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Wurlitzer Orgatron

How it works


Major S. J. Wright

(Wurlitzer Organ Co.)

To understand the difference between the Wurlitzer Orgatron and other electronic organs, and the reason for its majestic and beautiful tones, that have so long been associated with the finest pipe organs, one needs to be familiar with how these tones are generated.
The tone-producing unit, used in the Orgatron to produce its fundamental and harmonic frequencies, is in reality a "free" reed. These reeds establish their frequencies by means of narrow tongues of thin and elastic metal each of which is set in an individual frame provided with a rectangular orifice, above which the tongues, which are slightly smaller, are affixed at one end, so that when in movement the unattached end is free to oscillate into and out of the orifice in the frame without touching at any point. (note:- a more easily understood explanation of the free reed operation can be found on Wikipedia)
The harmonies which are created by the reed are obviously natural ones, inextricably bound up with the corresponding fundamentals. The tones produced are governed by precisely the same immutable laws of nature that control the making of tone in any wind-blown instrument. The combination of tempered fundamentals with natural harmonics.
Each reed is contained within a cavity, or reed cell, which coincides with a narrow oblong aperture morticed through the pallet board and sealed by a movable pallet which is affixed to the armature of an electro-magnet. When a stop tablet is operated and a key depressed, the electro-magnet operates the pallet, and direct communication is thereby established between the windchest through the reed cell to the atmosphere and sounds the reeds for as long a duration as the key is depressed and the pressure in the windchest maintained.
The Orgatron system of tone-production is not concerned about the tone of the reed itself, as only the fundamental and harmonic frequencies of the reed are used, so a method of silencing is applied to render the tone inaudible.
The reeds of the various Orgatron tonalities are all connected in parallel to adjustable polarizing voltages, and when they are set into periodic motion they constitute one side of a condenser microphone.
The electrostatic tone screws of the various stops are also connected in parallel and terminated to different input channels in the pre-amplifier. The tone screws constitute the other side of the plate of a condenser microphone. Between the tone screws and the reeds exists what is known as an electrostatic charge, The movement of the reed tongue varies this charge thus setting up an alternating current that is impinged upon the grid ot the first amplifier tube.
The tone screws, which are insulated from other assemblages in the tone producing system, are placed over a predetermined part of the reed tongue, according to the extent of the disconant harmonic elimination desired. The electrostatic impulses created by the various tone-producing elements are conveyed to the amplifier of the Orgatron and tone cabinets, where they are translated into sounds of different characteristics.
The Orgatron tone-producing principle may be more easily understood by comparing it to the action of a condenser microphone. The condenser microphone is composed of two metal plates, which are in close proximity to each other. One of these plates is rigid and stationary, the other plate, or diaphragm, responds to any vibrations which strike it, causing it to move to and from the stationary plate. The capacity of the unit is governed by the area and distance between the two plates. When the diaphragm is moving away from the stationary plate, the capacity is reduced to a minimum level, and as the diaphragm approaches the stationary plate, the capacity is increased to a maximum level.
This movement is produced alternately, the voltage which is connected with those plates rises and falls in direct proportion to these alternate changes in capacity, and it is this rise and fall in voltage that is conveyed to the amplifier. The tone screw may be compared to the stationary plate of the condenser microphone, while the reed tongue may assume the part of the diaphragm. The increase in capacity between the tone screw and reed results in an electrical impulse that is conveyed to the first tube of the amplifier, and from there to the speaker.

Extract from "Theatre Organ World" by Jack Courtney
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