During the total production run of Hammond tonewheel organs, there were several changes made in the number of outputs from the Tone Generator. This depended on the model and the year the organ was built:

The organ described in Lauren Hammond's patent application was never marketed and existed in prototype form only. The tone generator illustrated in the patent produced 89 frequencies starting at C0:

<pre> 7 blank tonewheels 5 tonewheels of 2 teeth C0-E0 12 tonewheels of 4 teeth F0-E1 12 tonewheels of 8 teeth F1-E2 12 tonewheels of 16 teeth F2-E3 12 tonewheels of 16 teeth F3-E4 12 tonewheels of 32 teeth F4-E5 12 tonewheels of 64 teeth F5-E6 12 tonewheels of 128 teeth F6-E7

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In the patent organ the number of teeth on the tonewheels change at F instead of C as in the production organs. Also note that there are twice as many 16 toothed tonewheels as the other octaves. This required two different sets of Gear Ratios, one set for E3 and below and a different set for F3 and above to make this odd arrangement of tonewheels work correctly. There are no tones above the sixth octave like there are in the production organs because the hobbing machine used to cut the sinusoidal "teeth" in the tonewheels could not cut 256 teeth. Hammond didn't solve this problem until just before the Model A was introduced (see Early Model info below).

The following organs were produced from the mid 1930's to the early 1940's and have a 91 frequency generator:

<pre> Model A: # 1 - # 2,676 Model B: # 4,000 - #10,549 Model C: # 1 - # 1,247 Model D: # 1 - # 3,143 Model E: # 8,000 - # 8,663 Player : # 9,000 - # 9,209

All 91 frequency models have:

12 tonewheels of 2 teeth C0-B0 12 tonewheels of 4 teeth C1-B1 12 tonewheels of 8 teeth C2-B2 12 tonewheels of 16 teeth C3-B3 12 tonewheels of 32 teeth C4-B4 12 tonewheels of 64 teeth C5-B5 12 tonewheels of 128 teeth C6-B6 7 tonewheels of 192 teeth C7-G7 5 blank tonewheels

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After the patent was issued and before production began, Hammond figured out a different combination of Gear Ratios that would change the number of teeth at the Cs rather than at the Fs. To simplify production he kept one standard set of Driving Gears and Driven Gears for each octave.

He also figured out a way to generate a top half octave without 256 tooth tonewheels. The ratio of a perfect fourth to unison is 4:3 or 1.333 times higher. Since 256/192 is approximately equal to 1.333, Hammond cut the top tonewheels to 192 teeth and changed the gear ratio between the driving gears and the driven gears to that a perfect fourth above the desired pitch. In other words, the gear ratio for the upper C was the same as that used on the F tonewheels below it. Because 256/192 is not exactly 1.333, the upper seven pitches are slightly sharper than the other other octaves (another reason a tonewheel generator sounds different than most clones with perfectly divided digital octaves). Only seven tones could be produced using these gear ratios so most generators produce 91 tones. But, because of the way the tone generator was designed, the number of tonewheels in a generator had to be divisible by 12. So five blank tonewheels with no teeth (and no magnetic pickups) were used to keep everything running smoothly.

Because it was impossible to build a "single-toothed" tonewheel that could rotate without unbalancing the Tone Generator the lowest octave in the organ had no "subfundamental" tones and the key contacts were left disconnected. If only the first drawbar was pulled out, the bottom octave in both manuals were silent if played. In other words, these early organs lacked Harmonic Foldback in the lower octave.

The folowing models were produced during World War II and have an 82 frequency generator:

<pre> Model A: # 2,677 - # 2,711 Model B: #10,550 - #17,074 Model C: # 1,248 - #17,074 Model D: # 3,144 - #17,074 Model E: # 8,664 - # 8,739 Model G: # 4,101 - # 7,349 Player: # 9,210 - only

All 82 frequency generators have:

9 blank tonewheels 3 tonewheels of 2 teeth 12 tonewheels of 4 teeth 12 tonewheels of 8 teeth 12 tonewheels of 16 teeth 12 tonewheels of 32 teeth 12 tonewheels of 64 teeth 12 tonewheels of 128 teeth 7 tonewheels of 192 teeth 5 blank tonewheels

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In an effort to trim costs during WWII, Hammond reduced, for a time, the number of tonewheels from 91 to 82. The 82 frequency generators eliminated the lowest 9 tonewheels and the organs were rewired to accomodate these missing tones. Hammond used a technique common to pipe organs which produce bass tones lower than the longest available pipe by simultaneously playing two pipes tuned a perfect fifth apart. This produces a _resultant_ frequency an octave lower. By rewiring the organ to play a tonic and its fifth at the same time, a resulting frequency an octave lower was produced. However, this has a negative impact on the quality of the tone. Hammonds do not have perfect fifths and this further limited the quality of the tone. The 82 frequency models are probably not too common.

After WWII, Hammond returned to the 91 frequency generators and added ComplexToneWheels to the lowest octave that provided additional odd hamonics to the fundamental. This improved the sound of the bass pedals. The following models were produced from the mid 1940's to the early 1950's and have 91 frequency generators with complex tone wheels on lowest octave:

<pre> Model BV: #17,075 - #29,737 Model CV: #17,075 - #30,287 Model RT: # 1,001 - # 1,201 Model B2: #35,000 - #40,303 Model C2: #35,001 - #40,459 Model RT2 # 1,300 - # 2,150

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A short time before the introduction of the 3 series, Hammond redesigned the tone generator to be narrower. These generators can NOT be used to replace the 91 frequency generator above because of differences in the output terminals. The following models have 91 frequency generators in a narrower frame (note that all B3's have the narrow generator):

<pre> Model B2: #40,304 - through B3 to end of production Model C2: #40,460 - through C3 to end of production Model M3: #??,??? - through end of production Model RT2 # 2,151 - to end of production

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M series SpinetOrgans were produced from the late 1940's to the early 1960's. To keep production costs down, these organs have an 86 frequency generator:

<pre> 12 tonewheels of 2 teeth (ComplexToneWheels but different in shape and harmonic content than the other organs) 7 tonewheels of 4 teeth (Tonewheels for generators 13 through 17 are blank with no pickups) 5 blank tonewheels 12 tonewheels of 8 teeth 12 tonewheels of 16 teeth 12 tonewheels of 32 teeth 12 tonewheels of 64 teeth 12 tonewheels of 128 teeth 7 tonewheels of 192 teeth 5 blank tonewheels

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The M series SpinetOrgans use an 86 frequency generator. Since the lower manual ends at F instead of C, generators 13(C) through 17 (E) are not needed. Tonewheels 13 through 17 are "blanks" (i.e. perfectly round) and the pickup magnets are missing. Early M organs use the "wide" generator (but unlike the other organs with the "wide" generator, the output terminals are not in consecutive order) but it's not known for sure how many. Later M, M-2 and M-3 organs used the narrow generator.

Used in later console organs such as the H-100 and X-77.

Only one Hammond model used a 12 frequency tonewheel generator, the X-66.

See ChorusGenerator

For an in-depth description of the different kinds of tone generators and how they are connected to the manual key switches: http://www.dairiki.org/hammond/wiring/

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