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JTM 45 negative feedback with Mercury 'Radiospares Deluxe' OT

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  • JTM 45 negative feedback with Mercury 'Radiospares Deluxe' OT

    Hi All,

    Id like to wire up a Mercury Radiospares OT to a JTM45 circuit
    for 4 ohms. The stock JTM45 schematic has the negative feedback wire running to the 16 ohm tap, on the standard output transformer. Since this OT can't be wired for multiple impedances at the same time, what would happen if I ran the negative feedback to the OT wired for 4 ohms? Is there a fixup circuit I could use?

    Thanks
    MP
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  • #2
    The voltage on 4 ohm tap will be half the amplitude of 16 ohm tap (because the impedance is proportional to voltage squared)
    So you want to decrease your feedback resistor (27k) to half the value. Try 15k or 12k standard value resistor

    Comment


    • #3
      Which OT in particular? It doesn't have a 16 ohm tap, or the 16 ohm tap will be unused?
      If it's there but unused, you still wire up the NFB to it.
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

      Comment


      • #4
        Originally posted by frus View Post
        The voltage on 4 ohm tap will be half the amplitude of 16 ohm tap (because the impedance is proportional to voltage squared)
        So you want to decrease your feedback resistor (27k) to half the value. Try 15k or 12k standard value resistor
        In general, moving from one tap to a tap with half the impedance, divide the feedback resistor value by the square root of two. This backs up frus, moving two taps lower means dividing by twice the square root of two - two.

        Comment


        • #5
          Apparently the OT suffers the same limitation as the older off the shelf Hammond 16xx series in that it's prohibitively difficult to switch secondary impedance. It's probable that Mike can't derive a 4 ohm and a 16 ohm tap from the secondary. In which case he should follow frus's good advice in post #2
          "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

          Comment


          • #6
            Here you go:
            Attached Files
            "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

            Comment


            • #7
              Everything was nice and simple in the old days!

              Comment


              • #8
                Originally posted by Malcolm Irving View Post
                Everything was nice and simple in the old days!
                Well, original Radiospares OTs of early JTM45s had exactly this kind of secondary terminals.
                - Own Opinions Only -

                Comment


                • #9
                  Mike P, have you already bought the Mercury "RadioSpares Clone"?
                  This isn't the future I signed up for.

                  Comment


                  • #10
                    I guess if you want to clone something, you can't neglect the flaws.
                    This reminds me of when the MusicMan amps came out and didn't include the captive nuts in the chassis that newer Fender's had evolved to. Getting your fingers up in there is part of the blackface mojo I guess.
                    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                    Comment


                    • #11
                      Getting your fingers up in there to get the nuts. I'm surprised Leo didn't jump on this opportunity. He's been enjoying some more "colorful" humor of late
                      "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

                      Comment


                      • #12
                        Sorry for the late response, thanks.

                        Leo, yeah I already bought the Mercury RS clone. There are quite a few bundles of wires coming out of that thing! I checked out some of the original JTM45 amps with the original RS and wow, looks like they hacked the holes in the chassis with a meat cleaver. I had read that they had some problems with arcing between the pins since they were all packed so tightly on that little board.

                        It has the wiring diagram Chuck posted above.
                        So, it can be wired for 4 ohms, I got the diagram on how to do that, but then its 4 ohms, and there are no other "taps".

                        Thanks frus, will stock up a few similar range values for alternate feedback resistors. (And I have an old clunker scope now, so maybe I can actually see the damage i inflict!)
                        The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                        Comment


                        • #13
                          Originally posted by Leo_Gnardo View Post
                          Mike P, have you already bought the Mercury "RadioSpares Clone"?
                          Yep! Already bolted in. Not sure if I did the "right thing" at this point, with all these wires. But it got a bunch of very good reviews. (Maybe it was the designers cousin).
                          The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                          Comment


                          • #14
                            Originally posted by mikepukmel View Post
                            it got a bunch of very good reviews. (Maybe it was the designers cousin).
                            Likely so. Wouldn't doubt Mercury has a claque audience right under their own roof to write "reviews."
                            This isn't the future I signed up for.

                            Comment


                            • #15
                              Originally posted by Leo_Gnardo View Post
                              Likely so. Wouldn't doubt Mercury has a claque audience right under their own roof to write "reviews."
                              so, then, more wires coming out of a transformer isn't better?
                              The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                              Comment

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