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Each Hammond console organ key has 9 thin metal leaf switches, each connected to the output of one tonewheel pickup corresponding to the nine harmonics for that key. There are 9 conductive busbars, stacked vertically and insulated from each other, running underneath each keyboard. They are arranged from the Sub Fundamental (16') on the bottom busbar to the 8th harmonic (1') on the top in the same order as the drawbars are arranged from left to right. As the key is pressed, each switch connects the proper tone for that harmonic on that key to its corresponding busbar. The busbars are then connected to the drawbars, which control the level of each harmonic. If a drawbar is pushed all the way in, any switch on the keyboard which closes on that busbar will not include that harmonic in the tonal mix. When percussion is used on the upper manual, the top busbar (8th harmonic), is disconnected from the drawbar and used to key the percussion circuit in the preamplifier. For a detail description of the busbars and key switches see patent 2,099,204.

Because there are nine switch contacts under each key and the key rotates through an arc, all nine switches do not close simultaneously. They close somewhat sequentially (but not necessarily in the same order as the busbars are stacked). Which means if a key is "bounced" or "slipped" and not depressed to close all its switch contacts to their busbars, some harmonics may not be heard. In addition, the switches "bounce" a bit when the contacts close depending on the velocity that the key is pressed and this also affects the sound. These "defects" actually enhances the organic sound of a tonewheel Hammond. Most clonewheel Hammonds lack these "features" which is another reason they sound somewhat sterile. For a technical study of the Hammond key switching see Dynamic Temporal Behaviour of the Keyboard Action on the Hammond Organ and its Perceptual Significance by Giulio Moro, Andrew P. Mc Pherson, and Mark B. Sandler in The Journal of the Acoustical Society of America, November 2017:

https://asa.scitation.org/doi/pdf/10.1121/1.5003796https://asa.scitation.org/doi/pdf/10.1121/1.5003796

The pedalboards on the console organs are similar but there are four busbars and each pedal closes two switch contacts on each busbar. This "forces" a certain harmonic mix on the pedals that can't be altered as readily as the manuals. The four busbars are mixed by a fixed resistor network and harmonic levels are adjusted by just two drawbars. One drawbar adjusts the level of the Fundamental and 3rd Harmonic (basically odd harmonics) and the other adjusts the level of the remaining harmonics (even harmonics, 2nd through 12th). Note that the lowest octave of tonewheels on the console organs are not "hobbed" as pure sine waves but are more complexly shaped to add odd harmonics to these tones. For a detailed description see patent 2,508,514.

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