Tone Generator

Excerpted from the OriginalHammondLeslieFaq

Each tonewheel Hammond organ has a main tone generator within it, and in some cases, depending on the model, a Chorus Generator. The *tone generator* assembly consists of a drive shaft on which are mounted 24 Driving Gears, two each of 12 different numbers of teeth. Meshed to each driving gear on either side of the drive shaft are two Driven Gears, 4 each of 12 different numbers of teeth. Each driven gear turns a small shaft on which are mounted two Tonewheels. Each tonewheel has a number of smooth, sine-shaped "bumps" or "teeth" and rotate adjacent to a Magnet and Coil assembly (very similar to an electric guitar pickup). Twenty-four driving gears _times_ four driven gears _times_ two tonewheels equals *24 x 4 x 2 = 96 tonewheels* in nearly all tone generators (only the model X-66 used a tone generator with 12 tonewheels and derived the rest with frequency divider circuits).

An AC Synchronous Motor is connected to one end of the drive shaft which can only run "locked" to a multiple of the the AC mains frequency of 50 or 60 Hz. At 60 Hz, the motor turns 1200 RPM or 20 times a second. But AC synchronous motors are not inherently self-starting. In the original patent drawings, the organ is shown with a hand crank in the side of the organ to start the motor! Fortunately, all production models have included an AC induction Start Motor to start the organ. The start motor is mounted at the opposite end of the drive shaft from the synchronous run motor. On some later models, a self-starting "Blue" synchronous run motor is the only motor. The drive shaft on the start motor side of the shaft also turns the scanner Vibrato.

The number of high spots on each tonewheel times the Gear Ratio of the driving and driven gears times the rotational speed of the motor determines the pitch produced by a particular tone wheel assembly. The pitches approximate even-tempered tuning, (it's done with integer math after all). But not all the tonewheels had "teeth". Some were left perfectly round or "blank" and included on the tone generator solely to keep the gear train balanced. Depending on the model and the year it was built, Hammond tonewheel organs had different types of tone generators that differed in the number of blank tonewheels. Of the 96 tonewheels in the most popular Hammond Console Organ models (B-3, C-3, A-100 etc.), 91 are 'true' tonewheels - being used for sound generation - with the other 5 being blank balance wheels. A few Hammond models (H-100, X-77. X-77GT) used all 96 tonewheels for sound generation and some models (including the spinet organs and console organs produced during WW II) used less than 91 'true' tonewheels. The X-66 model used 12 tone wheels which were used to generate the frequencies for the highest octave, with dividers being used to produce the lower octaves.

The gear train, while carefully constructed, did not rotate smoothly. The AC synchronous motor "cogs" a bit while rotating and is connected via a resilient Coil Spring coupling to the drive shaft. To further smooth the rotation each brass driving gear is also mounted on coil spring couplings. The bakelite (an early type of plastic) driving gears also tended to smooth out the rotation. Cross-talk was also an issue where adjacent tonewheels would "bleed" low levels of their tone into their neighbor's signal. Each driving gear, its two driven gears and four tonewheels run in a compartment magnetically shielded from the others by steel plates which divide the generator into a series of bins. All four tonewheels in a bin turn at the same speed but have different numbers of bumps so the tonewheels in a bin are therefore harmonically related. The tonewheels were arranged so that the tonewheels on the same side of the drive shaft within a bin were four octaves apart.

Note that the tone generator assembly is very difficult to disassemble. So broken or stripped gears are extremely difficult to replace. It is almost always easier to replace the entire generator. But be sure to replace the generator from a similar organ. Don't drop an M-3 tone generator in a B-3 because the M-3 generator is missing five tones and the lowest octave has tonewheels which are shaped differently than a B-3. A B-2 tone generator may or may not work in a B-3 as the tone generators were modified to be narrower just before the introduction of the "-3" series organs.