HAMMOND ORGAN
In any electric
or electronic musical instrument, the tones originate as a series of alternating
current waveforms. Because the pitch range of traditional keyboard instruments
usually covers a wide range of frequencies, the electronic substitute must
likewise generate a considerable number of alternating currents, each having
a different frequency. In these classic Hammonds, there are 91 tone generators
that run all the time when the instrument is powered on and their resulting
AC audio signals are present on normally open switch contacts under every
playing key and pedal. Pushing any key or pedal connects the appropriate tone
generator output(s) to the subsequent sections of the instrument for further
processing, amplification and application to loudspeakers where the electrical
signals become sound.
Knowing this, we can look at the Hammond’s
tone generating system, figure three, which produces the 91 different alternating
currents. Each individual generator makes the frequency of just one note of
the musical scale beginning with the lowest bass frequency of the instrument’s
pedals (32.70 Hz.) and continuing up to the highest available frequency of
5919.84 Hz. Below, in figure three, we show a Flash animation of a typical
tonewheel. In the animation, the wheel turns fairly slowly, and to the right,
the moving sinewave represents the electrical output signal. In actuality,
the tonewheels turn much faster, and would appear blurred like the blades
of an electric fan.
Figure 3. Flash ani-mation showing a typical Hammond tone wheel generating the AC signal for one note or pitch of the musical scale. The electrical out-put signal is repre-sented by the moving sinewave at the upper right.
Hammond’s
tone generating system consists of a series of 91 toothed or lobed steel
wheels, called tone wheels, each about two inches in diameter. The tone
wheels for the lowest octave of generated tones have two lobes. Those
for the second octave have four; for the third octave, eight; for the
fourth, sixteen, and so on up to those for the highest complete octave
of available frequencies which have 128. There are also seven additional
tone wheels which have 192 lobes to generate the very highest available
frequencies.
Figure three shows a typical arrangement
for generating one frequency of the instrument. The tone wheel rotates
close to the edge of a small permanent bar magnet about half the size
of an ordinary pencil. The tip of the magnet is ground to a chisel edge
and there is a coil of wire on a small spool close to the end of the magnet.
As the wheel rotates, its teeth or lobes disturb the magnetic field. This
causes an AC voltage to be developed in the coil, and this becomes the
signal for one tone of the instrument.
If, for example, we want to produce the
pitch A 440, this tone wheel (figure three) will have to make 1650 RPM,
which will result in 440 teeth passing the tip of the magnet every second.
Since a standard octave of a keyboard instrument includes twelve pitches,
it follows that there must be twelve different rotational speeds for the
various tone wheels. It is both the number of teeth and the rotational
speed which determine the output frequencies.
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Figure 4. A complete Hammond tone generating unit.
Figure four
is an overall view of a complete Hammond tone generating system. The device
contains the 91 different tone wheels, their associated permanent magnets
and coils, the necessary gearing, bearings and lubrication system. It also
has a shaded-pole induction motor to start the unit and get everything up
to speed, a synchronous driving motor to turn the main drive shaft at exactly
1200 RPM, suitable LC resonant filters for the output signals, and a 16 pole
rotary variable capacitor whose function I'll tell you about later.
One of the most important criteria for the instrument’s
correct operation is for all of the tone wheels to spin at a constant speed.
Although the various wheels run at twelve different speeds, each wheel must
rotate exactly at its correct design RPM. To maintain this constant speed
rotation, the entire series of tone wheels is driven from a central line shaft
which is turned by a synchronous motor. A synchronous motor is one which runs
in exact lock-step with the frequency of the AC power that supplies it. By
its design, the Hammond tone generator synchronous motor makes exactly one
revolution for every three cycles of incoming AC. On 60 Hz power, this is
1200 RPM.
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