International projectionist (July-Dec 1934)

December 1934 INTERNATIONAL PROJECTIONIST 15 ent name-plate. In the case of power transformers, resistors or condensers, the makers of which are really numerous, an order so worded would result only in a request for further information. However, orders for speech windings hased on type numbers must be sent to a really large maker of such windings (not to a dealer) and must carry the type number of the coil or transformer, as given on its case, not the type of amplifier in which it is used. Even then it may happen, occasionally, that the supplier will need further data. Speech circuit choke coils are rated in current, in inductance, and in frequency range (which last is not necessarily the same thing as the frequency range of the amplifier as a whole) . Exact duplication, from any other than the original source, may not always be possible. It can be approximated, however, by stating the circuit in which the coil is used, its function in that circuit, and the types of tubes in that stage of the amplifier. Where the coil is used as part of a speech band filter it will be necessary either to state the inductance or to submit a precise description and drawing of the filter, with all condenser values, and tell accurately just what bands of frequencies it is intended to pass or to stop. From this data and a standard filter formula the supplier can calculate the inductance required. It will check his calculations, and the projectionist's data, to give the diameter and number of turns of the original coil, and, if possible, to send him its core. But all this is make-shift at best. In the case of some filters the exact inductance will not matter very greatly, but in other instances precision in this respect is vital, an error so small as five per cent being sufficient to destroy the filter action. Where the filter coils have to be obtained from any source other than the original supplier, it is very desirable to obtain exact data in advance. Speech transformers are rated as to each winding, which must be known with respect to current, inductance and frequency range. In addition, the ratio of primary to secondary turns must be given. It is also common practice to state the impedance (at 1,000 cycles) of each winding. Such data is seldom Table C OUTPUT POWER Watts Decibels 2 25 3 27 6 30 10 32 20 35 24 36 40 38 48 39 completely available in the projection room, and cannot be obtained there by any practical methods of measurement. Roughly satisfactory results can be had by ordering, say, "an output transformer, secondary impedance 15 ohms, to work from a pair of 245 tubes in push-pull, in an amplifier having a frequency range of from 50 to 8,000 cycles". Inter-tube coupling transformers may be specified in the same way, omitting any mention of impedance and stating merely the types of tubes to be coupled, and whether they are single tubes or push-pull pairs. But if the amplifier is of recent design, there is a possibility that some of its tubes are operating as Class B. That possibility must be checked, as explained hereafter, since it will make a great difference in the constants of the speech transformers. On the whole, it is fortunate that in so many cases it is possible to order merely a replacement for P company's type Q transformer, and secure perfectly satisfactory results. The rating of an amplifier in frequency range can often be improved bv changing the input, output and coupling transformers. The details may prove involved, but a transformer company is often willing to solve them for the asking. Send such a company a circuit drawing of your amplifier, giving all the constants you know or can find out, including socket terminal voltages and currents, tube types, resistor values in ohms, condenser values in micro-farads, and transformer and choke coil type numbers. If improvement be practicable, the transformer maker can advise the exact changes needed and specify new transformers. But it is always necessary to remember that increased frequency response in the amplifier is no help unless the optical system can put a wider range of frequencies into the photo-cell, and the speakers can make use of a wider range when the amplifier delivers it. The frequency range of those parts does not, generally, rate nearly as high as that of even a moderately good amplifier. Rating of Amplifiers Amplifiers are rated in power by their wattage output, which means the power in watts supplied to the speakers. This last has nothing to do with the power consumed by the amplifier. For example, referring to Table A, the amplifier there rated as having an output of 40 watts undistorted audio power consumes 800 watts of line a.c. for its operation, or twenty times its speech wattage. The amplifier noted in the eighth line of Table A. with 6 watts of undistorted audio output in its speaker circuit, adds 300 watts an hour to the theatre's monthly electric bill, or fifty times its sound power. The difference is lost in Industry Ills Greta Garbo, whose popularity as a motion picture star seems definitely on the decline, will receive $300,000 for her next M-G-M picture. This is a tidy increase over payment for her last, a paltry $270,000. Meanwhile Claudette Colbert will have to struggle through six pictures in the next two years for the comparatively insignificant salary of $480,000. heating filaments, in the relative efficiency of power transformers, in the voltage drop across resistors and coils, and in the tubes themselves. The power output of amplifiers is also rated in decibels. Table C shows the output level, in decibels, for amplifiers having the wattage outputs given in Table A. Study of Table C reveals that doubling the output power in watts always adds three decibels to the level. For example: the second line shows that an output power of 3 watts is equal to the output power of 27 decibels, just as 3 feet equals one yard. But a 6watt output, twice as great, is equal only to 30 decibels, a gain of just 3 db. Then a 12-watt output should equal 33 db., and one of 24 watts, 36 db. The sixth line of the table shows this last, and the bottom line shows that increasing the level in watts from 24 to 48 raises the output in decibels from 36 to 39. Referring to the fourth line of Table C— 10 watts, 32 db ; fifth line, 20 watts, 35 db; seventh line 40 watts, 38 db; Power doubles each time; decibel rating increases in steps of 3. Therefore, in the case of any volume control calibrated in decibels, every increase of three db. in the volume setting doubles the sound output in watts. Now, if the volume control indicates that the theatre needs an amplifier with, say, 3 additional db. of volume, then the new amplifier must be twice as powerful as the old one, in wattage rating; if 6 additional db. are needed the new amplifier must be four times as powerful; eight times as powerful for 9 additional db., and so on. Amplifiers are rated according to gain, or amplifying power, as well as by output power. Amplifying power is commonly measured in decibels, but decibels of gain and decibels of output power are two different things. The output level of 39 db., shown in Table C, may represent a gain of 79, inasmuch as the original photo-cell current was, possibly, 40 db. below zero. In that event the 48 watts output power corresponding to an output level of 39 db. represents an amplification of 90,000,000, in watts, over the original wattage output of the photocell. Finally, amplifiers are also rated as to (Continued in Col. 1, next page)