Reed organs

Reed organs were the first type of electronic organ offered by Wurlitzer. The technology first appeared in the Orgatron, designed by Frederick A. Hoschke and commercially released by the Everett Piano Company in 1935. Wurlitzer acquired the technology from Everett in 1945.

Unlike later organs, notes in a reed organ are not generated by electronic oscillators. A reed organ is an electronic adaptation of a pump organ. In a pump organ, the player presses a foot pedal, which operates a bellows. The bellows pulls or pushes air past a metal reed, which vibrates, generating acoustic sound. (Whether the air is pulled or pushed differentiates an American reed organ from a harmonium.)

The electronic reed organ operates under a similar principle, but its sound does not come from the acoustic tones generated by the vibrating reed. In fact, the vibrating reeds are muffled by an enclosure, so that the user cannot hear them at all. Instead, the vibration is converted into an electronic signal using an electrostatic pickup. This is the same principle used by condenser microphones and, later, electronic pianos. All three technologies are premised on the fact that any two conductors separated by an insulator form a capacitor. (Note that “condenser” is an old word for capacitor.)

In a reed organ, the each reed is positioned next to one or more tone screws. The reed is mounted so that it vibrates freely when air is blown over it, while the tone screw remains stationary. The tone screw can be brought closer or further away from the reed, changing the timbre of the note, but it will always be separated from the reed by an air gap. The end of the screw and the tongue of the reed each form one plate of the capacitor, while the air gap is the dielectric. As the reed vibrates, the distance between the reed and the tone screw changes, so that the capacitance is continuously changing.

When the organ is powered on, the tone screw is connected to the amplifier’s power supply through a large resistor, while the reed is held at ground potential. Now, when the capacitance of the reed/screw assembly changes, a small signal voltage appears across the resistor. This signal voltage is amplified by the electronic circuit until it is strong enough to drive the organ’s speakers.

Although both electronic pianos and reed organs use an electrostatic pickup, the geometry of the reeds is more complicated in reed organs. In the electronic piano, the pickup is a large, comb-shaped electrode that is mounted at a fixed position from the reed. The capacitance between the vibrating electronic piano reed and the fixed pickup generates the electronic signal. The signal can be modified in the amplifier through filters and effects, but the pickup geometry can only generate one type of sound.

In contrast, the tone screw forms the positive plate in the reed organ circuit, and each reed can have multiple tone screws positioned along different points of the reed. This makes it possible to generate multiple organ tones from a single reed. And because the reeds vibrate through the motion of air instead of being physically struck by a hammer, each note can be associated with multiple reeds. The National Music Museum at the University of South Dakota notes that the Everett Orgatron in its collection has four reeds for each note, and each reed has two tone screws, which allows each note to generate eight possible tones.

Furthermore, electronic piano reeds are mostly flat, with a slight taper to ensure adequate sustain. Organ reeds are twisted. In Electronic Organs (1953), Robert Eby explains that the shape of organ reeds is intended to “eliminate unwanted harmonics and impart to the reed a fuller tone.” He writes: “The twisting and bending of the reeds is known as voicing. The voicing of the reeds is done at the factory by experts.”