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Thread: Hammond Tone Cabinet Reverb Drive Transformers

  1. #11
    ppp Pianississmo 2Tracker's Avatar
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    Well fortunately, the crystal element is intact! It was a bit fussier ascertaining this than I hoped, but treating the 2-pin output as "hot, neutral, ground" did the trick (for whatever reason I assumed otherwise). So, good news!

    Now I have to figure out a simple circuit around the tank. I haven't dug into the circuits you suggested yet, David, and while I'm assuming they service other parts of the organ (or are otherwise overly complex for my needs), I'm open to learning otherwise.

    I've looked over this document a bit too: http://theatreorgans.com/hammond/faq...TSBs/TSB49.pdf

    which details replacement of the original oil-damped reverb with a smaller unit, but I'm thinking this isn't really suited to my needs.

    This unit will be used in a recording studio, so unbalancing and balancing transformers on the input and output should do the trick.

    Thanks, again, for the truly helpful tips and advice!

  2. #12
    fff Fortississimo David Anderson's Avatar
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    2 members found this post helpful.
    As you've already ascertained, the spring drive transducer is basically an 8 Ohm speaker without a cone. Hammond drove it with a 6SN7 running push-pull into an output transformer.

    Since Hammond was a proponent of balanced audio, the crystal pickup unit plugs into a balanced gain stage for recovery. The earliest recovery stages used a pair of 6J7 or 6SJ7 pentodes, but for the rest of the oil-tube reverb run, they used a single 6SC7 dual triode, somewhat similar to a 5751 miniature dual triode. The circuit is very simple.

    So, the output of the crystal pickup would be Signal +/Signal -/Ground, like you'd see on an XLR connector.

    HR-1.jpg
    I'm David. 'Dave' is someone else's name.

  3. #13
    ppp Pianississmo 2Tracker's Avatar
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    Thanks David, this is most helpful! So I'm inferring that the 6SN7 receives more or less "line level" signal from a preamp elsewhere. A Google search doesn't immediately turn up specs for the transformer driving the reverb but I suppose I can extrapolate from the fact that it's being driven by triodes in push-pull.

    For that matter, I suppose I could also build a generic Fender (or Ampeg, or whomever) style circuit around it too. In your experience, is there anything unusual about the Hammond setup that would require specific treatment?
    Last edited by 2Tracker; 03-20-2017 at 03:27 AM.

  4. #14
    fff Fortississimo David Anderson's Avatar
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    This is Hammond elementary level information, but you are new to this.

    To keep the signal/noise ratio low when driving a tone cabinet via a 40 foot long cable that also supplied AC power, Hammond's original 1935 design involves the preamp in the organ sending a balanced-line signal to the power amplifier that's considerably hotter than what is now standard line-level, so the input to the reverb driver is fairly high.

    I can test one of the reverb drive transformers for you, but you are correct that there's nothing particularly unusual about the circuit. That the output of the crystal pickup was intended for a balanced recovery stage may or may not end up being significant.

    What you may need to do some experimenting with is what kind of damping oil to use in the tubes. I've read a number of suggestions, and I've tried one type of mineral oil, but wasn't thrilled with the results. I tend to think that a very light oil might yield better results. That's where I'd personally like to know more. I know about these things, but I don't get to work with them directly very often. I've never owned one.
    I'm David. 'Dave' is someone else's name.

  5. #15
    Moderator Wes's Avatar
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    Where did you source the mineral oil, David? Do you know the viscosity? Did it seem thicker or thinner than Hammond generator oil? Typical drug store mineral oil comes in three viscosities, but usually they only have "medium" on the shelf.

  6. #16
    ppp Pianississmo 2Tracker's Avatar
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    Quote Originally Posted by David Anderson View Post
    ...Hammond's original 1935 design involves the preamp in the organ sending a balanced-line signal to the power amplifier that's considerably hotter than what is now standard line-level, so the input to the reverb driver is fairly high.
    Interesting....by comparison, I believe the “standard” Fender approach is to use a 12AX7 with linked plates to drive the reverb. Just on paper, I’m thinking this is a lot more gain than a 6SN7 in similar configuration, but if the Hammond preamp is already providing a higher gain….


    I’m trying to puzzle out what would be best in a +4 dBm studio environment, and whether or not the 6SN7 will provide sufficient gain to drive the reverb. I could add a gain control, but I’m thinking it’s not best to place this in front of the first gain stage (in this case, the 6SN7 or 12AX7) for noise considerations….

  7. #17
    fff Fortississimo David Anderson's Avatar
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    Quote Originally Posted by Wes View Post
    Where did you source the mineral oil, David? Do you know the viscosity? Did it seem thicker or thinner than Hammond generator oil? Typical drug store mineral oil comes in three viscosities, but usually they only have "medium" on the shelf.
    I guess I got "medium" from the drug store.
    I'm David. 'Dave' is someone else's name.

  8. #18
    mf Mezzo-Forte MihevicB3's Avatar
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    There are a total of 5 springs in the Hammond fluid reverb system.

    The 3 springs in the long oil filled tubes are just suspension springs that support the whole system. Oil in these tubes isolates the suspension system from the other two springs that are in free air. For these long tubes, Most of the damping fluid is required by these long springs. I would not be too concerned about the type oil used in these tubes. I have seen Type A transmission fluid or mineral oil as recommended substitutes for Hammond damping fluid.

    The two free air springs are the reverberation signal springs. One spring goes to a crystal pickup (via a short ribbon strip) and the other spring goes into a short tube that has damping fluid . In operation, a first direct signal goes to the voice driver at the top. Immediately, the signal is transferred to a “lever” where it travels down both springs. The “lever’s” primary function is a signal direction changer. The first signal to hit the crystal pickup is delayed 1/15 second. That signal also hits the fluid in the small damping tube at the same time and is reflected back up the spring with a little less energy, through the “lever” and down to the crystal pick up. These reflections occur 4/15 second apart and continue until all the energy is dissipated by the fluid in the short damping tube. Changing the level in the short tube will change the true delay time of the repeated reflections. Less fluid means produces a longer delay time. Note that no spring unit (to my knowledge) has this feature. The spring necklace and tank units have fixed delay times determined by the spring construction. Another vintage unit, built in the ’60’s, was the Schober “Reverbatape”. It consisted of a continuous tape loop, a record head and 3 playback heads. A 3 gang adjustable pot that attenuated the signal from the playback heads adjusted the delay time.

    In my opinion, it is the fluid in this short damping tube that is critical. Hammond Tone Generator Oil can be used as a substitute for the damping fluid, however, its viscosity is higher.

    I did a crude viscosity test. See my pic below of my “lab” setup. I still have the damping fluid I ordered from Hammond years ago. For the test, I used the 4” medicine dropper you see in the pic. It holds 1/8 fluid oz. I filled it first with Hammond Damping Fluid (the “real stuff”) and then with tone generator oil. I timed each until the dropper was empty. The Damping Fluid took 3.1 sec to empty and the TG oil took 4.8 sec. Just by pouring, and the feel between my fingers, I could tell the Damping Fluid was thinner.

    John M.
    P1020640.jpg
    1956 Hammond B3
    1963 Leslie 122
    Two Pr40ís
    One JR-20 (for fluid reverb signal)
    Hamptone LEQ3B
    Trek II Reverb
    Trek II String Bass

  9. #19
    Moderator Wes's Avatar
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    I googled "how to measure viscosity" and found this experiment:

    http://www.wikihow.com/Measure-Viscosity

    Then I had a flashback to 1992, PHYS 105 - we measured the viscosity of glycerine with this exact method. Did not expect that memory. I miss physics lab.

    Wes

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