Theater Organ TechTalk -


A Series of Articles on Theater Organ technology - how things were (and are) done - and why.


The Relay - and Robert Morton Relays.


Extension and Unification offered the potential that Theater Organs could be extremely versatile and competent instruments - while at the same time affordable and small enough to fit in relatively small spaces. However - before that potential could be realized, a very complex issue had to be solved. While the number of pipes required were considerably reduced, the complexity of connecting the keys of the manuals (and pedals) to those pipes became increasingly difficult - particularly when one considered the additional requirement that any rank be playable to some degree from any manual (or pedals). As the number of manuals and ranks increase - the matrix of possible connections increases exponentially. Even modest organs with two manuals and five or six ranks have a connection matrix that numbers in the thousands of connections. A mechanism was needed that could handle dozens -to nearly a hundred connections at a time- and be able to connect and disconnect those connections in a fraction of a second; be easy for the organist to operate, be reliable enough to operate tens of thousands of times - and finally - not be cost prohibitive. Pretty tall order for the technologies available in the late 1800s and early 1900s. However - such a device did exist - though at the time not at the scale needed for this task. That device is the relay.

As it's name implies - a relay is a device - which passes on something - based on what is passed to it. In this case - the electrical signal from a keyboard contact - to a pipe which is to sound is passed to the contact of a relay - which either passes it to a pipe to sound - or not - depending on whether or not the relay is being sent a "connect" signal.

Connecting a signal - or not sounds just like what a switch does - and true enough - in it's simplest form - a relay is just that - a switch - but with an extra element. Unlike a switch - which is operated by it's contacts being directly operated- the contact (or contacts) of a relay are operated remotely by the presence of an electrical signal. This has several benefits: The relay can be remotely located - for instance - far from where it's being controlled - close to what it's is controlling. Because it can be remotely located (rather than having to be within reach of the organist) - it's size isn't a factor at either the location from where it's being controlled - nor the location where whatever is being controlled is located. This is important in a console - as a relay - which may be relatively large - can be mounted away from the switches which control it - allowing more switches for the operator (organist in this case) to be located within easy reach. Another benefit of relays - is that the actual opening and closing of Pipe valves - particularly for some of the very large pipes - can be difficult and take a lot of time. Some of the lowest pedal notes on a Classical Organ are very difficult to operate. However - in an organ with relays doing the work - turning a rank on or off; - or sounding a note requires little effort by the organist - or stop-tab. It's the relay's job to take care of any "work" that needs to be done, allowing the organist to concentrate on his performance - rather than the mechanics of making the correct pipe(s) sound.

So how does a relay work? In it's simplest form - a set of contacts are mounted to an electromagnet such that when a current passes through the coil of the electromagnet - the contacts are moved -- from open to closed - or closed to open - either is perfectly valid depending on the contacts design and purpose.

In the example to the right - one can see that when the circuit through the battery circuit through the coil is completed by closing the switch - the armature at the top is pulled down - closing the circuit through the contact on the armature and the fixed contact below. In this case - the controlled device is a lamp - which lights when it's circuit with the batter is closed. If the lamp were replaced by a valve - then the pipe would sound instead of the lamp lighting.


Why not just hook the pipes directly to the manuals (and pedals) with a switch - rather than using a relay?

There are several reasons: While this simple relay has only one contact that is either on or off - the relays used the Stop Relays switch anwhere from 20 to 61 contacts - and the fixed contacts may contain as many as 97 individual contacts. Most Theater Organ relays are compound relays - in that they have a single "set" of fixed contacts - and there may be several armatures (complete sets of moving contacts) that connect to (share) those fixed contacts. The rank of pipes being connected to is wired to the fixed contact set; and the manuals and pedals (at various pitches) are connected to the moving contacts (armature).

