Certain modern sampled electronic models go some way towards copying the basic sound. but they are not entirely convincing.

Every tone is produced by rotating a toothed steel wheel near a magnetic pickup. The resultant frequency is a product of the number of teeth and the rotational speed.

The wheels are mounted in pairs, flexibly coupled to a driven gear, each pair producing tones four octaves apart. Except for the top octave, where the interval is reduced to allow for unused tones.

For Console types, such as the A, B, C, D, E, G and RT models the generator is a steel box divided into twenty-four compartments. A drive shaft, divided into flexibly coupled sections, runs through all compartments. At one end is a synchronous run motor and at the other a starting motor. Drive gears, two each of twelve sizes, are mounted on this shaft.

Each drives two pairs of tone wheels in one compartment via 12 sizes of driven gear & 'slipping clutches'.

The wheels come in eight types, twelve each of 2, 4,8, 16, 32, 64, 128 & 192 'teeth'. One of these runs at each of twelve speeds via the various gear ratios. This combination can potentially produce ninety-six frequencies. On most models five high frequency wheels are replaced by blank discs, with no pickups. This gives a total of ninety-one tones.

Spinette model generators are similiar, but with eighty-six or eighty-seven tones.

A historic split 1956 M100 Spinette, restored in our Mercia Workshops.

A 'Whiter Shade of Pale' was recorded using an M102, not a console model as many think.

The H100 series, the HX100 and the X77 use the full 96 wheel range. With twelve 256 tooth wheels. There is a 25th compartment on the console generator, leaving room for four more wheels & pickups. This allows one hundred tones to be implemented, still not enough to eliminate treble foldback. The G100 was equipped with the maximum range of tone wheels.

Some wartime models have only eighty-two tones, the lowest 9 pedal tones being omitted for economy.

Middle & high tones are fed through L/C (Coil/Capacitor) filters, calibrated for each frequency, to restrain harmonics & crosstalk. These deliver a fair approximation to a sine wave. This results in the distinctive Hammond sound, obtained without recourse to electronic effects or processing.

The filtered outputs are wired to the manual contacts, nine to a key. Each manual has nine busbars, one for each harmonic. On the full organs selector keys or buttons connect these to one of the sets of nine drawbars or to preset combinations set up on the recorder board. Spinette models use tabs & 'hardwired' presets.

Drawbars allow silence or one of eight possible levels for each harmonic. Enabling complex tone colours to be built up from individual sine waves, one of the earliest forms of additive synthesis. Not all harmonics are available, only those forming useable musical intervals. It was calculated in the late 'thirties that over two hundred & fifty million different timbres can be created. But they all sound like a Hammond.

The 25 key pedal board has fixed harmonic combinations, 16 & 8 foot via two drawbars. Early spinettes have 12 pedals, M100 & later have 13. These use complex Tone Wheels & a single drawbar.

On certain early models intended for church use, for example the BC, D, K & E a second generator is fitted. Paired tone wheels are set, together with gearing, for slightly sharp & flat tuning.

A battered 73 year old E Type. Chorus generator, twin swells, preset & pedal pistons.

Switching in this generator creates a 'Chorus' effect, similiar to slightly detuned ranks of pipes. A mechanical tremolo system is used on some models.

A 1940 blonde oak model K during restoration at our Mercia facility. This instrument had previously been 'modernised', fitted with a scanner, selective vibrato & percussion.

It came to us in a sad state. After an attempt to bleach the cabinet, without any dismantling!
On vibrato models, BV, BCV, CV, DV, B2, B3, C2, C3, RT2, RT3, A100, DR100, M3, G series, M100 series and T series a Vibrato scanner is coupled to the run motor. Using a rotating multiple section variable capacitor, it reads the outputs of a sectioned L/C delay line, alternately forward and reverse. Giving a phase, hence frequency, variation at a 6.8 cycles per second (HZ) rate.

This creates the characteristic Hammond vibrato, quite unlike any other instrument. A switched bypass resistor allows a proportion of non-delayed signal for a Chorus effect. The L series employ an electro-magnetic vibrato system and were the first to use a self starting run motor.

This motor was subsequently adopted for the T series with the re-instatement of scanner vibrato.

On the B3, C3, RT3, A100, D100 and M100 series percussion is also provided on the second or third harmonic. This gives a louder tone, decaying whilst a key is pressed. The valve circuitry creates a pleasing bell like effect. The transistor spinette models (T series) and the L100 series are not so good in this respect.

Since it is based on electric clock technology the tuning of a Hammond is always accurate (see note below), provided the Mains supply frequency is correct. However the loosely coupled tone wheels create an essentially 'free phase' instrument. This hard to quantify or reproduce characteristic is shared with pipe & reed organs.

Modern electronic organs & sample players produce pitches by arithmetic division of a master clock. The resultant accurately locked frequency & phase of all notes produces a clinical & 'soul-less' nature. Such instruments do not convincingly replicate the living sound of a pipe organ, accordion or Tone Wheel Hammond. The difference is apparent to most people after a time.

This XB-2 model, in our Mercia workshops, is an example of the later breed of electronic organs carrying the Hammond name. Featuring a sampled tone source, with a Leslie simulator using CCDs.

Note: Strictly speaking there are built in errors in the tuning of a Tone Wheel Hammond. The relationship between notes in the musical scale can not be shown as a simple whole number, many decimal places are required. Since each gear & tone wheel must have a whole number of teeth the result is inaccurate.

So, in effect, the Hammond organ uses its own version of the tempered scale. To some degree this contributes to the unique Hammond sound. At least one of the clones has settings to approximate either tone wheel or concert scaling.

This basic error mechanism is shared by electronic instruments using a master clock, since dividers can only use whole numbers. Both Laurens Hammond & modern designers decided the result is close enough to satisfy the human ear. For this to be so the division must be from a sufficiently high starting number. Some electronic instruments fall short.

Sub Note: For those who like numbers, I calculated the ratio between adjacent semi-tones in the equally tempered scale. It is 1.0594630943592953 to sixteen decimal places. It would take a lot of gears or dividers to get that close.

Electrical Safety.

Console tone wheel Hammonds are generally fitted with a two pin American mains plug. They have large areas of exposed metal, All were built in the days before effective modern safety standards.

When used with a Leslie, matters are worse, the connecting lead carries mains, chassis & signal wiring together. A damaged lead, possibly due to being tripped over, can connect mains live to the organ metalwork. In addition the favourite 122 model left the factory with 2 large capacitors between the mains & amplifier chassis. These are lethal, especially on 230 Volts, & should be removed if still present.

Safety rating: '0'. It will be '2' if fitted with 3 conductor IEC plug & earth. Provided the Leslie "death caps" mentioned above have been removed.