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BC 2 tube preamp modifications, and nuances

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  • BC 2 tube preamp modifications, and nuances

    I've got a 2 tube preamp in for a bit of work. It came out of a 7,000 range serial number Hammond BC, but I do not know with certainty if the preamp is original to this organ. This thread seeks to answer two questions.

    1. This preamp has a non-original 50uf 25v capacitor attached to the cathode / grid3 (pin 4 and 5) of the 57 tube, the other side of which is attached to ground. Was this a common modification, perhaps done to increase bass response?
    2. There is a choke or transformer mounted between the 57 and 56 tube. Part of this is connected, but part of it is not. Half of its wiring is cut off. While I can tell this is a somewhat common component looking at photos of these preamps, none of the schematics I have of this preamp indicate what this is for. Is it a choke intended to aid with the power supply, or a transformer used as a coupling for signal?


  • #2
    Are you sure it's a choke or transformer there and not a chassis-mounted oil capacitor? That would explain why it's wired between the two tubes. Could it be C13?

    I'm not sure about the cathode bypass capacitor. The very first two-tube preamp did have electrolytic cathode bypasses, but they were deleted in later versions since Hammond seemed determined to avoid using electrolytic capacitors to the extent possible in these early preamps. Have you verified which of the five versions of the early preamps this is? By SN, it should be the fourth.
    I'm David. 'Dave' is someone else's name.


    • #3
      Thank you David. There is blue pencil markings dating it at 11/13/37.

      It has a Webster Electric label inside, serial number 25311, type 6121A2. (this last number is a little hard to read.)

      I stand corrected about the transformer being half-connected. It is not connected to anything, except for one wire attached to the 57 tube metal base (so… not really ground but the isolated ground of that metal plate.)

      There is indeed a very tall oil can capacitor, two 4uf caps. These caps are right on the money, but they each have about 27 ohms ESR, so I do not know whether it's wise to remove the new 4uf axials someone put in and re-attach the old oil can.

      I printed out all 5 variants of this preamp to compare. I seem to have pieces that are borrowed from three different series of the preamp (I will call them series 2, 3, and 4.)

      The 50uf 25 cap on the cathode of the 57 tube appears only on the Series 2. However the resistor in parallel with mine is 380 ohms, which is the one from Series 3 and 4.

      I have a 0.1uf cap that is shown as C6 on the Series 2 and Series 4 preamp.

      I have 10k resistors attached to pin 3 of the 57 tube, which are represented on the Series 3 preamp only. I also have a 40k resistor across the 4uf capacitors, a feature of the Series 3 and 4 preamp.

      I have a 100k resistor attached to B+, a Series 4 feature, and a 35k resistor attached to that, a feature of the Series 3 and 4.

      If you take all of this into consideration, I have a combination of Series 3 and 4 preamp, with a 50uf capacitor unique to the Series 2.

      Attached Files


      • #4
        To start off, none of the capacitors in the picture is original. Some are from the 1960s, and some are more recent, so this may have been worked on multiple times, or someone was using old parts. Only the resistors are original. We can't assume that whoever installed the capacitors followed the original circuit exactly.

        In terms of the various versions, the big difference between Version 3 and 4 is that on 3, the 57 screen voltage is determined by a resistor divider chain (10k), while on 4, it's now determined by a 2M resistor and the current draw of the screen. And sometimes schematics are not entirely accurate in terms of when running changes occurred.

        In technical terms, I tend to like the 4 & 5 circuits better on paper, and 5 has the filaments elevated to reduce him (see center tap of filament secondary). On 4 & 5, the 57 screen is bypassed to ground via a capacitor, which will shunt resistor noise to ground.

        At a certain point with these, you have to decide whether it makes more sense to rebuild or to go with a Trek II SSP-1, which also has an onboard Leslie 6H connection and does not require external B+. You know I'm kind of a tube purist, but we are dealing with 1930s transformers in these.

        Also, just thinking out loud, in certain ways, Circuit 1 is kind of the cleanest, more like what I'd design if given this task, and I think a lot of the later changes were made to avoid using electrolytic capacitors. That part is obvious, but were electrolytics unreliable at the time or expensive?? We don't really have to worry about that today.

