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Los Angeles Theatre: stage

615 S. Broadway Los Angeles, CA 90014 | map |

The Los Angeles Theatre pages:  history | vintage exterior views | recent exterior views | entrance | grand lobby | main floor inner lobby | lobby - 1st balcony level | basement - intermediate lounge | basement - main lounge | ladies room and nursery | men's room | vintage auditorium views | recent auditorium views | stage | boothretail and support spaces |


The view off toward the lockrail stage right as seen from the lower level of the house right organ chamber. Photo: Bill Counter - 2013



Looking into the auditorium from downstage right. Photo: Bill Counter - 2013



A plan of the stage at the Los Angeles. Thanks to Broadway Theatre Group. The plan appears on their Los Angeles Theatre website.

Note the two columns -- they mark the dividing line between the downstage (with a full height grid) and the upstage -- a "bustle" starting at 32' high and tapering down to 28' at the back wall. Veteran downtown projectionist Tom Ruff notes that it wasn't to save money (it probably didn't) but to provide a lightwell for tenants facing east in the William Fox Building directly behind.


Proscenium width: 60'

Proscenium height: 32'

Stage width wall to wall: 99'

Wingspace SL: 19' 6" from proscenium to sidewall.

Wingspace SR: approximately 16' proscenium to lockrail.

Obstructions: Both stage left and right we get columns about 30' from the centerline that support the wall at the back of the full height grid. Upstage of the columns we're in the bustle area. See the plan.

Loading: Double doors upstage right lead out onto St. Vincent Court. Also a smaller door upstage left, out into the exit passageway and thus to the alley off 6th north of the theatre. Side auditorium exits also open onto this north alley and St. Vincent's Court.

Screen size: 27' x 55'

Projection throw: 136'

Orchestra Pit: It's currently covered with a thrust stage now extending way out into the auditorium. The pit was never on a lift.

Stage depth: 25' from smoke pocket to back wall

Thrust depth: 20' from the smoke pocket to the lip of the stage at centerline.

Asbestos curtain: Release stations are both downstage left and downstage right. Evidently it's a rather plain beige -- not decorated.

Grid height downstage: The theatre's venue specs page lists 68' for the downstage 15'. 75' is the number in their tech pdf.

Downstage rigging: 21 Armstrong-Power type linesets rigged from the full height grid are operated at stage level stage right. The sets are tracked, Armstrong style, both front and back. Set #1 is the asbestos. Set #2 is the decorative act curtain, originally motorized, now operated manually. Set #3 is an Austrian drape style teaser. A downstage traveler on set #5 is motorized. Set #6 is the screen frame. Set #7 is the lambrequin, downstage of the asbestos. Other sets are 500 pound capacity.

Loading bridge: There isn't one.

Flyfloor: None

Bustle height upstage: 32' with a taper down to 28' at the back wall.

Upstage rigging: 10 underhung sets in the bustle still have sheaves on the underside of the concrete slab but no headblocks or lockrail or pinrail. The last couple of sets upstage are rigged with sandbags hanging offstage right. The rest are not rigged.

Projectionist Tom Ruff speculates that these sets were operated stage left from a now-missing platform supported from the mid-stage column. It's unknown whether these were hemp or counterweight.

Traps: One approximately 4' x 8' in the middle of the stage at centerline. No longer accessible as the basement ceiling below has been plastered. There is a usable 2' x 8' trap USL.

House light control: Operated either in the booth or downstage right. Some pots on the original 1931 board control dimmers in a rack behind the booth.

Stage lights and dimmers: None except 1st and 2nd balcony rail spots (PCs and PARS) permanently hard wired. A number of pots on the original 1931 board have been rewired to control dimmers on a Strand SCR rack in the basement that houses 16 12 Kw stage lighting dimmers.

Company switch: 800A single phase 120/240 in the basement downstage right. There's a new 3 phase 4 wire feeder for lighting loads onstage. If more power is needed a 1600 A 3 phase 240 V Delta switchboard is on the north side of the basement near the kitchen.

