The Wurlitzer 4600 Series
Electrostatic Organ
Here is a Flash animation that illustrates
a typical three-pickup reed in the Wurlitzer electrostatic organ. The reed
vibrates continuously as long as the instrument is powered on. Applying a
DC voltage to any of the three electrodes causes a suitable AC voltage audio
waveform to develop. The electrodes and the reed together constitute three
variable capacitors that share a common plate (the reed). Electrically, however,
they represent three separate capacitors.
The reed vibrates constantly in close proximity
to the three elec-trodes or pickups as shown. Notice that the single electrode
at the moving front edge of the reed is a thin piece of sheet metal, as
is the brass reed itself. Because the time interval that the reed passes
by this thin sheet metal electrode is very brief, the resulting electrical
wave has two sharp spikes per cycle. A spikey waveform contains lots of
high pitched harmonics and has a somewhat violin-like tone. Notice also
by virtue of the front electrode's physical placement that its wave-form
is slightly out of phase with the waveforms developed by the two electrodes
above the reed.
Because the waveform from the front edge pickup is slightly lagging
the phase of the waveforms from the other two pickups, when the signals
are combined, some of the harmonics of the front edge pickup's waveform
slightly cancel a few of the lower harmonics in the waveforms of the
other two electrodes, resulting in a somewhat different composite tone
if two of these pickups are energized at the same time. The three wires
from the pickups shown in the picture as red, green and blue connect
to the instrument's key switch contacts via keying networks and a set
of multiple contacts and busbars under the keyboards and pedals of the
instrument
Audio signals
occur only when voltage is present on one or more pickups. The amplitude or
strength of these audio voltages also depends on how high the DC charging
voltage is that is applied. If the voltage is suddenly applied, the resulting
audio tone begins virtually instantaneously. If the voltage is initially high,
and then gradually decreases over time, the tone will likewise begin loudly
and then gradually fade out. If the voltage is high initially and then fairly
quickly drops to a slightly lower level, the resulting signal will begin percussively.
If the voltage is allowed to rise gradually to some value, the tones begin
softly and gradually increase their level.
These properties are all very useful because they allow the musician, via suitable controls in the instrument's keying circuits, to control the so-called "envelopes" of the instrument's tones to a considerable degree. By allowing the voltage to rise almost, but not quite instantly, to its steady-state value, this feature adds a very smooth and mellow onset to the tones of the instrument. One of the faults of some electronic instruments of this era is their so-called "telegraph-key" tone, where when you push a key, the sound is instantly on, and when you let go, it's instantly off. This instant-on, instant-off effect is completely unnatural in that real (non-electronic) musical instruments don't do that. Even percussive tones such as piano tones or xylophone notes take a slight amount of time to develop, even if less than a millisecond. Likewise, the tones of all instruments do not decay instantly but take at least a few milliseconds to die out completely.
The electrostatic system lends itself to very easy control of the voltages applied to the reed pickups by means of various simple resistor-capacitor networks which control the application of the charging voltage to the various pickups on the reeds. The reeds, by way of their continuous mechanical vibration, are always "on," in the sense that they are ready to create AC waveforms as soon as the DC charging voltage is applied.
These properties are all very useful because they allow the musician, via suitable controls in the instrument's keying circuits, to control the so-called "envelopes" of the instrument's tones to a considerable degree. By allowing the voltage to rise almost, but not quite instantly, to its steady-state value, this feature adds a very smooth and mellow onset to the tones of the instrument. One of the faults of some electronic instruments of this era is their so-called "telegraph-key" tone, where when you push a key, the sound is instantly on, and when you let go, it's instantly off. This instant-on, instant-off effect is completely unnatural in that real (non-electronic) musical instruments don't do that. Even percussive tones such as piano tones or xylophone notes take a slight amount of time to develop, even if less than a millisecond. Likewise, the tones of all instruments do not decay instantly but take at least a few milliseconds to die out completely.
The electrostatic system lends itself to very easy control of the voltages applied to the reed pickups by means of various simple resistor-capacitor networks which control the application of the charging voltage to the various pickups on the reeds. The reeds, by way of their continuous mechanical vibration, are always "on," in the sense that they are ready to create AC waveforms as soon as the DC charging voltage is applied.
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