International projectionist (Jan-Dec 1945)

Providing Auxiliary Sound Requirements for Motion Picture Theutres By HENRY B. SELLWOOD In a previous article on this subject we discussed the difficulties currently facing motion picture theatres in obtaining, operating and servicing auxiliary sound equipment for both entertainment and advertising purposes. It was pointed out that a number of projectionists are increasing their incomes by making their talents and training available to theatres in this connection. The discussion is herein resumed. FIGURE 1 shows the schematic diagram of a mobile amplifier used in a truck or automobile or in a horsedrawn wagon or carriage, for purposes of exploitation. The only power source it needs is an ordinary 6-volt storage battery— a common automobile battery. Power input terminals are shown at the lower right hand corner of the drawing. They run through a fuse and a switch to a 6-volt dynamotor. A receptacle connected across the line makes the same six volts conveniently available for powering a d. c. loudspeaker field. or for any other purpose, such as a small string of 6-volt lamp bulbs used decoratively on the sound truck or wagon or carriage. A dynamotor is a very special kind of motor-generator, the details of which are not shown in the drawing. An ordinary motor-generator (such as many projection rooms use for arc current supply and — in some cases — for sound system current supplies) consists of two machines. The motor and generator are individual units, one driving the other. Each contains a rotary element, the armature, and in addition each has field windings. These two units may be coupled together, or they may be mounted inside the same casing with their rotary elements built around the same shaft, but they still are two complete units. The dynamotor does not contain two complete units. It is a kind of motorgenerator in which there is a motor armature and also a generator armature, entirely distinct from each other, but there is only one field structure, and that field serves both armatures. The result is a very compact machine, and an inexpensive one,' but one that is difficult to adjust as to results. The field rheostat used with ordinary motor generators does not work out well with dynamotors, because in the dynamotor one field serves two entirely different purposes. Dynamotors in consequence are seldom controlled either as to speed or output. They are correctly chosen for the work they are to do, connected with the apparatus they are to serve, and merely switched on or off. They are seldom equipped with motor starters. In other words, they are very simple devices, but only the right dynamotor can be used for any given apparatus. As to maintenance, they need the same care as any motor-generator. There are two separate commutators, one for each armature; they require the same looking after as any commutator. The bearings need lubrication from time to time. The projectionist is familiar with all these requirements. Power Circuits of Figure 1 All tubes of Figure 1 have 6-volt heaters. The filament circuit, therefore, is merely a parallel connection to the six-volt storage battery, and is not shown in the drawing. Plate power is delivered by the dynamotor, which provides high voltage d. c, hence there is no rectifier circuit in Figure 1. Also, the filter circuit is extremely simple. It consists only of the 8 mfd. condenser drawn to the left of the dynamotor. which removes the commutator ripple. The upper plate of that condenser may be regarded as the positive d. c. source for the balance of the circuit. The negative side of the d. c. line is grounded, and return to negative is through chassis or ground. From the upper side of the condenser trace up, right and up to the primary of the output transformer — through the two halves of that winding to the plates of the 79 tubes. The cathodes of those tubes are grounded, providing the return circuit. From the top of the condenser trace straight up, through the primary of the coupling transformer to the plate of the 89 tube. The cathode returns to negative (ground) through a 1250-ohm grid bias resistor. From the top of the condenser up, left, and up through x/2 megohm completes the connection to the plate of the 77. Similarly, follow from the top of the condenser up, left, up, left through y^ megohm and up to the screen of the 77 — which, although a pentode, is used here in tetrode connection. The Y^-meg. resistor just traced and the 25,000 ohm resistor immediately to the left of it constitute a voltage divider. The 25.000 ohms bridge the screen-cathode circuit of the tube. Trace the cathode to ground, down through 3,000 ohms and left to a ground connection. This 3,000 ohm cathode resistor provides control grid bias for the 77, return of its negative side to grid running through two parallel paths — the 100,000 ohm volume control and the secondary winding of the input transformer. Bias for the 89 is provided by 1,250 ohms in series with the 89 cathode, and returned to grid through V2 megohm. The 79's operate at zero grid bias. Therefore those grids swing positive during part of the input cycle, increasing output power at the cost of some distortion. The grid bias or cathode resistors of the two voltage amplifier tubes are bypassed by 2 mfd. condensers. Speech input enters at the left, and FIGURE 1 MAY 1945