Each tonewheel is a disc of soft iron with a diameter of approximately 2 inches. The rim of the disc is "hobbed" or cut with a specific number of smooth, rounded bumps or "teeth". As the disc rotates, this alternation of the radius leads to a modulation of the magnetic field of the Magnet and Coil pickup. This induces an alternating voltage in the coil of the pickup. There are 96 tonewheels in the generator of most Hammonds, however, some tonewheels are blank and included only to balance the geartrain.
The shape of tonewheel teeth are generally designed to produce a pure sine wave. However, the lowest octave of tonewheels in late model organs do not produce a sine wave. This octave is used only in the pedals. With only two Drawbars to control the pedal waveform, the pedal tones in early Hammonds (A, B-1 & B-2, etc.) sounded a bit dull. In the most popular model Hammonds (B-3, C-3, etc.) the tonewheels of this octave are cut with a more complex pattern to improve the sound (See Complex Tone Wheels).
While each output above the first octave of the Tone Generator should be a pure sine wave, there is some distortion:
- The Synchronous Motor pulses slightly as it rotates. Left unaddressed, this would cause a throbbing quality to the sound. Hammond effectively limited this by the use of a resilient Coil Spring coupling between the synchronous motor and the drive shaft to absorb the pulses.
- Eccentricities in the shape of the tonewheels can create a subtle low frequency amplitude modulation. The frequency of this modulation will be the same as the rotation speed of the tonewheel. Usually this modulation is low enough not to be objectional but if too apparent, the only solution is to replace the offending tonewheel. For a more in-depth discussion of the effect that eccentricities in the shape and mounting of the tonewheels has on the sound, read the patent application for No. 2,314,496. This patent describes a tonewheel generator that apparently was never commercially produced. Start reading about the end of the third page for Hammond's description of how the rotational speed of the tonewheels adds undesirable harmonics and how the "improved" generator disguises these by making them more "vibrato-like".
- Eccentricities in the gear train can create subtle low frequency amplitude and frequency modulation. It is impossible to precisely machine the Driving Gears and the Driven Gears. Out of round gears, irregularities in the teeth, backlash and the clearances between the teeth can cause these modulation problems. Laurens Hammond describes this problem at some length in the original patent (nearly all of page 6 - See Hammond Patents). This is why each driving gear is isolated from the drive shaft by a resilient coil spring. Eccentricities are dampened but are not completely eliminated by the springs. Like eccentricities in the tonewheel, if the sound is too distorted the only option is to replace the offending gear(s).
- Tonewheels can become slightly magnetized. This rare event can happen in organs which have not been operated for a long period of time. If the tonewheel is kept in one position too long, the magnetized pickup slowly magnetizes it. This results in a subtone which is twice the rotating speed of the tonewheel (because every magnet has a north and south pole). Over time, the tonewheel will gradually lose this magentic orientation if operated regularly. However, if this subtone is too objectionable to wait, the tonewheel needs to be demagnetized. This is a job for a skilled Hammond tech since the risk is very high that you could also accidently demagetize the pickup.
- Some harmonics are produced by magnetic crosstalk Harmonic Leakage from adjacent tonewheels (See Causes of Cross Talk). Most pickups are fitted with passive filters to limit these harmonics. Many people feel these leakage harmonics are part of the Hammond "sound". However if the leakage is too objectionable, the tone generator may need a Capacitor Replacement.
- Tonewheels can become damaged. The pickups were carefully adjusted at the factory to be a specific distance from the tonewheel. If an attempt is made to adjust a pickup closer to the tonewheel it's possible for the tonewheel to hit the pickup. This is going to cause the soft metal of the tonewheel to become deformed which is going to distort the sound. If the damage is bad enough, the tonewheel could also be out of balance. A tonewheel replacement is the only way to eliminate this distortion.
- Distortion from the AC mains frequency (See Causes of Hum). The AC mains frequency of 50 or 60Hz is present throughout the organ. To a certain extent you can't do too much about this. However, if a Hammond Clock or similar mains powered device is set on top of the organ, it can induce a great deal of hum. The solution for this is simple, move the clock.
The number of bumps or "teeth" on a tonewheel is 2, 4, 8, 16, 32, 64, 128, and as an exception of the power of 2 series, 192. Each successive change in the number of teeth in octaves 1-6 represents a change of one octave:
Octave No. of Teeth Frequency Note Range
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0 2 1 - 12 C0 - B0
1 4 13 - 24 C1 - B1
2 8 25 - 36 C2 - B2
3 16 37 - 48 C3 - B3
4 32 49 - 60 C4 - B4
5 64 61 - 72 C5 - B5
6 128 73 - 84 C6 - B6
7 192 85 - 91 C7 - F#7
</pre>
In the most popular models of the Hammond organ, the highest octave is limited to the first 7 tonewheels of 192 teeth. These tonewheels use 192 teeth instead of 256 because the machinery used to cut the tonewheels could not cut 256 bumps in a diameter of only 2". That makes these 7 pitches slightly out of tune (averaging 1.64 cents sharp) from the octaves below because the gear ratios are not precisely correct.
In most Hammonds, the last 5 tonewheels of the upper octave are not cut, have no pickups and are included only for mechanical balance. In spinet organs, such as the M-series, the first 5 tonewheels of the second octave are also not cut, have no pickups and are included only for mechanical balance. Since the range of the lower manual on spinet organs stops at F, tonewheels C1 through E1 are not needed.