F.O.H. patchable circuits: None

F.O.H. mounting positions:
No box booms. No pipes on either balcony rail.

F.O.H. hanging points: A number of holes below roof trusses are usable.

House sound system: None, except for film sound.

Backstage access from the auditorium: Just go down the house left side aisle and you head directly onto the stage. No access house right.

Dressing rooms: Eight, all in the basement, down one level. Access is via stairs offstage right. Or out the exit door upstage left and down the exit stairs coming up from the basement.

Wardrobe / green room: It's the trap room -- a large usable area downstage of the dressing rooms, which are strung along the upstage wall. This level is 11' 3" below stage.

Crossover: There isn't one inside. The dressing room exit stage left goes up and outside to the exit passage north of the theatre. From there you can come back in via a door USL. Stage right has stairs down to dressing room level that continue down to the main lounge level.

Some of our information here comes from the theatre's fine website. See their venue specs page as well as a 17 page downloadable tech packet in pdf format. Also check out the very interesting venue comparison page with an overview of the Los Angeles as compared to the Tower and the Palace.



Another view from offstage right. The horizontal beam is part of the column structure that's the dividing point between the full height stagehouse and the lower height upstage "bustle." Photo: Bill Counter - 2013



Looking offstage right. Photo: Bill Counter - 2013



The dimmerboard -- an early thyratron electronic control system instead of the more typical resistance dimmers of the era. At left, the original 1931 control panel. At the right is a 1944 retrofit. Since this photo was taken, the wing on the right has been disconnected and moved over to stage left. Thanks to Floyd B. Bariscale for his 2009 photo. Check out his Big Orange Landmarks blog and his Los Angeles Theatre article. Also see his 93 item Los Angeles Theatre album on Flickr.

Thyratron controlled dimmers were also used in the systems at Radio City Music Hall (1932), Rockefeller Center's RKO Center Theatre (1932) and the Civic Opera House, Chicago -- to name three of the more famous installations. These early systems used two thyratron tubes to regulate the DC control current to each saturable reactor dimmer.

The system at the Los Angeles allowed for remote control of dimmers in two racks -- one in the stage basement for the stage dimmers, one in the electrical room behind the booth for the auditorium dimmers.

The original 1931 Westinghouse control panel had 5 presets, variable speed motorized fading, and automated changing between presets (as long as you wanted them in numerical order). Each dimmer had a rehearsal fader as well as 5 small sliders above for setting levels on the 5 presets.

At the far left are various sub masters. To the right (below the IATSE logo) are controls for flashing the various presets. The wheel below is the board's grand master. The switches underneath the master wheel are for various stage worklight circuits. The meters on the board (labeled 1-5) are not voltmeters but indicators to advise which preset is active.

There's a similar board in the booth. House lights and some stage dimmers could originally be controlled from either location. The small DC wattage needed to run the system was provided by a multi-unit motor generator set in the electrical room behind the booth.

On the right in the photo above is a 1944 retrofit panel, also by Westinghouse. The thyratron tube system was abandoned as it was getting undependable. Here the dimmers were controlled by resistance plates that directly varied the DC control current. With the abandonment of the original control scheme, there was no lighting control from the booth.

There are now Strand Century SCR dimmer racks adjacent to the 1931 saturable reactor racks in both the booth and stage locations. A number of faders on the 1931 boards in the booth and on stage have been rewired to control those dimmers.

Head down to the bottom of this page for more details about the thyratron / saturable reactor dimmer system. Thanks to dedicated long time projectionist and all-around theatre wizard Tom Ruff for information on the system at the Los Angeles.



Another photo taken before the 1944 retrofit board was disconnected. We're looking offstage right with the lockrail beyond. Thanks to to Ed Baney and the theatre's team for this photo. It appears in the stage section of the Los Angeles Theatre website's photo gallery.



