International projectionist (Jan-Dec 1950)

double the amount of light available. Unfortunately, however, since we are talking about screen area, instead of doubling the amount of light we have to multiply it by four to retain the same light level over the total "area of the screen. This crying demand for more light on drive-in screens has resulted in more powerful arc lamps. In general, as the amount of light at the aperture increases, a point is finally reached beyond which it is dangerous to go because of the damage to the film. 'Certain manufacturers have introduced heat filters which may remove approximately 40% of the heat with a 20% loss in light. Questions Filter Efficiency Frequently, it is found that excessive costs for both carbon and power consumption can be avoided by reducing the operating amperage and eliminating the heat filter without decreasing the light on the screen. In other words, there is no point in having a high light level only up to the heat filter and then having a 20% loss in light, unless the over-all amount of light transmitted to the projector is appreciably higher than would be the case of the whole setup operated at a lower amperage without a heat filter. One other important feature is that a heat filter may have a 20% loss in light on the day of installation, but this light loss may appreciably increase as time goes by, due to two causes: (1) the efficiency of the heat filter may decrease with age; and (2) dirt on the surfaces of the heat filter will reduce its light transmission. Consequently, a heat filter is a unit which is continuously getting worse with age. In general, then, very much more effective operation can be obtained if a conventional heat filter with its light absorbing properties can be omitted. There are two general classes of arc lamps currently used in drive-in theaters : (1) the reflector type, using approximately 80 to 85 amp on a 9-mm black high-intensity positive carbon; and (2) condenser-type arc lamps, using amperages ranging from 130 to 180. Drive-in Amplifier Systems The essential difference between the amplifier systems designed for drive-in theater use and those for indoor theater use is the higher audio power required for distributing peak signals without distortion to large numbers of in-car speakers, often totaling 1000 or more. A typical drive-in theater amplifier has a total power output of 250 watts. It is a dual-channel system with the inputs connected in parallel but with the output from each channel connected to one-half of the total number of in-car speakers. At the top of the amplifier rack is the terminal strip for making external connections. Directly below is the channel selector switch and test panel. Next follow the two voltage and two 125-watt power amplifiers. Voltage, Power Amplifiers The amplifiers can be turned down on their hinges for easy access to the circuits when servicing. The channel selector switch makes it possible to .operate with both channels simultaneously as a dual channel system or, in the event of trouble in either channel, to switch the entire speaker load onto the output of the operating channel. At the same time, this switch changes the output transformer tap to match the speaker load. The disabled amplifier is automatically disconnected from the a.c. power source and the output load, so that it can be repaired without interrupting the performance. Monitoring and testing facilities are also included on the selector switch panel. The voltage amplifiers are two-stage units having high impedance inputs and transformer-coupled outputs. The soundheads are connected to the inputs by means of low-capacity cables. Coupling between the voltage and power amplifiers is accomplished through a 500-ohm "H" type variable attenuator which serves as the volume control. The attenuator is connected as a dual 250-ohm variable "T" attenuator so that both channels are equally controlled. The power amplifiers are three-stage Class "B" units utilizing four 809-type tubes in the output stage and are rated at a 125-watt output each with less than IT'S TIME TO CHECK UP ON THOSE DRIVE-IN NEEDS Then Buy PROVEN Satisfaction By Buying / MOTIOGRAPH Model "AA" Double Shutter Projectors for beautiful, rock-steady pictures / MOTIOGRAPH SOUND for lifelike reproduction / MOTIOGRAPH ARC LAMPS for an abundance of screen illumination V MOTIOGRAPH MOTORGENERATORS for a dependable source of arc current / MOTIOGRAPH IN-CAR SPEAKERS for exceptionally fine sound and ability to take abuse Order them now from your Motiograph dealer or write MOTIOGRAPH, INC. 4431 W. Lake St. Chicago 24, III. Export Division (Except Canada) Frazar & Hansen, Ltd., 301 Clay St., San Francisco 11, California INTERNATIONAL PROJECTIONIST • MARCH 1950 29