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NFB Switch for 5F2A

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  • NFB Switch for 5F2A

    Hi First post here thanks in advance for the help.
    I just finished my 5F2A princeton build and wanted to add the NFB cutout switch. I have a spdt toggle in between the wire from the 22k res and the OT secondary to break the connection. However, when I break it loud SQUEEL. Engaged all is good. Any suggestions on what I am doing wrong? Thanks.
    Attached Files

  • #2
    Are you sure it squeals when you cut the switch? (rather than when you engage the switch?). Sounds like you are getting positive feedback with the switch engaged, resulting from having the OT primaries the wrong way around. Even tho' manufacturers will say their SE OT primaries are a certain colour, it doesn't mean they always get it right. The solution is to swap the OT primaries around. Unconnect the plate wire and connect it to the B+, and unconnect the B+ wire and connect it to the plate
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo


    • #3
      If there's no problem when the neg.fdbck. is ON than the OT windings are connected correctly.

      If the loop is DISconnected and you'll get problems:
      - double check the wiring and layout, try moving some wires and be sure no earlier parts of the circuit can interact with the wiring around the OT/
      Love, peace & loudness,


      • #4
        Got it! Reversing the leads from the OT was the ticket. I put MM's in: PT choke and OT-for some reason I thought the black lead was ground...

        Thanks so much for the help and suggestions. Being an amp novice, a resource like the collective knowledge of this forum is priceless.


        • #5
          Originally posted by fili123 View Post
          for some reason I thought the black lead was ground...
          Your kidding

          I would have made the same mistake. In fact I have.

          I've noticed that transformer MFGs just don't really seem to care what color wire they use as long as there are different colors to differentiate from one another. I've seen different runs from the same reputable MFG where on the first all the 4 ohm taps were green and all the 8 ohm taps yellow. Then on the very next run all the 4 ohm taps yellow and the 8 ohm taps green It took me a while to figure out why the first amp sounded good and the second just didn't sound right. Really frustrating.

          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A