International projectionist (July-Dec 1934)

14 INTERNATIONAL PROJECTIONIST December 1934 Table B FILAMENT CHARACTERISTICS OF THEATRE-TYPE TUBES Type 224 226 235 256 257 2A3 210 250 245 845 864 280 281 866 102D 205 211 242 264 Fil. Volts 2.5 1.5 2.5 2.5 2.5 2.5 7.5 7.5 2.5 10.0 1.1 5.0 7.5 2.5 2.0 4.5 10.0 10.0 1.35-1.65 pliers cannot be expected to know that manufacturer's design, and orders to them commonly specify either wattage or current, as well as resistance in ohms. In this respect current and wattage amount to the same thing for practical purposes. The heating effect is, of course, the result of the wattage consumed in the resistor: Watts = I x E But E = I xR Therefore I x R can be substituted for E (since it is the same thing as E) in the first equation given, making that read: Watts = I x I x R Consequently, the heat loss of a resistor, which is an effect of the wattage consumed, is also an effect of the I x I x R, or PR, of the resistor, since T'R and watts are the same thing. It follows that resistor requirements are specified equally well by citing the resistance and the current, or the resistance and the wattage. In either case the supplier will be able to furnish an instrument suited to the purpose. In selecting a resistor type from a manufacturer's catalogue the projectionist may apply the foregoing equations to the choice of the exact part he needs, regardless of whether the catalogue lists those parts under current or wattage. To find the wattage of a resistor when only the current is known, multiply current by current resistance. To find the current when only the wattage is known divide wattage by resistance, and the figure obtained will equal current multiplied by current, or I2. Then, if I2 turns out to be 100, I will equal 10, and so on. There is no harm — on the contrary, an advantage of safety — in choosing a resistor of somewhat more than the required current or wattage rating, so long as the resistance in ohms is duplicated exactly. Condenser Ratings Condensers are rated for voltage as well as for capacitance. The voltage rating of a condenser is that potential difference which the instrument can withstand safely for prolonged periods of time, and cannot be judged by the rated capacitance. A 2 micro-farad filter condenser designed for work with a 110-volt line will break down instantly in smoke and smell if substituted for a 2 micro-farad filter condenser in the 1.000-volt plate circuit of a large theatre amplifier. The working voltage that any condenser used in an amplifier must be able to withstand can be judged by considering the wiring diagram and the known constants of the socket terminals. Tracing the wires backward from socket terminals to the condenser in question, and studying the circuit, should indicate in every case the precise voltage to which the condenser is subject, taking into consideration any voltage drop which may have taken place between the condenser and the socket. Such voltage drop may be caused not only by a resistor, but equally well by the resistance of a choke coil or of a transformer winding. Condensers are rated by both a.c. and d.c. voltage. The reason is that insulation can puncture in a fraction of a second — namely, during that brief instant of time when the a.c. voltage has reached its maximum, and therefore is nearly half again as high as its rated value. The voltage of a 110-volt line reaches momentarily to 150 volts twice during each cycle, or 120 times a second in the case of 60-cycle current. The effective voltage, taking a second of time as a whole, is only 110, but a condenser that could safely withstand 110 volts of steady d.c. might easily be punctured during the instantaneous 150volt peaks of an a.c. line of identical rating. If only the d.c. rating of a condenser is known, choose one rated at least 4/10ths higher for use on a.c. voltage. If the a.c. rating of the condenser is known, it can with safety be used on d.c. voltage not more than 3/10ths higher. Electrolytic condensers cannot be used with a.c. of any voltage. Transformers and Inductors Power transformers are rated by primary voltage, and by the voltage and current requirements of every secondary. Low-grade transformers that meet these specifications will nevertheless show a tendency to burn out when used continuously for long hours, as in thea tre service. It is necessary, therefore, when ordering such parts from any except makers of known reliability, to specify that they are intended for continuous service. In the case of suppliers of dubious reputation (if it be necessary to deal with such) it may also be advisable to exaggerate slightly the current drain on the secondaries, taking care to give all the voltages precisely. This method of ordering should insure receipt of a satisfactory instrument. Choke coils used as part of a rectifier filter are commonly rated in henrys of inductance, and in the current they are required to carry. Where the inductance in henrys is not known, and replacement cannot be obtained from the original source, give the current and the number of turns of wire and the inner diameter of the coil. If it is not practical to state the number of turns, at least give the inner diameter of the coil, the outside diameter (less wrapping) , and the size and type of insulation of the wire, from which facts the number of turns can be calculated with some approximation of accuracy. But the inductance in henrys should be obtained from the original supplier before replacement becomes necessary. Measuring that inductance is not a projection room job. Speech Transformers and Coils Since the constants of speech windings are far from simple, it is fortunate that the number of manufacturers of such windings is not large. Few amplifier makers use their own coils, although they may use coils that bear their own nameplate. This is sometimes true even of the largest companies, who are popularly supposed to wind their own. The result is that a reputable specialist in speech windings is likely to know all about the constants of practically every make and type of coil and transformer in America, and it is only necessary to ask such a maker to supply a winding to replace Company Z's type soand-so. The projectionist is more likely to get exactly what he wants, and there is a reasonably good chance that he will get the identical instrument, built into a different shell and carrying a differ Still Another "I am glad to be able to send to you the names of all the projectionists and some of the stagehand members of this organization as a bulk subscription to International Projectionist. "We feel sure that this is one of the best investments our Local Union membership has ever made." John E. Murphy Sec. and Bus. Rep., Local 96 Worcester, Mass.