A power unit., utilizing two UX-281 tubes, supplies the necessary voltages for the filaments, grids and
plates of the amplifier tubes. This power unit also supplies the polarizing voltage for the photo-cell, and
contains a special Rectox rectifying unit, which furnishes current at 1 ampere, 27 volts for the exciter
lamps in the projectors. The volume control dial is mounted on the top of the amplifier, and is countersunk flush with
the top. All other switches and plugs are mounted at one end, and a hinged door forms one side of the
cabinet, thus giving easy access to the interior. Figure 130 shows the amplifier in place with two
projectors. Figure 136 illustrates the interior, showing the location of the tubes and amplifier parts.
The receptacles on the end of the amplifier provide for an input of 60 cycle alternating current at 110
volts; the output voltage for two exciter lamps, one for each
projector; the output for the voice coil leads to the loudspeaker; and the sound input from, each of the
two projectors. There is also a key-switch provided in case it is desired to lock the equipment
electrically. A fader switch permits the changing of the sound from one projector to another, and
accomplishes this by changing over from one projector to the other both the input circuit to the
amplifier and the exciter lamp supply at the same time. These plugs and switches are all illustrated in
Figure 138. A phonograph input jack is located between the input plugs, and permits the playing of nonsynchronous records.