Inducing Swirl With a Cathodyne PI

Discussion in 'Amp Central Station' started by Luthier Vandros, Aug 26, 2019.

  1. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    I gleaned a bit of incidental information while studying up on the ol' PRRI and it's ancestry. I can't recall where, but during a read on the cathodyne-style phase inverter, I read that at best that elusive swirly tone can be had, while at worst that nasty doubling effect will occur without sufficiently augmented grid resistance - and somewhere in the middle, most of us live with neither artifact.

    Well, call me crazy, but I'm in for the swirl. The only time I've experienced that heavenly condition was with my first build, a 5E3 clone (at or near "12") that I assembled using Hoffman's catalog and a Weber Blue Dog. I remember first switching it on using a broomstick as I sought defilade behind a chair lol.

    In fact, it was that sound that would eventually precede my rectifier or Brimar 6v6's melting down and taking the PT with it. There's gotta be a way to ride that edge without going nuclear.

    Have any of you successfully gotten a cathodyne PI to intentionally and safely behave this way? Is this swirly goodness simply the "double frequency" blocking distortion occurring while both frequencies are ever-so-slightly misaligned or out of phase? Or is it just the nature of the TwdDlx and it's marginal filtering?

    I've never had that sound at my fingertips since that amp melted down and it's making me wonder what's possible with this PI configuration. I dont plan on modding this PRRI; I've got a Univibe clone for the swirly stuff. Maybe another 5E3 is in order.
     
    Last edited: Aug 26, 2019
  2. moosie

    moosie Doctor of Teleocity Silver Supporter

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    I can almost smell another amp burning from over here... :lol:
     
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  3. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    Keep your eyes fixed towards the sunset side of the continent for the smoke signal!
     
  4. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    Are there other longer lived amps with this swirly sound we can reference?
    I'm just not familiar with it though I may have heard the term.
    It's not related to what gets called "3D" is it?
    And I'm familiar with ghosting in old (pre '68) 100w Marshalls with inadequate filtering, not really doubling like you mention above.
    Not helpful just curious...
     
  5. caspersvapors

    caspersvapors Tele-Meister

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    yea not sure what is meant by "swirl"

    anybody got a clip of it?
     
  6. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    Not familiar with other amps that might do it, but I had a Weber c12b with no dope on it that would produce ghost notes at full tilt.

    The swirling sounds like a phaser is running on a low mix setting. That's why I was thinking about the possibility that any frequency doubling occurring might be slightly offset.
     
  7. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    The swirl effect, due to the two phases shifting in relationship to each other, is something a cathodyne cannot, through the nature of its operation, do.
    This does not mean an amp with a cathodyne phase inverter is imune to, or incapable of producing a swirl, it is just not the phase inverter (alone) doing it.
    The effect has more to do with a bias shift in the power tubes. Cold biasing, driven to the level where a subsequent grid clamping will arise, will cause a bias shift between grid and cathode, that will create cross-over distortion and a mismatch in the phases, and hense a swirl. Due to the clamped grid, the preceding cathodyne will also be likely to distort, with the dreaded nipples.
     
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  8. Wally

    Wally Telefied Ad Free Member

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  9. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    The ol' dreaded nipples. Reminds me of college.
     
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  10. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    ....and to note his explination as I read it;

    That swirl is created by a midrange dip in the signal, shifting position in the wave form, as a clipped signal reverts from a square form, into a lesser clipped form as the input signal decays.

    ...sounds plausible:)
     
    Last edited: Aug 26, 2019
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  11. RottenTheCat

    RottenTheCat Tele-Meister

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    It sounded GREAT right before it blew up....
     
  12. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    Yep. That's clear and a gives enough ingredients for a builder who's after that recipe.
     
  13. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    What would be the effect of very mismatched valves?
     
  14. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    To a certain degree, more of a certain type of pleasing harmonics, I'm led to believe.
     
  15. Wally

    Wally Telefied Ad Free Member

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    I have heard swirl many times. It takes a good amp in proper working condition, a guitar that can play in tune, and a player who can play that guitar in tune. Then, the harmonics start flying around AFTER they leave the speaker. ....ime.
     
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  16. moosie

    moosie Doctor of Teleocity Silver Supporter

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    I thought I was following you, until that bit about the nipples. Huh?
     
  17. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    When you see the nipple distorting, you gotta pull back, man. And don't make eye contact.

    Edit: haha, it's referring to the shape of the signal on the scope.
     
  18. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    Pondering on the notion..... to enhance the imbalance of shift between the two phases, thereby increacing the effect of swirl, a mismatch of the output tubes bias, or rather their inherent mismatched current draw, would accentuate the phasered tone I should imagine. Harmonica players sometimes have there amps driven by a very unbalanced signal from the phase inverter, and it certainly does colour the tone in a positive way for them.
    I have an amp that I built with a volume control on one leg of the P.I. for harp, but I don't use it for guitar.....in fact I rarely use it at all nowadays. If I get time, I will try it out, fully cranked with a guitar, and see if the swirl effect occures, and changes with the imbalance......Get back to you then.

    EDIT; Thinking back to the circuitry of that harp amp, I built it with a very low gain structure in the preamp, to avoid feedback, and the two KT66 output tubes are biased quite hot, and I'm not sure if the cathodyne P.I. will actually drive them into a sqaure wave clipping state, so maybe it is not the ideal amp to test the hypothesis.....
     
    Last edited: Aug 26, 2019
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  19. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    upload_2019-8-26_23-22-4.png
     
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  20. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    Would be cool on a switch. My name is Randall S. and I hereby patent this idea. You're all being sent gag orders and NDA's.
     
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