When the organist operates a stop-tab - say FLUTE for the Accompaniment Manual - he flips the tab on the Stop-Tab rail - which is a small (and easy to operate) switch - which then causes the connected relay armature to operate. The switch energizes an electro-magnet that admits wind to a bellows - which then pulls it's attached linkage that in turn operates the armature of the Stop Relay. The moving contact set (armature) connected to the Accompaniment manual is operated - connecting it's 61 contacts to the fixed contacts wired to the FLUTE rank - at the desired pitch level. Each pitch for each manual (or pedals) has it's own armature - so if there are 4 pitches of Flute available to the Accompaniment and Solo manuals; and two for the Pedals - there would be a total of ten armatures sharing the fixed contacts going to the FLUTE rank. Note that the 8 armatures going to the two manuals would each have 61 contacts - located at the appropriate pitches - while the two armatures for the Pedals would have 32 contacts - again located at the appropriate pitches. In the Extension and Unification TechTalk - we explored how the manuals could be connected the ranks at the various available pitches - however - we were using hard-wire connections - which aren't practical in the real world. The Stop Relays make the task of connecting (and re-connecting) the Manuals (and pedals) at various pitches quite practical.

As noted - the switch operated by the tab on the Stop-tab rail is small and easy to operate. This is important in several respects. Being small - many can be included in the limited space of the Stop-tab rail. If switches large enough to do the job were physically located at the Stop-tab rail - there wouldn't be space for but a few- severely limiting the number of available stops. The space issue could be solved by mounting those switches elsewhere in the console - and connected to the Stop-tabs via linkage. The problem with that is large switches require a lot of force to operate - making it difficult for the organist to quickly make Stop changes. This would also require some sort of external operator to allow pre-set and combination action to be used (the ability to store and recall Stop-tab settings via push buttons and tow pistons). If such an external operator were employed - why not just let it operate the switches in the first place - i.e. as a relay? Obviously relays are the best solution. (Robert Morton Pre-sets and combination action will be covered in a future TechTalk).

Here are two relays located in the front of a Robert Morton Console. The upper relay has 73 fixed contacts that connection to the Violin Celeste rank. Four armatures share those fixed contacts - and connect (from top down) the Solo Manual at 4'; the Solo and Accompaniment Manuals at 8 ' and the pedals at 8' The lower relay connects the 49 notes of the Chrysoglott. The bellows are located in the lower section - their pull rods entending through the top cover.

Here is a "view" from the side of a relay - showing how the armature moves it's contacts into contact with the fixed contacts. The top and bottom armatures are not operated (and their contacts not connected) - while the middle armature alternately rotates it's contact back until it connects with the appropriate fixed contact, then once again back to disconnect the pipe (stop). Manufacturers each have their own ways of making the relays - however the basics are the same. They all have an armature that makes carries a manual (or pedals) and connects those key contacts to the appropriate pitch of a rank.

With all pipes of a rank available on the fixed contacts of a relay - then armatures spanning those contacts have contacts mounted on them - positioned to align with the appropriate pitch within the rank. Here is an simple relay for a TIBIA rank in a small organ. There are 97 pipes in our extended rank - one armature for the pedals (at 16'); one for the Solo manual (again at 16'); and four armatures for the Accompaniment manual - one for each pitch of 16', 8' 4' and 2'. By operating one or more of the armatures (each is controlled by a stop-tab) - the manual (or pedals) are connected to the appropriate pitch of the TIBIA rank. In this simplified drawing - the keys of the key boar - and the contacts in the armatures are wired 1 to 1 - that is note one on the manual - is contact one on the armature. But Note - that where contact one is located is NOT necessarily adjacent to fixed contact number 1 - Depending on the pitch - the armature contact start at the first note of the pitch. So 16' (C1) is fixed contact #1, 8' (C2) is fixed contact #13, 4' (C3) is fixed contact 25; 2' (C4 or Middle C) is fixed contact #37 (and is noted by the red fixed contact). The remaining "Cs" are: C5 is fixed contact #49; C6 is contact fixed #61; C7 is fixed contact # 73; C8 is fixed contact #85; and the top C is fixed contact #97. Pipes run the same numbers. If this rank started at 8' (rather than extended down to 16') then all fixed contact numbers would be shifted down by 12 -- illustrating once again why making the 8' rank the "Unison" rank was so important - the organist does not need to keep track of pipe numbers, etc. within the ranks - just what pitch is desired- the relay takes care of connecting the keys to the correct pipes.

To make this bit clearer - here is one showing the routing of the Accompaniment manual to the 8' rank of Tobias.

