How A Theatre Organ Operates
*Click a photo to enlarge*
The original Wurlitzer relay
Close shot of keying relay
Switch Stack
The Relay
This is the brains of the organ....an early computer. This is what allows the console from having to be directly and mechanically connected to the pipe chests them selves. When a person presses down a stop key at the console for a specific set (rank) of pipes to be played, a stack switch moves up and is activated. There is one switch for each stop key and one wire for each note (or pipe) in each switch. When a person presses down a key at the console, a pneumatic inside the keying relay (as seen behind the glass of the pressurized cabinet in the above picture) is exhausted. The pneumatic has a buss bar on it which is usually a piece of real silver rod that makes electrical contact with the wires it touches. There is one keying pneumatic for each note on the keyboard. There is a wire that runs from each pneumatic up to a contact representing that note on the switch. Confused yet? The electrical signal from that buss hitting the contacts for the keys being played is sent to the switch stack and which ever switches are activated are then sent to........
Main Chest
Tibia
....here, the pipe chests as pictured above. This is the main chest and independent Tibia chest. All of these are original (circa 1932) with the exception of a newer tuba rank which is in the front closest to the swell shades. The Tibias are what bring the ensemble together and are a key part of what makes a theatre organ what it is. These aren't completely original, but close enough to it. This is the rank that was added before being installed in the Northeast Temple Lodge in Buffalo, NY. There was a lot of time spent on "voicing" this rank that gives it the sweetest sound.
The signal from the relay activates a magnet under the pipes to be played and keeping with tradition, there is one magnet per pipe on each rank. The armature in the magnet is pulled up which changes the pressure in the channeling leading to the pipe and exhausts the primary pneumatic and thus exhausts the secondary pneumatic allowing the air out into the pipes. You can actually hear the clicking and thumping of the relay as the person plays. Current organ builders actually use a computer now which takes up no room at all. It just consists of computer boards that can be mounted right on a wall. Nothing can beat the ingenuity of early technology though.
Lets recap. Lets say the instrument consists of 1 keyboard, and 1 set or rank of pipes. That set of pipes is available at 16', 8', 4' and 2' pitches. So that means there are 4 switches since there are 4 stops. There are 61 keys on a keyboard so that means there are 61 pneumatics in the keying relay. Each of those pneumatics has 4 wires on them because there are 4 stops that control 4 switches. You are at the console and press down the 16' stop, that switch is activated. You play a single note, lets say low C. That collapses the first pneumatic in the relay because that represents that low C note and that plays the 16' switch that is on which results in playing the low C pipe of that rank.
The signal from the relay activates a magnet under the pipes to be played and keeping with tradition, there is one magnet per pipe on each rank. The armature in the magnet is pulled up which changes the pressure in the channeling leading to the pipe and exhausts the primary pneumatic and thus exhausts the secondary pneumatic allowing the air out into the pipes. You can actually hear the clicking and thumping of the relay as the person plays. Current organ builders actually use a computer now which takes up no room at all. It just consists of computer boards that can be mounted right on a wall. Nothing can beat the ingenuity of early technology though.
Lets recap. Lets say the instrument consists of 1 keyboard, and 1 set or rank of pipes. That set of pipes is available at 16', 8', 4' and 2' pitches. So that means there are 4 switches since there are 4 stops. There are 61 keys on a keyboard so that means there are 61 pneumatics in the keying relay. Each of those pneumatics has 4 wires on them because there are 4 stops that control 4 switches. You are at the console and press down the 16' stop, that switch is activated. You play a single note, lets say low C. That collapses the first pneumatic in the relay because that represents that low C note and that plays the 16' switch that is on which results in playing the low C pipe of that rank.
