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Hammond 125B multiple outputs

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  • Hammond 125B multiple outputs

    I'm building a project with a Hammond 125B OT.
    125B.pdf

    I want...if possible...4, 8, and 16 ohm speaker outs with a 22.5k primary impedance for a single 12AU7 output tube in push/pull.

    The 4 is pretty easy...use wires 5 and 6.

    The 8 and 16 options though share a wire....wire #4.

    What do you think is the best way to have independent 8 and 16 ohm jacks that have to share that wire?

    Should I put them on a DPDT switch? Or use switching jacks?
    Attached Files

  • #2
    Because the power level is so low, a 3 pole 3 position switch such as

    Link: https://www.tubesandmore.com/product...les-3-position

    should work. You only need two poles. If you can find a 4 pole 3 position version, use two poles in parallel.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

    Comment


    • #3
      Whats the design?

      I got 125B for building a Robinette Slo Micro Naked and decided against using that OT because the bass response drops so much.

      I am going to try a 12BH7 output tube and an Antek MP-15W75 OT which will be at 15k into 8/16 ohms

      Comment


      • #4
        Originally posted by tedmich View Post
        Whats the design?

        I got 125B for building a Robinette Slo Micro Naked and decided against using that OT because the bass response drops so much.

        I am going to try a 12BH7 output tube and an Antek MP-15W75 OT which will be at 15k into 8/16 ohms


        Push pull triodes with a torridal output trans, might not sound like a guitar amp. No low watt designs with a dual pentode?
        I don't have a answer to the OP question, just nit picking.

        Comment


        • #5
          Originally posted by tedmich View Post
          Whats the design?

          I got 125B for building a Robinette Slo Micro Naked and decided against using that OT because the bass response drops so much.

          I am going to try a 12BH7 output tube and an Antek MP-15W75 OT which will be at 15k into 8/16 ohms

          I'm building the micro 800.

          I already have several real full size 2203s and 2204s so I thought this would be a fun little project.

          Comment


          • #6
            Originally posted by loudthud View Post
            Because the power level is so low, a 3 pole 3 position switch such as

            Link: https://www.tubesandmore.com/product...les-3-position

            should work. You only need two poles. If you can find a 4 pole 3 position version, use two poles in parallel.
            I'd like a jack for each though, if possible. Instead of a switch. But I'll do whatever it takes to get 8 and 16 ohm taps.

            My other idea was to use switching jacks. If I could run the 8/16 ohm in-common wire to the tip of one jack and have it shunt to the other jack via switch when not in use. Then I could use the remaining independent 8 and 16 ohm wires to the sleeves of each jack respectively. Does that make sense? Maybe I need to draw it.

            Comment


            • #7
              +1 on what Mozz says.

              NO Guitar amp at any power uses power triodes, way too polite weak sissy no balls.
              You might as well build an SS amp.

              If anything , build a *single* pentode amp low power, say using 6AQ5, ECL82, etc.
              Not a Push Pull, of course, but the real thing: bite, growl, sustain, what the doctor ordered.

              TONS of single pentode amps around since forever, maybe there is a reason for that.

              But .... twin triodes? .... sounds like an armchair_designer/you tube thing to me.

              But ... but ... didn´t Marshall and Bugera add a twin triode 1W amp to their catalogs?

              Yes, most probably because they know they can $$$ell a few, no market niche is ignored.
              That doesn't make them sound better
              Juan Manuel Fahey

              Comment


              • #8
                For a similar project with a 12AU7 in pushpull I used that same transformer and a 3-position 4-circuit Alpha rotary switch using double circuits for safety. I leave you a small diagram below.
                Those types of amps, if well designed and with musical criteria, can sound surprisingly good.
                This is a small sample from a professional guitarist who uses it a lot in his studio.
                (If you do not have a Facebook account, you need to click on the x at the top right to access it and turn up the volume in the video window when it appears).

                https://www.facebook.com/10000082109...6218044415536/

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                Last edited by Pedro Vecino; 09-28-2024, 02:21 PM.

