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50 SOUND MOTION PICTURES
such single incidental details that research discloses its patient, tireless wisdom.)
The initial amplifier is connected to the output of the photo-electric cell and amplifies this output electrically through a series of vacuum tubes until sufficient power is available to operate the final or power amplifier. The initial amplifier is always placed in the projection room in a convenient location, generally at a porthole, so that the projectionist at the amplifier control board may view the screen. The placing of the equipment will be different for each installation because of local conditions. A typical arrangement of equipment is shown in Plate II. In all sizes of R. C. A.-Photophone equipment larger than Type D, the initial amplifier is provided in duplicate. In all forms of the final power amplifier the equipment is open and accessible, permitting replacement of the vacuum tubes when necessary.
By the use of special circuits and grouping of unit parts the burning out of an individual tube or even the unusual development of a fault in an amplifier unit in general does not stop operation. It may merely modify either the quality or the volume of the sound to some extent (thereby directing the attention of the projectionist to the need for a tube replacement or other change in operation) without, however, totally interrupting the performance. R. C. A. Radiotrons are used in all Photophone equipment.
The final or power amplifier supplied in the R. C. A.Photophone system may be placed in the projection room, or in a room closely adjacent to the projection room, or back stage, as may prove most convenient and desirable. In such cases, the R. C. A.-Photophone system permits the separate power amplifier to be installed wherever there is most room for it.
The power amplifier is connected to the loudspeakers which it actuates. An important unit in this system is the