International photographer (Jan-Dec 1934)

Four The INTERNATIONAL PHOTOGRAPHER October, 1934 photo-elec+nc Ce\\ Ariod Cathode Kj) PR ;iO Megohms n ■* = Gridbias battery I To succeeding tubes in the PEC amplifier Grounded on recording ~ ~ wacnme frame cp Fig. 3. Photo-electric cell input circuit. izing battery is connected through the series resistance to the anode of the cell; while the negative terminal of the battery is connected directly to the grounded cathode of the cell. The small percentage of the modulated light beam that the film permits to pass shines through the window in the photo-electric cell and strikes the sensitive cathode surface. This impinging of the light on the sensitive surface of the cell causes negative electrons to be emitted by the portion of the photo-electric surface exposed to the light. Within certain limits, this emission of electrons is at a rate proportional to the incidental light. The polarizing voltage that is applied to the PE cell causes these negative electrons to be drawn to the positively-charged anode (for opposite electrical charges attract), thereby setting up a minute current flow in the PE cell circuit. This current flow is practically identical to the speech current being recorded. The action is the same as that which takes place in a thermionic vacuum tube, which was explained in an earlier chapter. In a vacuum tube, the emission of electrons from the filament is caused by the passage of an electrical current heating the filament ; in the photoelectric cell, the emission of electrons is caused by the action of light. In a vacuum tube, the filament (the cathode) is in the center and the plate (the anode) surrounds it ; in the photo-electric cell, the disposition of these two elements is reversed. The minute current flow in the PEC circuit is proportional to the intensity and quantity of the light falling on the photo-electric surface. Since the light is from an incandescent lamp (the recording lamp) that produces a light of constant intensity, the current in the PE cell circuit varies directly as the changes in the quantity of light that the moving ribbons of the light valve permit to pass. The variation in current flow in the photo-electric cell circuit causes a variation in the voltage drop across the series resistor PR. This variation in voltage drop is fed through the coupling condenser, C, to the gridfilament circuit of the vacuum tube, where it produces a corresponding voltage drop across the grid-leak resistor, GL. This minute voltage variation is amplified by the tube in the manner previously discussed in the chapters on amplification. The grid bias battery is necessary to keep the grid at a definite negative potential. The PEC Amplifier The PEC amplifier is a three-stage affair employing small tubes of the "peanut" variety connected in a resistance-coupled circuit. The heavy metal box in which the amplifier is mounted is fastened directly to the frame of the recording machine ; and both the frame and the metal box are carefully grounded. The impedance of the PE cell circuit is very high, and the current in the circuit is so minute that any stray electric currents or disturbances ("static") picked up by the wire connecting the cell to the amplifier might be appreciable in comparison to the sound currents. Since these disturbing currents would be amplified in propor tion to the amplification of the sound currents, they might become extremely annoying in the monitoring circuit. To minimize the picking up of these stray currents, the PEC amplifier is mounted as close as possible to the PE cell and connected to it by a short, e'ectricallyshielded wire. The sound current is increased by the PEC amplifier to a level that can be safely transmitted before it is sent through the 500-ohm impedance output circuit of the amplifier to the monitoring circuit in the amplifier room. The three tubes in the PEC amplifier are held in spring-suspended sockets as a protection against mechanical shock, which otherwise would cause the tube elements to vibrate and produce an unpleasant ringing sound in the output of the amplifier. This resilient mounting is particularly necessary for these tubes, because the mounting of the amplifier directly on the frame of the recording machine exposes it to considerable vibration from that source. The amplifier has its own individual plate batteries, which also supply the polarizing voltage to the PE cell. These batteries are of the dry B battery type ; and they are set in a metal box sunk in the concrete of the recording machine base. The rheostat, filament on-off switch, and filament ammeter are all mounted on the large wall panel that carries the controls for the recording machine. The outputs of the two PEC amplifiers in each recording channel connect through variable attenuators that permit balancing of their electrical output levels to the contacts of the PEC relay, as shown in Figure 2. A close-up of the amplifying equipment that constitutes a single "channel." The four recording bridging amplifiers are at the upper right; the inter-communicating telephone is at lower left set in a panel; the handset below it. — Courtesy Paramount Productions, Inc. /;/ the next chapter, the constructional features, as well as the operations of stringing and tuning the light valve will be discussed in detail. It costs only $1.00 to own a copy of Fred Westerberg's CAMERAMEN'S BOOK OF TABLES. Please mention The International Photographer when corresponding with advertisers.