British Kinematography (1949)

22 may be placed at any point in or remote from the amplifier with complete freedom as to how and where the leads are run, hum or noise due to pickup on the volume control leads is non-existent, and the lead capacitance has no effect upon the frequency characteristic of the amplifier. Noise, inevitably produced by the first valve, is attenuated with the signal so that the signal/ noise ratio is always the maximum possible. This remote control feature has been found invaluable in major theatres, wdiere it becomes feasible to monitor any special show from the hall without the use of motor driven controls. The theory is simple, the current leaving the cathode at any particular value of bias on G\ is fixed by the voltage on the second grid, but the division of current between screen and anode circuits is controlled by the negative bias applied to the third grid, high values of negative bias (lOv.) giving zero anode current (and consequently zero gain,) zero third grid voltage giving maximum anode current and maximum gain. One valve is associated with each machine. Sound Change-over Machine to machine sound change-over is made by a miniature P.O. type relay in circuit after the first valve, where a signal of several volts exists. Trouble due to high contact resistance, dirt and contact potential " clicks," etc., are avoided by this high signal voltage, and the use of an hermetically sealed type of relay. The operating circuit for the relay is extremely simple. It is required that on pushing the change-over button on any machine, the relay associated with that machine should " close " and that the relays associated with the other machines should " open " if they are not already open. The conventional method of achieving this would be to add three other sets of contacts to each relay, these contacts and the inter-connecting wiring being required for the operating coil circuits alone. The desired end has been achieved in this equipment without any extra contacts or inter-connection leads. Each relay coil has in series with it a small metrosil resistor, all units being supplied in parallel by a substantially constant total current through a series resistor from a conventional rectifier. Metrosil is a silicon carbide' base material in which the resistance is a function of the applied voltage instead of being constant as for normal metallic resistors. Halving the voltage across a resistor of this type increases the resistance by approximately 8 times. The change-over push-button associated with each machine short-circuits the metrosil resistor in series with that machine's relay. On pressing a sound change-over push-button, the current in the associated relay rises from about 7 m.a. to approximately 2o m.a. and the relay closes. Short circuiting the metrosil with the push-button reduces the voltage across the relays by a factor of about three times. This reduction in voltage, combined with the large increase in resistance of the metrosil resistors in series with the other relays, reduces the current on all the other relays to about .25 m.a. and those relays " open." It is worth noting that further push-buttons can be added in parallel at any desired position in the theatre, only two small wires being required. This feature is used to permit change-over from " Film " to " Public Address," etc., and from a control point in the auditorium. The middle section of the amplifier circuit is straightforward and requires no particular explanation except for the tone control circuits. It is a matter of common experience that some local adjustment of frequency characteristic is required to obtain optimum results in each theatre. This requirement is covered by providing three " bass " levels and three " top " levels available by soldering two short jumper wires across appropriate tags. The optimum characteristic is selected by the installation engineer after hearing a few