        Edit: Bypassing a tube's cathode to ground is not only done to increase stage gain; it can also be done, in some cases, to reduce hum by shunting any 60Hz hum that might be picked up by the cathode from the heaters to ground. It's a balancing act. You might boost gain and increase one kind of noise, but eliminate another.
        I'm David. 'Dave' is someone else's name.


        • #5
          Thank you, David. I knew the resistors were probably original, but the rest of the caps are a mixed bag from later.

          If I move forward with a complete rebuild, I'll probably stick to version 4, since it more closely matches the date of the organ. I do not know whether it makes sense to keep the 50uf cathode bypass capacitor on the 57 tube in the design. It was never really intended to be part of any of the designs while in parallel with a 380 ohm resistor.

          We've discussed the qualities of the oil tub capacitors at length on this forum. The stock oil can has accurate capacitance with matching dissipation and no measurable leakage at 400V DC, but should the ESR being 27 ohms be a good reason to not use it any longer?
          Last edited by muckelroy; 05-03-2019, 08:47 AM.


          • #6
            I had some documents detailing the five versions of this preamp. I'll try to find a way of sharing it. I bought a version two last year and flew it to the UK only to find the output transformer was dead as a door nail.
            Hammond C3, M102, XB3, XB5, X5
            Lowrey Heritage DSO-1, H25-3, Yamaha E70
            Farfisa Compact Duo Mk2, Vox Continental 300, Korg BX3 Mk1, Leslie 122, 145, 910, 415


            • #7
              The ESR finding is curious. My impedance bridge tests capacitors at 1kHz, while my Anatek Blue ESR meter tests them at 100kHZ. Capacitors do vary in capacitance with frequency to some extent. That's well known.

              It is possible that via some mechanism I can't explain, those capacitors are still good at audio frequencies, but not at frequencies approaching the low end of the RF bands. A band-aid for that would be to parallel a large enough film capacitor to smooth out the frequency response, but oil caps I've measured in Hammond amps from the 1940s do test good with my ESR meter. I have no idea how the caps in a 1930s preamp would have tested when new, but not having low impedance at RF could result in oscillation.

              The interesting -- or weird part -- about having 4uF capacitors as B+ filters is that they will have about 6.6Ω capacitive reactance @ 6kHz, but 1.3k @ 30Hz, which is a big enough figure, potentially, to affect the response curve. Might be interesting to measure. They were also doing something odd with the bass frequency response of the 56 triode in the later designs with those two 1.0uF capacitors. Those capacitors couple a reduced output from the 57 to the 56 cathode, but then roll off the highs, which I think should limit the bass response. Remember as we've discussed before that the Hammond tone cabinets of that era probably couldn't handle really deep bass.

              That very first preamp design looks like it should be pretty flat in frequency response; they were definitely trying to play with the bass response in the later versions. But why?
              I'm David. 'Dave' is someone else's name.


              • #8
                I bought a 1940 model D last year that came with a few extra 2 tube chassis. I seem to recall one or two of them having an extra transformer on top, similar to what I am looking at here. I'll have to go find those and see what they're attached to. It's off site in storage, currently. The next time I have those chassis in my hand I'll photo document them to see what they're doing.

                The owner of this organ seemed rather happy with the way the organ sounded, so I don't want to drastically change the design. I do think adhering to one of the 5 designs would be a good idea if any work is done. After all, the tone control pot is there for a reason, and this organ does have bass all the way down. It will probably sound amazing no matter what preamp design is used. Your note on the 1.0uF capacitors possibly rolling off highs and limiting bass response is interesting though.


                • #9
                  Originally posted by muckelroy View Post
                  Your note on the 1.0uF capacitors possibly rolling off highs and limiting bass response is interesting though.
                  To clarify, I think those capacitors have little effect on the highs -- because 1.0uF in parallel with a 2.5k resistor is what's called a partial cathode bypass, preserving some negative feedback at lower frequencies. C13 is the odd part. I don't know exactly how to calculate what it does, but it might be interesting to sweep the preamp with a signal and observe the overall gain at various frequencies.

