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200hz filters in JR-20 bass/treble channels?

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  • 200hz filters in JR-20 bass/treble channels?

    Hi all. I've got a JR-20 tone cabinet here. I get that it's actively biamped and crosses over around 200Hz using RC filters in the amps. I'm hoping someone here can help me understand the circuit a little better. (This is partly curiosity, and partly that I'd like to get rid of the filters and run both channels full range.) I'm following the schematic from Captain Foldback here:

    http://www.captain-foldback.com/Hamm...R20_code_C.gif

    - I'm pretty sure I get what's going on in the treble channel. The blocking caps C9 and C10 form a high pass filter with R19 and R20 respectively---the RC calculator says .0035µF/180kΩ comes out to a cutoff of ~252 Hz. Does my reading of that seem correct?
    - The bass channel has me a little lost. I see R24/R25 along with C16 in front of the driver tubes, which could be a low pass, but the values look all wrong: 56kΩ and .0008µF come out to around 3.5kHz. R33/R34 and C22 form a similar arrangement in front of the output tubes, but 22kΩ and .01µF comes out to ~724 Hz. (Wondering if the negative feedback tap factors into this somehow?)

    Thanks!

    Click image for larger version

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    Click image for larger version

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  • #2
    With C16 and C22, you don't just have an RC low-pass filter to ground; you have frequency cancellation between the two sides of the balanced audio, which makes the calculations somewhat different. And then you have C20 and C21 in series with 22k resistors partially coupling the 6V6 control grids and cathodes. Only the difference between control grid and cathode will be amplified.

    Overall, the bass channel low-pass filtering is done in stages rather than all at once, but, mathematically, each filter builds on what the prior filter has done, so in total, they create a 200Hz low-pass filter. Hammond may have wanted a particular slope to the filter that was better accomplished by low-pass filtering in stages rather than all in one stage. Hammond may also have wanted to cancel any higher frequency noise produced by the 6SJ7 pentodes while avoiding presenting them with too low a load on the plates, which pentodes don't like.

    It's not an easy circuit to analyze.
    I'm David. 'Dave' is someone else's name.

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    • #3
      Got it---yeah, I was wondering how the two balanced legs complicated things. (And thanks for validating my head scratching.) In practical terms, to convert the bass channel to run full range, my first thoughts were:
      - remove C16 and C22---their associated resistors R24/R25, R33/R34 are functioning as grid stoppers and should be left in place? (I know with triodes the Miller capacitance issue can make the grid stoppers part of a LPF, but that's not the case with pentodes/tetrodes like these, correct?)
      - if necessary remove C20 and C21 to decouple the grid and cathode of the output tubes---I assume this would also increase gain

      Am I tracking this more or less correctly? Any other tips on disabling the filters? Thanks again.
      -E

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      • #4
        Also, the “Rs” of the RC filters would be the *sum* of r24 and r25 and r33 and r34, 1.2 meg and 44k ohms.
        Tom in Tulsa

        Fooling with: 1969 E100, 1955 M3, 1963 M100, Leslie 720

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        • #5
          I would just remove C6 and C22 and see where that gets you. As you note, Miller capacitance is not an issue with pentodes/beam tetrodes.

          If you have any noise issues from the pentode drivers after removing C22, there may be other steps you need to take.

          Edit: Just to put it out there, noise reduction might include bypassing the 6SJ7 screens with capacitors to ground or cathode. Look at AO10 preamp schematics to see what I mean. This shunts screen-dropping resistor noise to ground. The low-pass filtering probably gets rid of any hiss or popcorn noise you might get from those resistors run full-range.
          Last edited by David Anderson; 07-21-2021, 11:56 AM.
          I'm David. 'Dave' is someone else's name.

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          • #6
            Great, will do and report back (as soon as I make the time). Thanks!

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            • #7
              Also:

              Originally posted by tpappano View Post
              Also, the “Rs” of the RC filters would be the *sum* of r24 and r25 and r33 and r34, 1.2 meg and 44k ohms.
              Just plugged these into the calculator and got the much more plausible -3dB points of 166 Hz and 362 Hz respectively---thanks for pointing this out.

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              • #8
                You’re welcome!
                Tom in Tulsa

                Fooling with: 1969 E100, 1955 M3, 1963 M100, Leslie 720

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