                Comment


                • #9
                  Originally posted by Pedro Vecino View Post
                  For a similar project with a 12AU7 in pushpull I used that same transformer and a 3-position 4-circuit Alpha rotary switch using double circuits for safety. I leave you a small diagram below.
                  Those types of amps, if well designed and with musical criteria, can sound surprisingly good.
                  This is a small sample from a professional guitarist who uses it a lot in his studio.
                  (If you do not have a Facebook account, you need to click on the x at the top right to access it and turn up the volume in the video window when it appears).

                  https://www.facebook.com/10000082109...6218044415536/

                  Click image for larger version Name:	Hammond125.jpg Views:	0 Size:	52.5 KB ID:	1005113
                  Click image for larger version

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ID:	1005114
                  Wow this is cool, thank you.

                  Comment


                  • #10
                    I've recently become aware of another 12AU7/12BH7 (22.5k) OT here
                    https://primarywindings.com/product/...t-transformer/
                    They say its good to 85Hz and it is inexpensive, at less than half the 125B

                    they have a variety of OTs/PTs with big Union Jacks on them so theyre made...?

                    Comment


                    • #11
                      I've used the Hammond 125H with the secondaries mismatched for 2:4/4:8/8:16Ω for a primary impedance of 20k with good results.

                      Comment


                      • #12
                        Originally posted by tedmich View Post
                        I've recently become aware of another 12AU7/12BH7 (22.5k) OT here
                        https://primarywindings.com/product/...t-transformer/
                        They say its good to 85Hz and it is inexpensive, at less than half the 125B...
                        The sellers text just says "Typical bandwidth is 85Hz – 6kHz." That statement leaves a lot of unknowns. To properly compare the performance to the Hammond, you would need see the actual response curves and know the measurement conditions. Historically, manufacturers would specify bandwidth at the -3dB points. However, I have recently seen specs that used -10dB points. I'm also curious why the upper limit of the transformer you found is only 6kHz. Although it could be argued that higher frequencies are not needed for a guitar amp, the relatively low HF spec seems to me like an indication of a lower quality transformer.

                        Comment


                        • #13
                          If I want independent 4,8, and 16 ohm jacks, I need wires 2+4 for 8 ohm, and wires 1+4 for 16 ohm, can I just run #4 to a jack and put a switch on the #4 wire between two jacks?
                          Attached Files

                          Comment


                          • #14
                            Originally posted by Tom Phillips View Post
                            The sellers text just says "Typical bandwidth is 85Hz – 6kHz." That statement leaves a lot of unknowns. To properly compare the performance to the Hammond, you would need see the actual response curves and know the measurement conditions. Historically, manufacturers would specify bandwidth at the -3dB points. However, I have recently seen specs that used -10dB points. I'm also curious why the upper limit of the transformer you found is only 6kHz. Although it could be argued that higher frequencies are not needed for a guitar amp, the relatively low HF spec seems to me like an indication of a lower quality transformer.
                            on the opposite end of the spectrum, this Lundahl OT has impeccable specs! ; )
                            https://www.don-audio.com/Lundahl-LL...ut-transformer

                            this one (from these guys https://www.musicalpowersupplies.com/) is more reasonably priced https://www.ebay.com/itm/16695621999...Bk9SR7aW4ovLZA

                            I've asked for frequency specs.

                            that offer a few types
                            Click image for larger version

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                            Last edited by tedmich; 10-04-2024, 07:38 PM.

                            Comment


                            • #15
                              Originally posted by Greg_L View Post
                              If I want independent 4,8, and 16 ohm jacks, I need wires 2+4 for 8 ohm, and wires 1+4 for 16 ohm, can I just run #4 to a jack and put a switch on the #4 wire between two jacks?
                              Per your diagram, you have shorted the secondary pins #1 to #2. That's critical problem.
                              It looks like your thinking is to use the switch shown to connect the 16 Ohm jack but the circuit leaves the 8 ohm jack connected all the time.

                              I'd still recommend that you re-consider using a multi section rotary switch to select your output impedance.
                              What do you think?

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