                  If the owner likes how it sounds, I don't suppose it would hurt to replicate what was done to it. Did someone defeat C13 and fully bypass the 56?
                  I'm David. 'Dave' is someone else's name.


                  • #10
                    Very interesting discussion. Thanks for sharing, guys!


                    • #11
                      After thinking about it, I'm going to backtrack on one of my points above in terms of the circuit around the 56 tube. The network of those two 1uF capacitors is an odd one that I've never seen in any other tube circuit. I now think that the point of it is to try to approximate the effect of a full cathode bypass on the 56 without using an electrolytic capacitor. In other words, any AC signal at the 56 cathode (which will be in the bass region) is coupled back to the grid circuit in phase with the grid signal.

                      Like I said, I've never seen that done that way in any other tube circuit, but then, I don't know of any other manufacturer other than Hammond that was so determined, until the 2-series, to avoid using any electrolytic capacitors in its preamps.

                      But that reinforces one point I made above -- that, from a modern point of view, Hammond's first version of the two-tube preamp makes the most sense. It's a simple, standard pentode gain stage driving a simple, standard triode stage. I think the later versions were efforts to do the same thing without using electrolytic caps, like shunt-regulating the 57 tube's cathode voltage with a chain of voltage-dividing resistors instead of using a single bypassed cathode resistor.
                      I'm David. 'Dave' is someone else's name.


                      • #12
                        Thank you David. I know that my list of "modifications" seems complicated, but that was mainly because of the existence of two 10k resistors (which resemble R61 and R62 on the 3rd version). There is nothing connected to the junction of these resistors, so really, those are acting like a single 20k resistor, acting like R72 on version 4.
                        What I have really is version 4, with an added 50uf capacitor in parallel with the 380 ohm resistor (so attached to the cathode of the 57 tube.)


                        • #13
                          It just occurred to me that the "choke" in between the 56 and 57 tube is actually a dual 1uF capacitor. It must be an oil filled capacitor. I don't know why my dense brain too so long to come to that realization.


                          • #14
                            Originally posted by muckelroy View Post
                            It just occurred to me that the "choke" in between the 56 and 57 tube is actually a dual 1uF capacitor. It must be an oil filled capacitor. I don't know why my dense brain too so long to come to that realization.
                            Scroll up to my first reply in this thread :-)
                            I'm David. 'Dave' is someone else's name.


                            • #15
                              Right. For some reason, in your first reply, I thought you were talking about the dual 4uF can capacitor. That would explain why some preamps have this, and some don't. The first series I suppose does not.

                              Edit: The preamp is reinstalled back into the organ now. I should probably mention here what the problem was in the first place. I had a very stubborn POP when switching speeds. The problem was not being caused by the Leslie, or by the Leslie kit. The arc caps in the Leslie were new. On the bench, with a known-good Leslie 8000 box attached to it and an International 200V regulated power supply used for B+ voltage, with no Leslie connected to the kit, the pop still occurred when the speed switch was toggled. (Yes, the 10uF cap in the kits were new, and every resistor in the kits verified, and solder joint re-flowed.)

                              Every capacitor in the preamp was replaced, as well as the 4uF caps swapped with dual 22uF caps. I increased the value of C6 to 0.3 uF, and tested with the 50uF capacitor on the cathode of the 57 tube removed (to make it identical to the Series 4 in every way) but the pop persisted.
                              -- The only fix I could find was a 0.1uF across the tremolo switch. I don't like doing that, because if you ever want to install a solid state relay into the Leslie, it won't work properly with a cap across the switch like that. However for now, it works fine with a traditional relay.

                              The pop issue did persist with both tubes replaced.

                              The only thing that was left to do was a complete rebuild, which would have replaced the resistors. That may fix it here, but the owner was not willing to go that far, and I was on a time crunch to get this thing finished. So I didn't have time to find my extra 2-tube chassis to compare and diagnose further. I cleaned up the preamp, kept the replacement capacitors in play, and installed everything back, with a 0.1uF cap across the tremolo switch. The organ sounds amazing, and everyone is happy.

                              I did replace the 56 tube, as the old one was rather weak. The original 57 tube was still strong, even stronger than my NOS replacement 57 tube.