WURLITZER 4600 Series Instruments

North Suburban HAMMOND ORGAN Society

The Wurlitzer electrostatic organs of the 1950s are some of the most unusual organ-like electronic musical instruments in existence. Please read the Hammond article first, however, as much of the basic info that will make this article more understandable is included there.

They are sometimes referred to as Wurlitzer reed organs, however this term is not really accurate in the usual sense. A reed organ is an instrument which uses air-blown reeds for its sound production. An accordion is a reed organ in a portable form, for example. The musical tones that you hear from a reed organ or an accordion are the actual acoustical tones of the reeds. In the Wurlitzer electrostatic organs that this article is about, however, despite their having reeds, you do not hear the sound of reeds, because the reeds are not used as sources of acoustic sound, but instead are used as electrostatic generators of audio alternating current waveforms. These AC audio frequencies are then further processed and amplified, ultimately being converted to sound via conventional loudspeakers, just as the audio waveforms generated in Hammonds and other similar instruments are converted to sound via speaker systems.

In reed organs and accordions, the reeds remain at rest and only vibrate when actually called upon via the keyboards and associated valves to produce particular tones or notes as required by the music. In the Wurlitzer electrostatic organ, all of the reeds in the instrument vibrate all the time. In the typical 4600 series instrument, there are 85 reeds, covering seven full octaves plus an additional top C. The frequency range of the reeds is from low CCC of 32.7 Hz. to 4186.0 Hz.

The reeds in the electrostatic Wurlitzer are free reeds made of brass. As such they are similar to those found in accordions and reed organs, and indeed they sound the same. They are called free reeds because they are fastened to suitable frames called shallots and vibrate over very close tolerance openings in the shallots. The openings in the shallots are slightly larger than the reeds, and therefore the reeds vibrate into these openings during part of each cycle of vibration, but, except for the ends where they are fastened to the shallots, they do not touch, or are free of, the shallots for their entire vibrating length.

Tolerances on these reeds are very close, especially in the case of the smallest reeds which produce the highest generated tones of the instrument. Here, the clearance between the reed and the opening in the shallot is not more than 0.001 inch. Manufacturing, adjusting and tuning these small reeds requires the precision of watchmaking. The larger reeds which are responsible for the lowest frequencies are still close tolerance devices, the clearance between the reed and the opening is about 0.01 inch.

Needless to say, eighty-five brass reeds all vibrating simultaneously produce a lot of very unpleasant noise. The tones of free reeds are in general not the nicest of musical sounds by any means. You need only listen to an accordion to confirm this widely-shared opinion. There really is a reason for the multitude of accordion jokes around, such as this one: What is the definition of a gentleman? A man who knows how to play the accordion, but refrains from doing so.

If you have 85 reeds all sounding continuously, it is necessary to employ some rather elaborate sound- proofing, and indeed in the typical Wurlitzer electrostatic organ, there are no fewer than five different layers of material between the reeds and the outside world. The reeds are arranged in octave groupings in a series of seven steel pans on the reed unit. Each pan is lined with 1/8" thick rubber. Already this accounts for two layers of material.

Surrounding the reed unit is a special three-layer enclosure which consists of a steel-frame rectangular box with, from inside to outside, first a layer of Masonite, then a layer of 1 inch thick felt, and finally a layer of 5/8" Homasote. Thus, five layers of material; rubber, steel plate, Masonite, felt, and Homasote all combine to insulate us from the sound of the reeds, and such elaborate soundproofing is necessary to make these instruments playable, for the continuous racket of the reeds would otherwise render these electrostatic organs useless.

In order to keep all of these reeds vibrating, it is necessary to develop an air pressure differential across them, which function is accomplished by a centrifugal compressor which is an integral part of the reed system. All of the reeds are connected to a central low-pressure region which is routed to the compressor intake. The compressor discharge feeds channels which go to the interiors of the steel pans that surround the seven groups of reeds. The pans are held in place by suitable bolts and seat on gaskets on the main body of the reed unit, thus making the system hermetically sealed when all pans are properly bolted in place. Cleanliness is absolutely essential to the proper functioning of free reeds, and especially when they are used in an electrostatic generating system. Therefore, when all pans are in place, the interior of the reed unit is isolated from the outside air. The air within the reed unit is continually recirculated through the system by the compressor, a 3600 RPM centrifugal blower unit with a squirrel-cage type impeller which is powered by a two-pole induction motor.

In the earlier ES instruments, the compressor impeller was behind the reed unit and necessitated a right-angle gear unit as part of its drive. In later versions, this was eliminated by redesigning the reed unit so that the centrifugal compressor was on the top of the unit and in a direct line with the motor.


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