Behind the lighting control board. That horizontal shaft we see is part of the motorized drive to fade between presets. Photo: Bill Counter - 2013



A look at the original section of the 1931 lighting control board. At the extreme left of the photo are curtain motor controls. Photo: Bill Counter - 2013



The curtain motor controls. The big vertical lever at the bottom was for the decorative act curtain -- now manually operated. Photo: Bill Counter - 2013



A peek inside the box at the left in the previous photo. It's an electromechanical release that could have been tied into an alarm system to automatically release the asbestos curtain. Or the smoke vents. Or both. Currently it's not operational. The present asbestos curtain release on either side of the stage is a ring at the end of a piece of aircraft cable that's hooked over a peg. Photo: Bill Counter - 2013



The downstage right corner. The lit doorway goes onto the stairs to the stage basement for the dressing rooms or pit access. Or continue down another level (to the main basement lounge level) and you're at the back of the kitchen that's west of the lounge. The door just to the right of the end of the lighting board goes into the auditorium. Photo: Bill Counter - 2013



The view along the T-wall stage right. The concrete beams mark the upstage end of the full height grid -- and the end of the counterweighted linesets. Photo: Bill Counter - 2013



A lockrail view from downstage. Photo: Floyd Bariscale - Big Orange Landmarks - 2009



A downstage look to the grid. This is all there is of the full height stagehouse. Note the underside of the upstage bustle to the right. Photo: Mike Hume - 2015. More of his great work can be seen in his Los Angeles Theatre album on Flickr. Even better, head to the Los Angeles Theatre page of his Historic Theatre Photography website. He has many stage photos as well as fine views of other areas of the theatre.



Standing offstage right looking upstage at the bustle with its underhung rigging. Photo: Bill Counter - 2013



Looking onstage from the house. Note here we see the pit uncovered. It's a Gary Leonard photo in the Los Angeles Theatre website's photo gallery.



Looking up at the bustle and the rear of the movie screen. In the center of the photo note the full height grid downstage. Photo: Bill Counter - 2013



The back of the screen from upstage right. The sandbags at left are for a couple of the linesets upstage. We've got amps and subwoofers on the floor plus L, C & R channel speakers up on stands as part of a sound system designed by Tom Ruff for the LAHTF / Cinespia screening of Baz Luhrmann's "Romeo+Juliet."  Photo: Bill Counter - 2013

The screen frame appears to be a 1931 Armstrong Studios original. Mr. Ruff calls our attention to the two rectangular framed spaces up on the screen either side of center where the original speakers were attached. They flew with the screen. Some Altec "Voice of the Theatre" components from a much later speaker system are currently stored in the basement below the north retail space.



The exciting view in the upstage right corner. The loading door is just out of the frame to the left. Photo: Bill Counter - 2013



Another upstage right view. That's the loading door at right behind the sandbags. Photo: Bill Counter - 2013



A look in from the loading door upstage right. The unit with the green X is the 1944 retrofit dimmer control board, now disconnected. The unit with the light to its left is the 1931 original board, still in place. It's a c.2009 Gary Leonard photo appearing on the stage page of the Los Angeles Theatre website's photo gallery.



Outside the loading door upstage right. The Gary Leonard photo appears on the exterior page of the Los Angeles Theatre website's photo gallery.



In the exit passageway outside the loading door stage right. We're south of the theatre with St. Vincent Court off to the right. The stairs are coming down from the house left front 1st balcony exit. Broadway is straight ahead down the canyon. Photo: Bill Counter - 2013



The view upstage from stage left. Note the recent seismic bracing with the primer-colored brackets securing the sloped ceiling to the back wall. Photo: Bill Counter - 2013



Looking across from downstage left. Note the full height grid downstage and the reduced height bustle upstage with the sloped ceiling. Photo: Bill Counter - 2013



The house from downstage left. Thanks to Ed Baney and the theatre's team for the c.2009 Gary Leonard photo. It appears in the stage section of the theatre website's photo gallery.



