The sound produced by early HammondOrgans differed from pipe organs in one characteristic way. There was an attack transient that sounded like a click or pop when a key was pressed. This was considered a defect and was caused by the audio signal being routed directly through the key contacts. As a key was depressed, the nine contacts under the key closed against their respective busbars at slightly different times and ''bounced'' as they closed. The sine waves from the constantly running generators would be connected at random points in their oscillation.
Considerable design efforts were made to reduce it but it could never be eliminated. In the patent, key click was to be removed by a ''bypass''. In later console organs like the B3, the higher harmonics were pre-emphasized and the preamp designed with an upper frequency roll-off to help conceal the click.
Later rock and blues players found the key click characteristic to be desireable and some jazz organists consider it to be essential. Many Hammond organ simulators include a key click control to reproduce this characteristic.
For a detailed analysis of key click and the interaction of the key contacts on busbars see: https://asa.scitation.org/doi/pdf/10.1121/1.5003796
+
+
+
+
+!! Additional info from KonZissis:
+
+Whilst dirty key contacts or busbars and also deteriorated grounding connections can enhance the key click, the key click is always present even with perfectly functioning and clean key contacts and busbars and perfect grounding connections so therefore Laurens Hammond and the subsequent Hammond designers tried to suppress the key click sound right from the start in 1935 until the end of Hammond tonewheel production in 1975.
+
+Hammond reduced the key click by calibrating the tonewheel generator to have the "pre-emphasis" in the midrange and the treble TG notes which meant that the midrange and the treble TG notes were set to have a progressively upward rising output curve in their output levels.
+
+The secondary winding of the drawbar matching transformer on the early era organs of the 1930's and 40's such as the Model A, AB, B, BA, BC, BV, CV, D, E, G and the RT organs had a very simple treble cut filter consisting of a 0.001 uf ( 1 nf) capacitor wired in series with a 20K or 22K resistor between the output wire of the secondary winding and the ground.
+
+This 20K or 22K resistor/ 0.001 uf capacitor treble cut filter reduced the key click by rolling off the treble response, as well as bringing down the levels of the "premphasized" midrange and treble TG notes so that their volume levels through the preamplifier were better balanced with the lower frequency TG notes.
+
+The B2, C2, and the RT2 organs with the AO10 preamplifier, and the A-100 series, B3, C3, D150 series and the RT3 organs with the AO28 preamplifier and the M3 and the M-100 series organs with the AO29 or the AO67 amplifier used a treble cut filter consisting of very small value capacitors in series with very high value resistors wired as part of the negative feedback circuit between the input grid and the output plate of the input tube of the non vibrato channel and the input tube of the vibrato channel.
+
+The other tonewheel organ models including the E-100 series H-100 series, L-100 series, Porta-B, R-100 series, T-series, XTP and the X-77 organs also had treble cut filtering in the amplifiers acting as the "key click filter", and the treble cut / key click filtering on these particular organs was more severe than what it was on the previously mentioned organ models.
+
+A lot of us Hammond enthusiasts love the key click sound so therefore we like to undo Hammond's attempts to reduce the key click.
+
+If you have a copy of the Hammond organ schematics or the service manuals, and if you are competent and confident to work on Hammond organs, and especially when it comes to the organ preamplifiers with the dangerous high voltages inside them, then you can increase the key click effect by disconnecting the treble cut / key click filter.
+
+On the 1930's/ early 1940's Model A, AB, B, BA, BC, C, D and E organs, you can disconnect either the C4 0.001 uf capacitor or the R4 20 K resistor which are located inside the rheostat box.
+
+On the 1940's BV, CV, G and RT organs you can disconnect either the C1 0.001 uf capacitor or the R1 22 K resistor which are located inside the rheostat box.
+
+On the B2, C2, and RT2 organs with the AO10 preamplifier you can disconnect the C1 40 pf feedback capacitor of the V1 6SJ7 tube of the non vibrato channel, and the C7 25 pf feedback capacitor of the V2 6SJ7 tube of the vibrato channel.
+
+On the A-100 series, B3, C3, D-150 series and the RT3 organs with the AO28 preamplifier, you can disconnect the C2 39 pf feedback capacitor of the V1 6AU6 tube of the non vibrato channel, and the C7 24 pf feedback capacitor of the V2 6AU6 tube of the vibrato channel, and also the C33 0.0033 uf (3.3 nf) treble cut grounding capacitor of the percussion output filter so that the resulting brighter sounding percussion signal will then be better matched with the modified main organ signal as well as have more key click.
+
+On the L-100 series and the Porta-B organs with the AO42 "Percussion amplifier" which is also the straight organ signal preamplifier, you can disconnect the C203 39 pf feedback capacitor of the V1 12AX7 input tube, and you can disconnect the C208 0.01 uf (10 nf) percussion treble cut grounding capacitor wired after the Q201 2N306 transistor so that the resulting brighter sounding percussion signal will then be better matched with the modified main organ signal as well as have more key click.
+
+On the Model M organs you can disconnect the C2 15 pf feedback capacitor of the V2 6J5 tube.
+
+On the M2 organs you can disconnect the C6 25 pf feedback capacitor of the V2 6SJ7 tube of the non vibrato signal, and the C1 25 pf feedback capacitor of the V1 6SJ7 tube of the vibrato channel.
+
+On the M3 and the M-100 series organs with the AO29 or the AO67 amplifier you can disconnect the C6 30 pf feedback capacitor of the V2 6AU6 tube of the non vibrato channel, and the C1 20 pf feedback capacitor of the V1 6AU6 tube of the vibrato channel, and also the C33 0.0022 uf (2.2 nf) treble cut grounding capacitor of the percussion output filter so that the resulting brighter sounding percussion signal will then be better matched with the modified main organ signal as well as have more key click.
+
+Please note that with all the above mentioned modifications to disconnect the treble cut / key click filter, that this will result in the whole organ sound becoming brighter or more trebly which means that the recapped pre 1964 organs and the red mylar capped post 1964 organs can become too bright or too shrill sounding which would necessitate the tonewheel generator to be recalibrated in order to bring down the levels of the midrange and the treble TG notes so that they are better balanced with the lower frequency TG notes.
+On the other hand some people will like this brighter, more "edgy" or "biting" organ sound.
+
+The above mentioned modifications to disconnect the treble cut / key click filter are best suited for the pre 1964 organs with the original wax paper capacitors on the tonewheel generator, because the output levels of the TG notes 49 to 91 on the pre 1964 wax capped organs have noticeably decreased from what they were originally set at the factory because the wax paper capacitors have drifted up in uf value and therefore they are no longer properly filtering and emphasizing the TG notes 49 to 91.
+
+Because of this, the treble boost effect created by disconnecting the treble cut/ key click filter will have the dual beneficial effect of boosting the key click sound for a nicer, more "spitty" characteristic as well as boosting the treble response of the whole organ signal, which means that because the wax capped TG notes are at reduced levels, the organ will now have a nice spitty key click but without the excessive brighhtness or shrillness that would be produced with the recapped pre 1964 organs and the post 1964 red mylar capped organs.
+
+In fact these key click boosting modifications are a good alternative for those who consider their pre 1964 wax capped organs to sound dull or muddy, but they do not feel confident enough or they do not want to have to go to the very delicate and time consuming task of recapping the tonewheel generator.