Note that the lower armatures aren't "full width" - they are only as wide as they need to be. The same is true of the fixed contacts - if they aren't needed for anything - why waste the materials? This is very common - and explains why the armatures are grouped by "where" they connect - rather than together by "family" (say all of the Accompaniment manuals, then Solo, etc.).

Though not shown; an armature whose contacts start at some point OTHER than the start of an even pitch is known as a "Mutated Stop". These stops are at some odd interval (such as 2 2/3' and 1 3/5') - and are usually used to add "color" or texture to other stops through the mixing of their harmonics.

As noted before - this is a simplified representation of the system and circuits - and is only to help illustrate the concepts. For instance - here is a problem that would be caused by having a single contact on each key connected directly to the Stop Relays: Let's say you have the stop selected for the Accompaniment Manual for the 8' Tibia (as above) plus the 8' Diapason. Let's also select the 8' Tibia for the Solo Manual. Now we press down Middle C on the Accompaniment Manual. C4 Tibia pipe and the C4 Diapason pipes sounds as expected. Now press Middle C on the Solo Manual. We expect to hear C4 in the Tibia rank (and we do) - but we also hear C4 of the Diapson! Not Good. Here are some diagram ti explain what is going on:
Here we have a partial wiring diagram showing the accompaniment manual connected to the 8' stop of the Tibias and the 8' stop of the Diapasons. We've also selected the 8' Stop of the Tibias for the Solo Manual as well. Pressing the C key on the Accompaniment Manual sends the keying voltage through the 8' Stop of the Tibia relay to the electro-magnet that operates the C Tibia pipe - and it sounds. The keying voltage is also passed on through the 8' Stop of the Diapason relay to the electro-magnet that operates the C Diapason pipe - and it sounds - All is Good. Now - without changing anything - we press the C key on the Solo Manual. We expect to hear the Tibia - which, of course we do - BUT we also hear the Diapson - which isn't selected (at least not for the Solo Manual). What's the deal? Looking at the diagram - we see that the keying voltage is going through the Solo Manual, through the Tibia 8' Stop relay - and on to the C pipe. But also note that the Accompaniment Keyboard is also connected to the 8' Stop of the Tibias - AND to the 8' Stop of the Diapasons. This is where the trouble is - That common connection is supplying Keying power to a stop (rank) that isn't supposed to be sounding.
Robert Morton's solution on organs with up to 16 stops on a manual. By providing "isolation" between the Stop relays - the unintended path to other stops / ranks is eliminated.
The solution in principle is easy - we need to separate or more correctly stated isolate the stops from each other. In practice - the shear number of contacts involved complicates the matter quite a bit. There are two (well, three really) ways to accomplish isolation between the Stop Relays: First is to provide a contact on each key for each Stop that each key is connected to. With a typical 2/6 instrument - there may be as few as a half-dozen and as many as a couple dozen stops available to a manual - which also means to each key. If there are eight stops - then there needs to be eight contacts. More stops - and adding second-touch adds up very fast. Robert Morton made key contact assemblies that contained 8 contacts. Each key could accommodate three of these assemblies - two as "first touch" contacts - and one set up as "Second Touch" contacts. So equipped - Robert Morton consoles then can accommodate 16 primary and 8 secondary stops per key - which, of course means per manual.

Larger organs can have a huge number of stops - and it would be quite impossible to accommodate sufficient separate contacts on the keys for each. The solution for these organs, then is a Keying Relay. Much like the Stop Relay - a single control line from a key causes a relay to operate - closing as many contacts as needed. Since there needs to be a keying relay for every key - on a moderate sized console with four manuals - there needs to be (at least) 276 keying relays (4 times 61 keys PLUS 32 for the pedals)- more if the number of stops exceeds the available contacts on one keying relay. One can see how the complexity (and cost) of these larger organs can skyrocket.

A third solution was mentioned - and is common in newer installations - and that is a solid-state solution. Isolation can be achieved by placing a diode in the path between the key contact and each of the Stop relays. Since current can flow only one way through a diode- "reverse path" problems from the common connections are eliminated. Another solid-state solution is to "encode" each key of a manual - and send that data to some sort of processor. One common approach is to encode the manuals so that the keys generate MIDI messages - which can then be sent on a a solid-state relay in the chambers - or to a computer, etc. to control the digital reproduction of a Theater Organ by playing "samples". Such an instrument is known as a Virtual Theater Organ - and will be covered in another installment of TechTalk.


