Rotary Impedance Switch Schematic for 4, 8, 16 ohm

Discussion in 'Amp Central Station' started by Gazza74, Sep 20, 2021.

  1. Gazza74

    Gazza74 TDPRI Member

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    I noticed several others have been struggling with this design, so thought I would share what I've found. Hammond makes a rotary impedance switch to support their line of OTs. However, I think the schematic supports other OTs just cross reference your OT wire color to the Hammond color scheme. I also attached the technical document supporting this mod. Hope this helps.



    Impedance Schematic.JPG
     

    Attached Files:

    trancedental likes this.
  2. muscmp

    muscmp Friend of Leo's

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    i have one on my 5e3 build but rarely ever use it. thanks for the diagram and prompting me to use it more often.

    play music!
     
  3. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Attach the negative feedback wire to the normal impedance tap on the switch.
     
  4. Gazza74

    Gazza74 TDPRI Member

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    @schmee thanks for that advice on the NFB lead. Question, my amp build is a 5F2-A and I have a 22k resistor for the NFB. I believe this is the spec for a 4 ohm speaker which I have. Should I connect the NFB lead to the #1 lug along with the green 4ohm lead from the OT?
     
  5. 2L man

    2L man Tele-Holic

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    Yes!
     
  6. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Yes, usually you connect to the "standard" output, even if you dont use it. It's not super critical though, just changes the NFB a little is my understanding.
     
  7. Gazza74

    Gazza74 TDPRI Member

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    Thanks! @schmee I am going to install 3 jacks for switching to different speaker cabs with different ohms. Do I need to jumper the black common wire across all three jacks ground lugs?
     
  8. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Yeah that'll work. They often ground through the chassis.
     
  9. Gazza74

    Gazza74 TDPRI Member

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    Thanks!
     
  10. doug

    doug TDPRI Member Silver Supporter

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    What switch are you using?
     
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