Across from stage left. Onstage it's a group taking an LAHTF "all-about" tour of the building. The light behind tour leader Steve Gerdes is the open loading door. Photo: Mike Hume - 2015



Across from stage left with the screen in. That's the loading door to the right of the lights. Photo: Bill Counter - 2013



The view from upstage left. It's a c.1992 photo by Berger Conser Architectural Photography taken for the Los Angeles Theatre and can be seen on the stage page of the theatre website's photo gallery.

Anne Conser and Robert Berger are the authors of the great book "The Last Remaining Seats: Movie Palaces of Tinseltown," available on Amazon. The Robert Berger Photography website has a portfolio of 16 photos from "The Last Remaining Seats."



The upstage left corner. That's the end of the 1944 retrofit wing of the lighting control board parked temporarily just to the right of the exit door. The door goes out into the north exit passageway. Photo: Bill Counter - 2013



The front of the 1944 Westinghouse board. At the time of the photo this "retrofit" control board was parked stage left. Photo: Bill Counter - 2013



Part of the rear of the 1944 lighting control board. It has resistors to directly regulate the DC control current to the saturable reactor dimmers. They had given up on the earlier thyratron tube control system.  Photo: Bill Counter - 2013



Outside the exit door upstage left. We're just north of the theatre facing toward Broadway. The stage is behind us to the right. Through the gate we can exit into the alley that runs north to join 6th St. Photo: Bill Counter - 2013



Looking up with the William Fox Building on the right. Above us at center is the upstage bustle. Left of that is the full height stagehouse downstage. Photo: Bill Counter - 2013


In the basement:


What's left of the motorized mechanism for the decorative act curtain. We're in a little room on the landing half way down the stage right stairs to the basement. Down one flight more and you're at dressing room level. Continue down deeper and you're at the back of the kitchen area on the west end of the main basement lounge. Photo: Bill Counter - 2013



At the stage right end of the dressing room level. Here we've just come down the stairs and are looking across to stage left. At the right is the back of the dimmer rack. Along the left side is a whole row of dressing rooms. Photo: Bill Counter - 2013

Since these photos were taken the whole dressing room area has been elegantly carpeted.  



Center in the basement dressing room area looking stage left. The last doorway on the left is the star dressing room. Beyond are stairs up and out to the north exit passageway. There's no direct access to stage left. Off to the right is the entrance to the orchestra pit. Photo: Bill Counter - 2013



The stairs to the orchestra pit from the center of the dressing room area. The pit is currently covered. Photo: Bill Counter - 2013



Up in the pit looking toward house left. Photo: Bill Counter - 2013 



One of the dressing rooms. Thanks to Michelle Gerdes for this and her other 2009 photos that appear here. Check out her Los Angeles Theatre album on Flickr for 72 great photos.



Another one of the dressing rooms. Photo: Michelle Gerdes - 2009



A peek into the star dressing room. No stars were sighted. Photo: Bill Counter - 2013



In the star room looking north toward the toilet area. Photo: Bill Counter - 2013



The light fixture in the star room's toilet area. Photo: Bill Counter - 2013



The stage left stairs up from the dressing room level out into the exit passageway north of the theatre. Photo: Bill Counter - 2013



Looking back toward stage right in the dressing room area. In the cage straight ahead is the 1931 thyratron controlled saturable reactor dimmer rack for stage lighting. There's a similar rack in the booth for house lighting. The green thing on this end is a newer Strand SCR rack. Photo: Bill Counter - 2013



The front of the 1931 stage dimmer rack. Note the empty trays along the front of the rack. A plug-in module containing the thyratron tubes and the control electronics was originally in front of each dimmer. At the near end is the Strand SCR dimmer rack that has replaced the saturable reactor dimmers. On the floor at the left of the photo are several step-up transformers that used to be wired into the old system to compensate for the voltage drop across the dimmers even when fully up. Photo: Bill Counter - 2013 


More about the dimmer system at the Los Angeles:

Saturable reactors with the control current controlled by a resistance dimmer plate (instead of electronically) were common in big 1920s theatres to handle large loads. The Fox St. Louis, Fox Detroit and Warner Hollywood all used saturable reactors to control some of the large house lighting loads.

