The Wurlitzer 4600 Series Electrostatic Organ

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     The reason for connecting the load resistor to the shallot of the reed instead of to the pickup is as follows. Have you ever noticed how in your home stereo system, for example, you have just two wires going to either speaker? And yet, in spite of that, all of the extremely complex audio information of, for instance, an entire symphony orchestra is all represented by a single AC audio waveform that requires only two wires. Along the same principle, Wurlitzer found that they could "common" or tie together, all of the reed shallots and then just have one common load resistor and one blocking capacitor, and no matter how many notes the musician might use at any instance, and no matter whether he's using one, two, or all three pickups, the resulting multitude of individual notes and tones can all be combined into one single AC audio waveform that represents all of them. This of course makes the circuitry much simpler than if they had to use individual load resistors and blocking capacitors for every single pickup on each reed individually.
     In actuality, Wurlitzer divided the group of 85 reeds into two subgroups, called ap-propriately bass and treble. Reeds one through twenty-four, representing the first two lowest octaves is considered the bass group, and the remaining sixty-one reeds comprise the other or treble group. Although these Wurlitzer electrostatic organs are considered single channel or mono instruments, initially in the amplifier, the signals from the bass and treble reed groups are processed differently in separate channels.
     Bass is accentuated on the lower frequency tones, and for the remainder, there is some emphasis on higher frequencies. Unlike the Hammond with its upper limit of 6 kHz and the need to suppress its key clicks, the Wurlitzer electrostatic organ produces lots of high harmonics on the tones that originate from the front edge pickups. Also, there are no key clicks in its keying system. Therefore there is no need to limit high frequencies where they can be useful, thus the treble emphasis for signals coming from reeds 25 through 85.
     There is also the consideration of vibrato. This periodic and regular wavering effect which adds so much to vocal music and much popular instrumental music as well does not sound good on very low bass tones. Why this is so, I don't know, but I would guess that it is because on very low pitches, the frequency of the tones is not too much higher than the vibrato frequency itself. In all better-designed pipe organs, for example, the vibrato/tremolo effect is kept away from just about all pipes with fundamental pitches below Tenor C. In keeping with this tradition, the vibrato effect in the Wurlitzer electrostatic organs likewise effects only Tenor C and tones that are above it.
     In spite of their delicateness, these free reeds are extremely pitch stable. The few remaining electrostatic Wurlitzers in existence today are at least fifty years old. I have seen as of now five of them, and all remain in excellent tune. Free reeds, once manufactured and correctly tuned will stay in tune indefinitely, especially if they are kept clean, which is one of the features of the electrostatic Wurlitzer. Because the reeds are not used for acoustic sound production, they can remain in a hermetically sealed environment where they are kept free of dust and dirt.
     Free reeds are sensitive to temperature changes, surprisingly going slightly flat as temperature increases. However, the pitch change resulting from normal temperature fluctuations is so small that it can only be measured by sensitive electronic equipment. After the instrument has been running for a while, the air temperature in the reed unit will rise about five degrees F above ambient. This temperature rise is most likely caused by the centrifugal compressor. The effect, however, on the pitch of the reeds is a lot less than even one cent which is 0.01 semitone. The normal tuning tolerance for musical instruments is greater than that so we may therefore consider that a Wurlitzer ES instrument has essentially permanent and constant tuning.
     Metal fatigue of free reeds may occasionally cause a noticeable detuning, however in these instruments, the amplitude of reed vibration is fairly small, much less than what you would find in an accordion, because it's not necessary for these reeds to make loud noise as the reeds in an accordion have to do. This low amplitude mechanical vibration therefore is not conducive to creating metal fatigue, and the reeds' maximum vibration is much less than that required to approach their elastic limit. On the next page we'll look at the interior of a 4600 series instrument and see how all of this stuff is arranged. Below are typical examples of the waveforms that result from the three pickups of a typical reed in this instrument.

Left. The waveform from a pickup over the front of a reed shows some appreciable low-order harmonic development, the second har-monic being quite prominent and also the third. These harmonics are not exactly in phase. This shows that the motion of the front edge of the reed is not sinusoidal but undergoes changes in instantaneous velocity.

Right, the waveform from a pickup over the center of a reed shows a somewhat similar shape but with considerably less harmonic development. The waveform above and this one clearly demonstrate that the motion of the reed becomes considerably more non sinu-soidal as we get closer to the front end.

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