Phase reverser circuit - have you tried this simple P.I. ?

Discussion in 'Amp Tech Center' started by Bendyha, Sep 10, 2019.

  1. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    I can't recall ever seeing this PI in a guitar amp. I found this in a 1958 tech. sheet. Compared to a long tailed pair, it has about twice the gain, and about twice the THD, which is probably the reason it doesn't seem to turn up in HIFI amps, but since when has a bit of distortion ever worried us guitar amp builders. Does anyone have any experience with this circuit, or know where I can find examples of it either used or written up anywhere.
    upload_2019-9-10_23-22-16.png
     
  2. TequilaCaster

    TequilaCaster Tele-Holic

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    Does not the old Supro Thunderbolt use this PI scheme?

    [​IMG]
     
  3. LetItGrowTone

    LetItGrowTone TDPRI Member

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    With small differences, Ampeg B-15 and V4.
    Principle of operation, with a small variation on your circuit at the end of the page: http://www.valvewizard.co.uk/paraphase.html

    He says: "if the power valves are fixed biased then the paraphase will needs its own coupling cap and grid leak from the junction of Rf and R1". For an example see V4.
     
    Last edited: Sep 10, 2019
  4. wabashslim

    wabashslim Tele-Holic

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    Is that a switch that grounds out the signal from the upper V4? Why?
     
  5. TequilaCaster

    TequilaCaster Tele-Holic

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    Yes, it acts as a 'STAND BY' switch, by shorting the signal to ground, so it does not drive the following tubes' grids.
     
  6. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Nope, that is just a standard paraphase. Quite a different kettle of fish.
    upload_2019-9-11_9-37-40.png
     
    Last edited: Sep 11, 2019
  7. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Ah...Merlin....the "Floating Paraphase" thats more like it. Other than the occasional mention, one doesn't seem to find much written about it. Crowhurst as usual is a good source.
    I've never seen the gain and distortion levels listed, and compared to a LTP before.

    The B-15 schem I have is a different PI....So is the V4, which is just a Schmitt.
     
    Last edited: Sep 11, 2019
  8. danlad

    danlad Tele-Meister

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    A floating paraphase is on my list.

    It took me a decade to get round to my first push pull amp so you don't want to wait for any experience from me to try it though!
     
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  9. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Looking around a bit further, there are a few different ways, or rather, subtle differences between, various circuits that get the moniker "Floating Paraphase".
    Some are just subtle changes of resistor values in an effort to better balance the output (or rather the input to second tube to match that of the first) , other changes make the circuit quite different; sort of like the difference between a true Schmitt PI, and the Fender LTP. It seems the range between floating, non-floating, and side-branch PI's, and thier derivatives can be quite fine before one becomes the other, or unique in its own right.

    Still seeking a guitar amp schematic with a simple floating paraphase....will have to make dig through my archives again, and gatherwhat I find together.

    The raw symmetrical simplicity of the circuit in the OP is what I find so appealing.
     
  10. LetItGrowTone

    LetItGrowTone TDPRI Member

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    This, from V4 1971.
    Same thing except the signal for the second tube is taken from *before* the coupling caps, and so it needs "its own coupling cap and grid leak from the junction of Rf and R1".
    And instead of 1M/1M the split is 820K/1M, for symmetry.

    V4 1971 PI.PNG
     
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  11. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Closer, and different from the Ampeg, is the 1954 Fender 5D8 wide panel/low power Twin, with its self-biasing/ floating PI. But the cathode resistor is not shared, so it is not quiet the same sort of floating topology as the OP, or the one Merlin shows. This was a short lived circuit at Fender, but highly regarded by many.
    upload_2019-9-11_19-41-46.png
     
    Last edited: Sep 11, 2019
  12. timfred

    timfred TDPRI Member

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    Ampeg Reverberocket

    One feature or issue is that at the onset of clipping, often the first half of the triode clips asymmetrically while the second half clips symmetrically. Maybe that's a good sound to you, maybe not.
     
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