    110
    Joined:
    May 24, 2020
    Location:
    Vancouver, WA
    Does your scope have an FFT (spectrum analyzer) function? That could help you pinpoint a weird oscillation that would otherwise go unnoticed. Once you know the exact frequency that's giving you fits, then filtering it out is a cinch.
     
  12. itsGiusto

    itsGiusto Tele-Holic

    Posts:
    652
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    It does, but it's crappy. See here where Dave Jones says the FFT on the cheap Rigol scopes is "a toy".
     
  13. _Steve

    _Steve TDPRI Member

    Age:
    41
    Posts:
    83
    Joined:
    Jul 14, 2020
    Location:
    Los Angeles
    Its doesn't need to be precise. You just need to see if you have a big spike in energy at high frequencies.
     
  14. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    668
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    It seems the notch is 'normal'; at least not only present in your amp.
    The changing of the signal is a start-up effect.

    I'm hesitant to draw any conclusions atm.
     
  15. itsGiusto

    itsGiusto Tele-Holic

    Posts:
    652
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    If you wouldn't mind doing that, it would be very useful! Thanks!
     
  16. drew1d

    drew1d Tele-Meister Silver Supporter

    Posts:
    145
    Joined:
    Jun 30, 2014
    Location:
    Westchester NY
    *Furrows Brow and squints at your Scope waves...*
    "Blocking distortion from one of the caps. probably the treble cap."
    I figured, if I sound like I know what I'm talking about, people would listen and like me. But I have no idea. I don't even know how to hook up a scope or interpret what it spits out. Hope I gave you a laugh. Don't give up!!! Good luck.
     
  17. drew1d

    drew1d Tele-Meister Silver Supporter

    Posts:
    145
    Joined:
    Jun 30, 2014
    Location:
    Westchester NY
    I didn't even read this when I posted, I'm just at the blocking distortion section of a book I'm reading. Wasn't a goof on you, I'm just a goofball.
     
  18. D'tar

    D'tar Friend of Leo's

    Posts:
    3,329
    Joined:
    Jan 11, 2013
    Location:
    WNY
    within a perfectly balanced PI, I would suggest yes. some imbalance may prove to be sonically beneficial.
     
  19. itsGiusto

    itsGiusto Tele-Holic

    Posts:
    652
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    But when the PI is driven into distortion, is the symmetry still supposed to hold?
     
  20. itsGiusto

    itsGiusto Tele-Holic

    Posts:
    652
    Joined:
    Dec 31, 2017
    Location:
    Massachusetts
    One of my continual frustrations with trying to use the FFT on my scope is that the signal quickly descends into the noise floor, such that even by, like, the mids, I can no longer discern and signal apart from the noise. Maybe I'm using the scope wrong, IDK, but unless there is a strong, highly periodic signal on the scope, it all just looks like noise.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.