A saturable reactor is essentially a large choke coil with enough reactance to dim an AC lighting load fully out. The lights are brought up by putting a DC control voltage through a second winding around the same core, thus "de-saturating" it and allowing current to flow to the lamps. The more DC flowing in the control circuit of the reactor, the brighter the lights.

The innovation with the systems of the Los Angeles Theatre's vintage (and there were not many built) was that they used electronic tubes to regulate the DC control current to the dimmers -- instead of the resistance dimmer plates in a 20s looking board. Thus the controls could be miniaturized, pre-sets were possible, and the dimmers could be easily operated from multiple locations. In earlier boards we could "pre-select" which dimmers were to be turned on and off for different "scenes" -- up to 10 on some boards -- but not the intensity levels.

At the Radio City Music Hall the board was still huge but was designed to be in in a space in front of the orchestra pit so, for perhaps the first time, the operator could actually see the stage. At the Los Angeles the system was split with control either in the booth or backstage.

Two boards at the Los Angeles: One board for the house lights was located in the projection booth with the dimmers themselves in a room behind the booth. The stage board controlled the dimmers for stage lights located in a rack down a floor on the stage dressing room level.

The boards were similar 5 scene preset boards but controlling different banks of lights. But there was some overlap: the stage board also allowed color master control of the house lights. And faders to run up or down whatever was set on the 5 presets of the board in the booth.

And, switchable by tranfser switches so that control could be either in the booth or backstage (but not both simultaneously), were the dimmers for footlights, 1st border, orchestra pit floods, and stage floods on the balcony rail.

The control system: There were actually three tubes to supply the DC control voltage to each dimmer. An early amplifier stage and then the two thyratrons. These were all three-element tubes with the flow of current through the tube regulated by increasing or decreasing the voltage on the center element, the grid.

The voltage applied to the anode and cathode for the two big thyratron tubes actually hooked to the dimmer control circuit was obtained from a power supply with a transformer connected to the 120 volt AC supply. You needed two thyratrons "back to back" as each only conducts for half of the AC cycle. The two outputs are combined, rectified into DC, and sent to the dimmer.

What controls the flow in the two thyratron tubes was an earlier stage using a standard UX-226 amplifier tube. The current coming out of that UX-226 to go to the thyratrons was varied by the appropriate fader on either the stage or booth control board -- the fader output went to the grid of the UX-226 and thus varied the relationship between grid voltage and anode voltage. 

Control modules on the boards: Each dimmer (which might control a number of individual branch circuits that would be controlled simultaneously) had a separate module on the board. At left is a dimmer control module and, at right, a color master typical at the end of a row of modules.

At the top of the module, a "selector switch" had one position for "individual potentiometer" that made the fader at the bottom of the module active for independent or rehearsal use. In the switch's other position, we're in "preset" mode. The output from this "selector switch" was the control voltage (either from a preset or the individual potentiometer, as selected) that went to the grid of the UX-226 tube on the dimmer rack for that dimmer.

Just above the little sliders is the three position "pilot switch" that determined the assignment of this module's individual potentiometer. One position would be independent -- connected to the hot bus and then on to the dimmer. The center position is off, as seen here. But the dimmer could still be active if you had selected preset mode on the top "selector switch." The third position would put the individual dimmer on its color master instead of in independent mode. You could thus use it independently, or via the color master regardless of any settings of the preset sliders.

There's a pilot light between the two switches at the top of the module. It would be lit if the dimmer were up and you had selected the "individual potentiometer" mode on the "selector switch" above rather than "preset" mode. If your "pilot switch" were off of course it wouldn't light. But it would be lit in the other two modes if the dimmer were up -- both in independent or in color master mode, assuming the color master was also up.

