International projectionist (Jan 1963-June 1965)

Record Details:

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250 £ 200 IA c ? 175 Jj 150 O 0-125 <0 .^ 100 u -P ju 75 o $ 50 25 10 20 30 40 50 60 70 80 90 100 Ohms load resistance for 10-ohm source FIG. 2 — This graph reveals that the maximum transfer of electrical power (watts) is achieved when "source" and "load" impedances are the same. It is here assumed that the source impedance is 10 ohms; the number of watts developed when different load resistors are used may be read from the curve. Thus a 10-ohm load resistor gives a maximum power of 250 watts; both 5-ohm and 20-ohm loads (one-half and twice the source impedance, respectively,) give 221.77 watts. of the volume control. In other words, the volume control should be of the potentiometer, not the simple rheostat, type. If a simple dead-end reostat is used, the load on the amplifiers will increase with the number of speakers turned up loud, thus causing annoying variations in system output and, worse, in the frequency characteristics of the system. No matter how much is done to prevent it, a number of speakers are damaged and stolen every season. Deliberate theft cannot be prevented if the larceny-minded patron brings along a pair of wire cutters. Accidental severance of speaker cords can nevertheless be minimized by using waterproof coiled cord instead of the ordinary flexible cord which may loop down and catch on automobile bumpers and door handles. A number of spare in-car speakers are an absolute necessity to avoid losing parking spaces when something goes wrong with speakers previously installed. To play it safe, there should be at least one spare speaker for every hundred speaker posts. The projectionist must also have a spare speaker known to be in perfect working order for his own use in checking ramp and junction-box circuits — this in addition to the permanently installed projection-room monitor speaker. We do not believe that it is desirable to complicate the sound wiring to the extent of having a separate on-off switch for each and every ramp, together with loading resistors to preserve system impedance; but it is certainly necessary to provide switching for each individual power amplifier. The most convenient arrangement is a plug-in switchboard having one heavy-duty plug for the output term inals of each amplifier. If, for example, there are four amplifiers, there must be four output plugs, each supplying audio power to one-fourth of the drive-in. The plug arrangement permits very rapid replacement of an ailing amplifier with an emergency stand-by amplifier of the same type and power output. (Two other plugs must also be provided for each power amplifier, one for the current which operates the amplifier, and the other for the sound input from the preceding driver or voltagegain stage.) Even though the plug-in arrangement is preferable for drive-ins in which all the equipment is removed at the end of the season and re-installed at the opening of the next one, permanently wired emergency amplifiers connected through a main switchboard are recommended both for indoor theatres and for those drive-ins in southern climates which operate the year around. The presence of a spare power amplifier in good working condition does much to reduce the projectionist's worries, to prevent ticket refunds because of sound loss, and to allow repairs to the sound system to be made in a careful, unhurried manner in the daytime instead of under stress and the pressure of time while a show is in progress. Sound Service Indispensable The services of a good sound-maintenance organization are a "must" because the projectionist, already burdened with the duties involved in his work, cannot rightly be expected to jeopardize the condition of films and equipment or the presentation of the show to "double" as a sound engineer. Sensitive electrical adjustments in amplifier circuitry, the correction of noise and distortions arising from electrical, optical, or mechanical causes, and the balancing of amplifier outputs are not normally a part of the projectionist's job. It is nevertheless quite proper for the projectionist to familiarize himself with the types of vacuum tubes used in his system, and with the locations of their sockets in the rectifier and amplifier units. A full complement of all rectifier and amplifier tubes, together with soundhead photocells and exciting lamps, should be stored in the supply cabinets ready for instant use. And do not forget spare fuses! It is very doubtful that frequent or routine sandpapering or burnishing of vacuum-tube prongs is at all necessary. The prongs of tubes are made of a plated alloy which does not corrode. If the prongs are wiped clean when the tubes are inserted into their sockets, and if the socket contacts are sufficiently tight to begin with, the amplifier tubes can be left alone during the season — except, of course, for the testing and replacement of the few tubes which may go bad or be suspected of defects. If at any time the tubes are removed from an amplifier chassis for the purpose of cleaning it, cover the prong holes of the tube sockets with strips of masking tape or "Scotch" tape to prevent dust and dirt from going inside the chassis through the holes. Big Power Tubes Have Shortest Life Transistorized preamplifiers and voltage-gain "drivers" eliminate tube troubles and filament ("A") circuits from the first two stages of the system, and also reduce hum and distortion due to "microphonic" and gassy tubes. The power amplifiers, on the other hand, usually employ vacuum tubes to provide the audio power (watts) required by a large number of in-car speakers. Powergain tubes are larger and more expensive than voltagegain tubes, and they get rather hot in operation. They are more likely to deteriorate and cause distortion than International Projectionist May 1963