Just noticed this thread while searching for something else. Here’s how a Hammond works.
There are a bunch of “tone wheels” driven by a single motor on a complex gear train designed to approximate a 12 tone equal tempered scale (but it’s not quite exact due to the integer math of gear teeth – coming up with this was the real miracle of the Hammond organ). Every harmonic the Hammond makes comes from one of these wheels. The wheels are bumpy around their circumference and signals are picked up by a small, guitar-like magnetic pickup above each one. (The number of them varies on different Hammond models. I think 96 tone wheels is a typical example). Those pickups output high impedance (so very low voltage and current) signals just like an electric guitar does. They are not amplified yet.
Each key has 9 contacts corresponding to the 9 drawbars. These contact 9 “bus bars” beneath the key that I’ll get back to in a moment. Each of the tonewheels is wired to the appropriate contact on all keys that need that harmonic. So the same tonewheel gets used by many different keys in different bus bar positions. For one key it a particluar tonewheel might be wired to the fundamental, so it’s on contact/bus bar 3 (for 8’). For a key an octave down, that same tonewheel would get wired to contact 4 (for 4’) as it’s now the 2nd harmonic. Etc.
When you hit a key, all 9 contacts close. (Though not all at the same time. This is why Hammonds have a kind of reverse touch sensitivity, where the click is more prominent the slower the velocity the key is hit with. In fact, you can slowly lower a Hammond key with all the drawbars pulled out, and distinctly hear each harmonic come on one at a time, in no particular order.) But you only hear all 9 harmonics if all 9 drawbars are pulled out. Those drawbars are volume controls (again, completely passive like an electric guitar) for each of the bus-bars, which together carry the 9 signals from the combination of all the depressed keys. The drawbars mix how much of each bus bar’s signal we hear. Again, entirely passively.
Only then does this high impedance signal go to the pre-amplifier input. Some people, like Keyboard Mag’s editor Steve Fortner, will tell you there’s hundreds of volts under a Hammond keyboard on the bus bars and the key click comes from some kind of electrical arcing, which is of course completely ludicrous. There are high voltages in a Hammond, but only inside the amplifier chassis. If you touch the bus bars of a running Hammond all you’ll get is a really loud hum (like you touched the tip of a guitar cable going into an amp).
So the drawbars and keyboard together as a mixing matrix that selects amounts of the 96 wheels, which are all running, all the time. (And always locked in the exact same phase relationship to each other. Another important and unique aspect of the Hammond sound and part of why a Leslie sounds good on it, but bad on sampled or synthesized Hammonds that don’t model the locked phase.)
The percussion feature steals the 9th bus bar/drawbar and uses it instead for a trigger for the envelope of the percussion (which yes, is polyphonic, but single triggered). You can’t play the top (1’) harmonic and use percussion at the same time. (Apparently some people modify the percussion to use drawbar 8 (1 1/3’) instead of 9 as it’s less useful than the “whistle” at 1’.
Most of that design is lifted straight from a classic pipe organ, just with air pressure translated to voltage. But again, it was the fudged math to approximate the correct musical pitches with whole number ratios (because you can’t have a fractional gear tooth or tonewheel bump) that was the major technological innovation. Laurens Hammond, a complete non-musician, was also looking for another place to sell his company’s AC motors that synchronized to the line frequency. Previously all they made were clocks but music was deemed another application that could use a motor with accurate timing (at least as accurate as the generators at the power plant).
As for MIDI modding a Hammond for a MIDI out, you’d have to put new switches under the keys and run those to a MIDI interface. The existing key contacts are all tied up (and aren’t set up at two different levels to provide you with velocity timing) and can’t be made to do double duty as audio signal is actually running through them. To modify it with a MIDI in would be MUCH more work – you’d need to replace all 122 key contacts with relays controlled by a processor!