The 5 little sliders in the middle of the module set levels for each of the 5 preset scenes. The larger lever below the 5 little sliders is the individual potentiometer. It was active if you had the "selector switch" at the top the flipped to individual control. The other position of the switch would have you in preset mode.

Fading between presets with motor or handwheel: Behind the board and connected to each module was a ganged fader with a set of contacts for each dimmer, all interlocked. It would only fade sequentially: from 1 to 2, 2 to 3, etc. Set at position 1, for example, it would take the output of preset slider 1 for each dimmer and send it to the dimmer rack -- moving the ganged fader either by hand wheel or motor on toward setting 2 would fade out your preset 1 settings and move toward your preset 2 levels.

Controlling the motorized fader: Hit the start switch and it will travel to the next preset and it, because of a limit switch, will stop there. Hit the start switch again and it moves to the next preset.

"Flash" jumping to a preset: In addition to the ganged fader there were "flash" buttons allowing an instantaneous change from one preset to any other. There were 6 multi-contact relays controlled by a row of 6 push buttons marked 1-5 and "I."

Each of the 6 relays had a set of contacts for each dimmer. Using one button would cause the contacts controlled by all the others to open. Pushing button 2, for example, would cause the contacts for each dimmer to close for preset 2 and open for presets 1, 3, 4, 5 and also inactivate the ganged fader.

Pushing the "I" button disabled the direct connection of any of the preset sliders to the hot bus and put them in series with the rotary ganged fader. You then had a circuit from a dimmer module's preset fader (or two if you were in mid-fade) and through the ganged fader before going on the the dimmer control input in the thyratron rack.


 
A drawing of the 1931 thyratron control scheme that appeared with "Thermionic tube control of theatre lighting," an article by Westinghouse engineer Burt S. Burke describing the system at the Los Angeles that appeared in the January 1932 issue of Projection Engineering. He's showing two dimmer modules, the ganged fader, and the relay scheme. At the top, the hot bus "P" connects to the 5 preset sliders for the dimmer as well as to the larger "individual potentiometer" on the module used for rehearsal or color master control. Thanks to Bob Foreman for finding the Burke article. It appears on his site Vintage Theatre Catalogs.

Note the output of each of the preset scene sliders going through a resistor and a set of relay contacts on its way to the module's output. And the output of the preset sliders also connects to the ganged fader, shown here as a round unit with taps marked 1-5. This is a ganged assembly with a similar set of taps for each dimmer on the board. Its output goes through a set of relay contacts as well. That switch at the output near the wire marked "grid lead" is the "selector switch" to determine if this dimmer is controlled from the presets or the individual potentiometer.

There were 6 multi-contact relays -- one for each preset and one (marked "I") for the rotary fader. They're shown at the bottom of the drawing. The contacts controlled by them are those we see in series with the outputs of the 5 preset potentiometers and the ganged fader.

Note: In the drawing that individual potentiometer is connected directly to the hot bus. Omitted is the "pilot switch" which would have three positions: hot bus / off / hot bus via color master.

Color Master / Grand Master control: On the color master section of the board you, of course, had a fader for each color. Like on the individual dimmer modules there was also a "pilot switch" for each. So your red master, for example, could be set to independent, off, or controlled by a grand master fader.

The stage board also had duplicate color masters for the booth dimmers that controlled the auditorium lighting. Whatever was preset on the 5 presets of the booth board could be controlled from the stage as well.

On the booth board the color masters were in series with the presets. Thus, as Burke says regarding the house light circuits: "...by mean of the color masters, it was possible for the operator to dim out any particular color from any scene that had previously been preset thus giving a very flexible control." Thus one gathers that the booth board was wired differently. As he explains the control scheme for the stage board, the color masters aren't in the circuits during preset mode.

Problems: Evidently there was difficulty getting parts to keep the system running -- or it was just unreliable. In 1944 Westinghouse built a retrofit board to control the dimmers. All the tube modules were junked and DC control voltage to the various dimmers was supplied instead by a bank of rotary resistors.

