The Wurlitzer 4600 Series
Electrostatic Organ
In these particular 4600 series instruments,
some reed pans have three pickups per reed instead of two. This is so
that the pedal tones may be independent of the lower pitches on the keyboards
and not be borrowed, so that when you play a pedal, if you're already
holding a lower note on one of the keyboards, you will still hear the
pedal note come in, which would not happen if it was borrowed from the
same pickups that provide the keyboard tones. This also allows the pedal
tones to have less harmonic development, because the pedal tone pickups
are both larger and also closer to the centers of the reed. On at least
some of the spinet versions of these instruments, the pedal was derived
from the tones available on the keyboards, and there was no 16' or first
octave of reeds. In the early version of these instruments, the center
pickups provided a softer flute tone for the accompaniment. Later versions,
including these 4600 series consoles provided a soft flute tone for the
accompaniment by applying a reduced voltage to the normal flute tone pickups
over the ends of the reeds.
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Left. This is the interior
of the pan for the second octave of reeds which shows the two pickups
above each reed. There is also a flat sheet metal pickup at the front
edge of each reed. Roll cursor over picture to see individual parts captions.
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Right, Closeup of a three-pickup
reed. This is the third octave of reeds which generates notes beginning
with tenor C through the next B. The larger pickup over the center of
the reed is for a pedal note. In addition to the two above the reed, there
is also the flat sheet metal pickup at the front edge of each reed which
is clearly visible in the picture and held in place by a Phillips head
screw. This picture was taken with the instrument in operation. Notice
that the top reed in the pictures is at the outward end of its vibration.
At rest it would lie flat with the shallot.
The
above pictures and those on the preceding pages give you a very good look
at the Wurlitzer electrostatic tone generating system where the vibrating
reeds act as variable capacitors to generate AC audio waveforms. The pickups
above the reeds can be screwed in or out to move them closer or farther from
the reeds which adjusts the level of each note, so that when the instrument
was initially being set up at the factory, it would be possible to scale the
output levels of each note for best overall sounding results. The front edge
pickups mount on a black plastic block. The pickups themselves are slotted
and can be moved closer to or farther away from the front edges of their respective
reeds. In order to generate waveforms with significant harmonic development,
it is necessary for the front edge pickups to be very close to the reeds.
It appears from what checking that I have done that this distance is within
a few thousandths of an inch.
If a reed should accidentally touch a pickup, the result is a loud bang over the speakers, and if the reed repeatedly hits a pickup as it vibrates, the result is loud, useless distorted noise that will completely overpower any possible musical signals. Typically, the DC voltage that is applied to the appropriate pickups by the contacts under the playing keys is +160 VDC. Some additional tonal effects are available by keying the pickups with a greatly reduced voltage which then decreases the amplitude of the resulting signal. If the DC is suddenly applied to the pickups, the tones would begin with a fairly loud pop. Therefore, it is necessary to apply the keying voltage through small resistor-capacitor networks which delay the voltage rise-time on the pickups. Even when on the fast percussive mode, there is still a few milliseconds' worth of buildup to prevent noisy keying transients. On the next page we will look at the keying system to see how the DC voltage from the keys and pedals of the instrument gets to the appropriate pickups on the reeds.
If a reed should accidentally touch a pickup, the result is a loud bang over the speakers, and if the reed repeatedly hits a pickup as it vibrates, the result is loud, useless distorted noise that will completely overpower any possible musical signals. Typically, the DC voltage that is applied to the appropriate pickups by the contacts under the playing keys is +160 VDC. Some additional tonal effects are available by keying the pickups with a greatly reduced voltage which then decreases the amplitude of the resulting signal. If the DC is suddenly applied to the pickups, the tones would begin with a fairly loud pop. Therefore, it is necessary to apply the keying voltage through small resistor-capacitor networks which delay the voltage rise-time on the pickups. Even when on the fast percussive mode, there is still a few milliseconds' worth of buildup to prevent noisy keying transients. On the next page we will look at the keying system to see how the DC voltage from the keys and pedals of the instrument gets to the appropriate pickups on the reeds.
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