Robert Morton relays aren't different so much in how they are made - or operate - as much as where they were installed. Relays manufactured by various companies might differ in details, etc. - however they accomplished the same thing - providing easy and competent access to the resources of the instrument. Where Robert Morton and many of the manufacturers were different - was the most mounted all of their relays in a rack - or cabinet that could be installed anywhere between the console and chambers - or inside the chambers themselves. During the height of manufacturing at Robert Morton - the instruments were built in an almost assembly line manner - using many techniques common to mass manufacturing. This was also a time when there was a rush to get organs installed in motion picture studios - and competition for sales was intense. Robert Morton developed what would become a "standard" small theater organ that would be price competitive, taking little space and requiring little "custom" work to fit into most any building space - and could be installed very fast. This was their 2/6 model - of which more than 100 were installed in a fairly similar configuration: The console was self contained - all contacts are on the keys (and pedals) without the use of keying relays. The switch stack (Stop Relay) is also mounted within the console - as is the combination action controls. While this makes the console a bit larger than other consoles of similar features (and very much heavier) - the lack of having to find space for - and the much greater ease of installing these externally more than made up for the those issues. Another unusual step was to make their unit chests individually for each rank. While that required a bit more doing to pipe wind, etc. - the flexibility in being able to fit into unusual and cramped spaces again more than made up for that issue. The ranks usually included in these organs were Tibia, Trumpet, Vox Humana, Flute, Violin and Violin Celeste. Having the relay right inside the console provided another benefit - less connections to hook up when installing. The cable between the console and chambers came pre-assembled, and had to be attached (and hopefully) soldered to the fixed contacts of the relays. The other end connected to the chests and the organ was (save for some auxiliary connections) wired. See the next "TechTalk" about Robert Morton standardized connector system.

One last detail of "Extension and Unification" and relays - - Foldback. Recall that one of the main purposes of Extension and Unification is to provide the most "bang" for the buck. During the hayday of Theater Organs - the business was quite competitive. In order to keep the price down - ways were found to cut corners without compromising performance too seriously. One area where costs could be contained were in the upper end of some of the "Extended ranks". Take a rank such as the Vox Humana. The standard rank is 8' pitch - 61 pipes. Unification makes it easy to also play the rank at 4' as well - save for one small problem - the "Usual" way to extend a 61 pipe rank to reach 4' is to add 12 notes to the top end. However - that requires more pipes, larger chest, more wires, etc. However, the top octave of most ranks (and the octave below that for many ranks) all sound alike (much like a child's toy flute). This is were Fold-back comes in. When a 61 note rank is played as a 4' stop - one runs out of pipes before one runs out of keys on the Manual. If both ranks are being played together - the missing last 12 notes of the 4' stop would cause a small drop in volume and fullness - but not be all that obvious. It's when the stop is played by itself that it becomes an issue - as the top 12 notes of the manual are silent. Not good. To accomodate this - the last 12 notes are repeated - called "foldback". When playing the top two octaves of an 8' stop the notes play as:
C4, D4, E4, F4, G4, A4, B4, C5, D5, E5, F5, G5, A5, B5, C6 -
while the same two octaves played on the same rank at the 4' pitch would sound:
C5, D5, E5, F5, G5, A5, B5, C5, D5, E5, F5, G5, A5, B5, C6. Playing typical songs - unless one is listening very close - it's highly unlikely that it would be noticed. And when played with other ranks - near impossible to hear. Fold-back is accomplished very easily by the Stop Relay - where the top octave of the manual is wired in parallel with the octave below. The isolation of separate key contacts (or keying relays) keep the commoned notes from corrupting other stops - so the solution works well.

Return to TechTalk

Next page: Theater Organ Tech: Robert Morton standardized connectors

 Return to Main Page


If these mighty instruments interest you - come on and join us in keeping these wonderful gifts from the past making beautiful music!

Copyright © 2009 The Magnolia Chapter ATOS. All rights reserved.