Later solutions at the Los Angeles: The thyratron racks (minus their control modules) are still in place. Those lighting circuits in use were later connected to SCR dimmers. The 1931 control boards are still in place and a few of the potentiometers have been rewired to control the SCR dimmers.

Lots of different manufacturers: The system at the Los Angeles (opening January 1931) was a Westinghouse design. Rockefeller Center's RKO Center Theatre had a thyratron system designed by Hub Electric Co when it opened in 1932. Evidently it had many problems. The system at the Radio City Music Hall (1932) -- again with about the same technology -- was by General Electric.

Things were evolving rapidly. Within two years, the Hub thyratron system at the RKO Center Theatre in New York was replaced with a new Westinghouse board. It was still using reactor dimmers, but the new design by Westinghouse employed a Ward Leonard "hysterset" control system using a little reactor to control the big saturable reactor and only a single full-wave rectifier tube instead of the 10 (!) evidently temperamental tubes per dimmer previously in the system.

Although not separated far time-wise, the look of the Westinghouse control console at the Center was quite different from the one at the Los Angeles. After the Los Angeles installation, Westinghouse figured out how to fade between presets randomly, rather than having to do it sequentially. The dimmer racks also looked different but perhaps they were a retrofit of the original Hub racks.

Other installations of this type included the Civic Opera House in Chicago (GE, 1929), Cleveland's Severance Hall (Westinghouse, 1931), Earl Carroll Theatre in New York (GE, 1931), RKO Plaza in Schenectady (GE, 1931), RKO Palace in Albany (GE, 1931), RKO Orpheum in Denver (GE, 1932), Metropolitan Opera (GE, 1934), Billy Rose Aquacade (GE, 1939) and the Horace Robinson Theatre at the University of Oregon (1948).

Bob Foreman's Vintage Theatre Catalogs article "1954 General Electric Lighting Switchboards," has some great information on electronic boards, especially the 1952 installation at Jones Beach. And thanks to Bob for researching the installations listed above. In addition to GE, Westinghouse and Hub, other manufacturers including Ward Leonard and Frank Adam Electric also offered similar systems in the 30s and 40s.

Later thyratron systems: While these early systems used tubes (and sometimes added reactors) to control saturable reactor dimmers, later systems (primarily developed by George Izenour at Yale University) used bigger, higher power tubes to control the loads directly. He had a working system in 1948 and it was on the market in a partnership with Century Lighting in 1949 as the Century-Izenour System. See the Wikipedia article on George Izenour for a bit of discussion of his work.

Controlling the loads directly with the thyratron tubes meant big, expensive and high heat producing tubes but got away from two of the big problems with the saturable reactor: slow reaction time and restrictions on loading. Like a resistance dimmer, the saturable reactor only dimmed well within a certain range.

Saturable reactor systems also had a problem with a noticeable voltage drop across the dimmers even when fully up. Frequently these systems used a step-up auto-transformer to supply power to a rack so the output at full bright was actually 120 volts. Los Angeles projectionist and theatre technology buff Tom Ruff notes that the step up transformers can cause serious over voltage conditions at the bulbs in a dimmer circuit when the circuit is under-lamped. The more bulbs that burn out, the higher the voltage gets, causing even more burnouts.

Solid state dimmers: Both of these types of thyratron tube systems allowed compact remote control but were rendered obsolete with the invention of the silicon controlled rectifier (SCR). There was an attempt to hang on to the old saturable reactor by renaming it a "magnetic amplifier dimmer" and controlling it electronically rather than with tube circuits. Ariel Davis and other manufacturers sold these into the 60s.

Century was still around when SCR dimmers were becoming the definite answer. One of the early low-cost modular systems for portable use was their EdkoTron. The name, of course, derived from the name of the head of Century Lighting, Ed Kook. Century was later sold to the British firm Strand and later yet became a part of Philips.

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