Motion Picture News (Oct-Dec 1929)

October 5 , 1 9 2 .9 1233 EQUIPMENT i)i 1 1 i-i Nil ■ \u s OPERATION l> by JOHN F. RIDER o o MOTION PICTURE NEWS THE PLATE VOLTAGE BATTERY ELIMINATOR 600.4 THE plate voltage battery eliminator is nothing more than the conventional B eliminator so common in the independent and in some of the other installations. Its function is to replace the battery or generator form of plate voltage supply. The eliminator as a unit consists of three sections. The first is the transformer-rectifier system, wherein a power transformer supplies A. C. to the rectifier tubes and the tubes convert this energy into pulsating D. C. as mentioned in page No. 600.3. The second section consists of a number of chokes and condensers so arranged that the complete system constitutes a filter circuit, its function being to separate the A. C. and the D. C. found in the rectifier output and to pass the D. C. to the output system of the eliminator. This act of separating the A. C. and the D. C. is the act of filtering. The third section is the voltage divider, consisting of a number of resistances arranged to distribute the eliminator output voltage according to the requirements of the tubes employed in the amplifier. The power transformer as a rule is of such design that it supplies filament voltage for the tubes in the amplifier, in addition to the plate voltage. Whereas individual windings are employed to supply filament voltage for tubes of different voltage and current specification, the lone plate winding supplier voltage and current for all the tube plate circuits. After rectification and proper filtering, the voltage is apportioned by producing definite voltage drops at certain values of current output across sections of a number of resistances connected in series, the entire combination being connected across the filter circuit output. The voltage divider as t:ie 31 . :i i network is known being predesigned for use with certain tubes and for certain current loads and employing fixed resistances of predetermined value, does not permit the haphazard changing of tubes nor the haphazard replacement of resistance units. Furthermore, the current and voltage output at any one tap are definitely predetermined and tubes constituting a load cannot be added simply because they will be operated at the voltage available at the tap or binding post. In contrast to the battery type of installation the magnitude of current available from the eliminator displays an effect upon the voltage. In other words the vacuum tube rectifier is not a constant resistance structure. The greater the current drain, the less the output voltage. A definite value of current and voltage are available, but as either i increased or decreased, the other varies inversely, although not exactly in proportion. o o MOTION PICTURE NEWS TESTING TUBES. 700.4 WHAT is the best method of testing tubes? has been a frequent question. The answer must be qualified for the men who are not in a position to make their own determinations but who are advised as to the correct method of testing the tubes furnished with the amplifier. Sometimes we cannot help but wonder how a simple tube testing device is not a part of the amplifying equipment. The cost is non-prohibitive to say the least. Inquiry along these lines solicited the information that the average operator would not know the function of the tester. . . . This is difficult to believe. If a man can operate and test or measure voltages on a standard sound amplifier panel and interpret the meter readings, he can operate a set tester. However, the usual test for plate current is quite satisfactory, considering that the two best tests are beyond the average operator because he does not possess the required equipment. Relative to the plate current test, many men are confused in the correct interpretation of the readings secured on the plate milliammeter in a push-pull stage. "Which tube is the best, the one which has the greatest effect upon the meter, or the one which has the least effect?" has been asked. . . . The tube with the greatest effect, assuming normal operation and not extreme load when one of the tubes is inserted. The tube with the least effect shows minimum emission, consequently lowest plate current. In the event that two tubes are employed in a rectifier and one overheats (the plate becomes red during operation) whereas the other remains cool, the tube with the cool plate is defective and is causing overload of the remaining rectifier. Queries have been voiced relative to the correct interpretation of plate current indicated with normal filament potential and with less than normal filament potential. Suppose that two tubes A and B show normal plate current at normal filament current. However when the filament current is reduced, say 20 per cent tube A shows higher emission than tube B. Which of these tubes is suitable for use. Under the circumstances both tubes are suitable for use. The fact that one tube shows higher emission at the low value of filament current indicates a superior filament, but this condition does not signify that the othet filament is defective. If the emission at the rates value is satisfactory the tube is satisfactory. Of course the tube with the low emission will "go" first but under the condition stated above, the tube is still suitabfe for use. Without a special tester, the suitability of a tube is usually determined by observation of the normal plate current. While this method serves its purpose, the meter reading is not an absolute indication of the condition of the tube because a defect in the plate circuit of the system will cause a similar reading.