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31H Type 2 Field Coil to PM conversion

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  • 31H Type 2 Field Coil to PM conversion

    Hey all, it's been a while.

    My dad and I are working on finally getting one of our garage Leslie projects fixed up and out of the garage. This one is a chopped 31H. We're planning on using the original 31H Type 2 amp, but we don't want to give up our last field coil speaker. I'm looking for options to replace the FC.

    The two options I'm currently thinking of:
    1. Replace the FC with a high wattage resistor
    2. Replace the FC with the FC from my M2


    For Option 1, I've seen in M2 threads to replace the FC with an equal value resistor (looks like 2650 ohm) of at least 25 watts. Is this wattage right for the 31H amp?

    For Option 2, I have a field coil speaker from an M2 that died on us years ago. Via the schematic, it looks like this speaker has a 700 ohm coil. This is a good bit lower than the 2650 ohm coil in the 31H. Because of that difference, I'm a little wary about dealing with it, since I assume I'd still need a resistor to make up the resistance.


    Thoughts from the crowd? Anybody have experience using a resistor with the 31H amp?
    Keyboards: 1972 Fender Rhodes Stage 73, M-111, M-3, A-100, M2 Desk(!), B3!
    Spinny things: Wurlitzer Tone Cabs (500 and 420), PR-40, 31W, 31H, chopped 31H, 125 (empty cabinet), 30A!!!

  • #2
    The field-coil in a 31H setup dissipates around 30W continuously, I'd get a 50W resistor ~3k. The field coil increases in resistance at it heats up. One Leslie manual states that it can rise to 3,100Ω hot. (As copper wire heats up, its resistance increases.) If going this route, mount it very carefully. I have seen situations where a load resistor almost set the cabinet wood on fire.

    It should also be possible to re-bias the amp to operate without a load resistor -- if you knew what you were doing.

    The 12" M2 FC speaker would not be a suitable replacement for any number of reasons. It's not rated for that much power. It's a full-range speaker, not a woofer. It simply won't move as much air as a 15".
    I'm David. 'Dave' is someone else's name.

    Comment


    • #3
      The resistor 2650 ohm will work fine then use a good quality 15 in woofer type speaker , I had no problem with mine, I also found that at least the series 3 amp will run without it, but I still used the resistor and had no problem.
      Frank

      C-3, solovox 2 hr-40 Hammond tone cabinets a 31-H leslie and a 21 -h .

      Comment


      • #4
        I'm not familiar with this circuit, but many amplifiers use the field coil as a power supply choke. In that case, it should be replaced with a coil of similar inductance to the original field coil. In this arrangement the inductance is usually more critical than the resistance.

        Bear in mind that technically due to power supply variations that happen due to audio amplification, a permanent coil speaker should never sound exactly like the electromagnet coil speaker it replaces.

        Comment


        • #5
          Originally posted by KC9UDX View Post
          I'm not familiar with this circuit, but many amplifiers use the field coil as a power supply choke. In that case, it should be replaced with a coil of similar inductance to the original field coil. In this arrangement the inductance is usually more critical than the resistance.
          I'm an expert on this circuit, and the coil does not function as a power supply choke in the 31H amp. I know it does in some guitar amps, but not in this case. The 31H amp has a separate power supply choke.

          - - - Updated - - -

          Originally posted by Baldwin51 View Post
          The resistor 2650 ohm will work fine then use a good quality 15 in woofer type speaker , I had no problem with mine, I also found that at least the series 3 amp will run without it, but I still used the resistor and had no problem.
          Though the amp will run without the field-coil resistance in place, the output tubes will be running too hot because the field-coil is in parallel with the output tubes and sharing the same cathode resistor.
          I'm David. 'Dave' is someone else's name.

          Comment


          • #6
            Thanks for the responses, y'all! High-wattage resistor it is.

            David, mostly out of curiosity, what would rebiasing the amp entail? I don't have the chops to know what to change to rebias it, but I definitely have the chops to do modifications if I know what they are.
            Keyboards: 1972 Fender Rhodes Stage 73, M-111, M-3, A-100, M2 Desk(!), B3!
            Spinny things: Wurlitzer Tone Cabs (500 and 420), PR-40, 31W, 31H, chopped 31H, 125 (empty cabinet), 30A!!!

            Comment


            • #7
              Rebiasing would likely include changing the cathode resistor value and compensating for the absence of field-coil current in setting the 6L6 screen voltage. And you'd have to watch that the B+ voltage didn't exceed the limits of the power supply capacitors. Plate voltage on modern 6L6GCs can go higher than the original 6L6Gs the amp was designed around with no problem, up to around 500V, as long as you keep the screens lower. This approach would likely increase the output power of the amp somewhat.

              Of the 333 milliamps of cathode resistor current, a bit over 100mA of that is used for the field coil, so, basically, you have to recalculate the operating points of the output stage, taking into account that reduced draw from the power supply will, in and of itself, raise B+ voltage. You have to keep all these parameters in mind and come up with a solution that works from all angles.

              My personal recommendation is a chassis-mount style resistor mounted with thermal paste on an appropriate heat-sink, which can be just a piece of aluminum. The dimensions of an appropriate heat-sink are given on the resistor datasheets, as in how many square centimeters it needs to be. My reasoning for that is that with the larger heat-sink area, no one point gets that hot. A smaller resistor dissipating 30-35 Watts will get very hot on its surface.
              I'm David. 'Dave' is someone else's name.

              Comment


              • #8
                Awesome. Thanks for the help!

                Hoping to get started on this soon. I'll post pictures as they come (and any other questions :P )
                Keyboards: 1972 Fender Rhodes Stage 73, M-111, M-3, A-100, M2 Desk(!), B3!
                Spinny things: Wurlitzer Tone Cabs (500 and 420), PR-40, 31W, 31H, chopped 31H, 125 (empty cabinet), 30A!!!

                Comment


                • #9
                  Yes sir you are 100 % right, I never would give up my field coil speakers because they sound better, only reason I mention this is because a few years ago I bought a Leslie which had a missing 15 in field coil speaker . I built a 32 series amp for it and experiment with the speaker, later replacing it . hard to find now days. at the end of the day I like things set up like they are suppose to be. I like the sound of both of my leslies , 21-h with 30 amp 5881 tubes and the same amp in my 30 -h which is the best sounding Leslie I ever seen. I have no idea why as I have owed several tall leslies and none sounded like this one with a super mellow sound you just love every time you pass over that sweet spot. I can tell David that you are a experienced tec. have a great day

                  Comment


                  • #10
                    Originally posted by torea View Post
                    Hey all, it's been a while.

                    My dad and I are working on finally getting one of our garage Leslie projects fixed up and out of the garage.
                    I was hoping your Leslie project was a taller, older cabinet. Good luck with the 31H shorty project.

                    Comment


                    • #11
                      Originally posted by markm View Post
                      I was hoping your Leslie project was a taller, older cabinet. Good luck with the 31H shorty project.
                      Ooops, totally missed this reply. If I'm thinking of the same taller, older cabinet you're thinking of, it's still on the docket. Current sitting in my dad's garage with the parts all stacked up in it.

                      One of these days...
                      Keyboards: 1972 Fender Rhodes Stage 73, M-111, M-3, A-100, M2 Desk(!), B3!
                      Spinny things: Wurlitzer Tone Cabs (500 and 420), PR-40, 31W, 31H, chopped 31H, 125 (empty cabinet), 30A!!!

                      Comment

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