IEEE Power & Energy Magazine - January/February 2017 - 77

january/february 2017

advantage, however, resulted from the
fact that the controlling device was a
very small, very low power potentiometer. This meant that more than one
"pot" could be provided for each dimmer so that the desired dimmer setting
could be preset for upcoming cues. The
changeover from one cue to the next
could then be made simply by operat-

ing a single master controller that fed
the individual dimmer controllers.
This presetting technique remained
in use for stage lighting control until the development of computer-type
lighting boards during the late 20th century. That innovation meant that a huge
number of upcoming cues could be stored
in the computer memory. Manual preset

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vacuum diode used for rectification
a "kenotron," where "kenos" means
empty space, a reference to the high
vacuum in the tube. Finally, a common
three-element vacuum tube used extensively in radio circuitry was christened a
"pliotron," from "pleion," which means
more, a reference to the tube's use as
an amplifier.
Figure 2 is a schematic of typical
thyratron-reactor dimmer circuitry from
this era. a resistive potentiometer (MCB)
is shown at the far left. apparently, MCB
stands for main control board, which was
the location for this device since it was
the means by which the light board operator controlled the dimmer.
a dual-plate kenotron tube (K) is
shown that acted as a differential rectifier. It combined the signal from the
"pot" with a feedback signal to derive
a resultant signal for the grid of the
thyratron tube (T). Grid capacitors (FI
and FB) introduced a time constant that
determined how long into a half cycle
of the impressed ac it would take for
the thyratron to conduct and feed current to the control winding of the saturable core reactor. This, of course, was
a function of the position of the control
potentiometer. The phanotron tube (P)
handled the inductive current in the
control winding during the alternate
half cycles when the thyratron was not
conducting. at the right are shown the
saturable reactor dimmer (sR), a line
contactor (LC), and the lamp load being dimmed (L).
The control board could be located
anywhere where it was convenient for
the operator to see the stage as he ran
the lighting cues. Most often this was
someplace in the house so that the operator had the same view as the audience. This location usually had not
been possible with resistance dimmer
or simple reactance dimmer boards
because of the plethora of high current
electrical connections to the boards. as
such, they were stuck backstage where
the operator had a very limited view of
the stage itself.
Flexibility in control board location was a big advantage of thyratronreactor installations. another huge

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2017