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How to trace blocking distortion with a scope

Discussion in 'Amp Tech Center' started by _Steve, Oct 16, 2020.

  1. _Steve

    _Steve TDPRI Member

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    Hi!

    I have a homebuilt Marshall 1959 that is making a "buwhah" sound when i hit a big power chord - ie the initial attack is heavily attenuated then after a second or so the full sound comes through in a swell-like fashion. At first I thought this might be extreme B+ sag but someone told me this was a classic symptom of blocking distortion.

    So my question is, how can I trace blocking distortion with my oscilloscope so I can see what stage its occurring in? What do I look for? And how do I tell the difference between acceptable BD and excessive BD?

    Thanks in advance!
    Steve
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    Probably due to the output valves running into Class AB2 or the coupling capacitors in the pre amp stages are too large.
     
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  3. Peegoo

    Peegoo Poster Extraordinaire

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    I've never encountered this problem, but the intuitive thing to do would be to start at the guitar input on the amp and run a signal trace in stages, hopping over each component in the chain until you find the component that's pulling the signal down.

    I'm not an expert on this stuff; just thinking out loud.
     
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  4. loopfinding

    loopfinding Tele-Afflicted

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    yeah first thing's first check all the coupling cap values and make sure you didn't get off by a multiplier (like 220n instead of 22n). if there isn't something silly there, run a 250mV sine wave into the amp at like 80hz, probe each of the stage's output side of the coupling caps, stage by stage and see what happens when you turn the gain up. you might see some weird cutoff or fold over at some stage output that's happening way too early for where your gain pot is set. if it isn't in the preamp, then you've isolated it to the power section, so have a look-over there for screw ups. given the nature of the problem it sounds like it could be something simple.

    i don't have much experience with marshalls personally, but a buddy of mine used to install a PPI master in the back and knock it down ever so slightly just to reign in the front panel gain range, since there was always a hair too much of "bloom" for his taste with the non MV types. obviously what you have is much more extreme than this.
     
    Last edited: Oct 16, 2020
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  5. _Steve

    _Steve TDPRI Member

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    Thanks I visually double checked all the coupling caps and they are the correct values.

    What does the 'fold over' you describe look like on the scope?
     
  6. loopfinding

    loopfinding Tele-Afflicted

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    my guess is you'll probably see something that looks sort of like rectification, but could possibly have some weird discontinuities or spikes.
     
  7. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    I'm not an expert in Marshalls, but is this a faithful clone of the very first 1959s from the late 1960s, or the JCM 800 sold as a 1959? I ask because the earliest schematics only show grid stoppers on two of the four output tubes. I've read of momentary oscillation happening in amps where this is done (like the 5F8A tweed twin), so if you don't have grid stoppers on all four tubes, that could be something to try.
     
  8. andrewRneumann

    andrewRneumann Tele-Holic

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    I experienced something similar and I think a lot of it was screen sag. My screen stoppers were pretty big and their supply capacitor was too small to source the current. By increasing the screen supply capacitance I was able to keep things running smoother when those big power chords attack.
     
  9. tubegeek

    tubegeek Friend of Leo's

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    It can look like an "M" shape at the peak of a sine wave. In other words, the part that's SUPPOSED to continue up then come down reverses direction partway and goes down then up.

    It's different from flat-topped clipping.
     
  10. _Steve

    _Steve TDPRI Member

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    Got it - Thanks! I'll take a look this evening!
     
  11. _Steve

    _Steve TDPRI Member

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    That's interesting and I can see how it would be consistent with the issue I have.

    Is there a way I test to see if this could be the issue?
     
  12. andrewRneumann

    andrewRneumann Tele-Holic

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    The only way is to clip large capacitor(s) in parallel with the screen supply capacitor(s).

    I have to say my situation did not occur on a 1959. It was a home brewed design and 2x6V6, but the description sounds similar. If you followed a known good schematic, it’s hard to understand why you have this problem. So my confidence is low... but thought I’d chime in.
     
  13. _Steve

    _Steve TDPRI Member

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    Well I didn't *exactly* follow the schematic ;) Everything is 1959SLP except its running only 2 output tubes and a 50W OT. I have 50uF of screen filter cap, made up of 4 50uF caps in series/parallel per the 1959SLP spec.
     
  14. dan40

    dan40 Friend of Leo's

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    Do you have the power tubes properly biased? Have you tried replacing all of the tubes with known good ones?
     
  15. _Steve

    _Steve TDPRI Member

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    Yep unfortunately tried multiple good tubes and tried different bias settings..
     
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  16. _Steve

    _Steve TDPRI Member

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    So I scoped each section and I didn't see any M shaped signals.

    However I was thinking about it, and as blocking distortion caused a build up of charge on the preceding coupling cap, wouldn't I see a DC offset (possibly shifting) at the grids?
     
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  17. tubegeek

    tubegeek Friend of Leo's

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    That's basically (as I understand it) what blocking distortion is - a bias shift which moves the tube into grid current, and which doesn't settle quickly.

    Good info as always from Randall Aiken.

    https://www.aikenamps.com/index.php/what-is-blocking-distortion
     
  18. David Barnett

    David Barnett Doctor of Teleocity

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    What value did you use for the negative feedback resistor? And which tap is it connected to on the secondary of the output transformer?
     
  19. D'tar

    D'tar Friend of Leo's

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    So 2x el34? What Ot? Assuming you have accommodated the changes proper. Please post photos as you progress. Thanks
     
  20. dan40

    dan40 Friend of Leo's

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    I have had positive feedback cause unusual symptoms beyond the normal squeal that we expect when the OT primaries are reversed. Have you tried disconnecting the nfb wire from the output jack to see if it helps? Might be worth a quick test just to rule it out.
     
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