Radio broadcast .. (1922-30)

416 RADIO BROADCAST ADVERTISER Tht AmerTran DC Luxe Approaching the Ideal 'T^HE AmerTran De Luxe J. now makes possible a transformer coupled amplifier which excels all other form of amplifiers. When us d in connection with the better loudspeakers of the C9ne type and the new tubes permitting the use of higher plate voltages in the last stage to prevent overloading, this new transformer provides faithful reproduction over the entire audible range. It so closely approaches perfection that further developments cannot be noticeable to the human ear. The AmerTran De Luxe is the result of twenty-five years' experience in transformer building, and beyond all question sets an entirely new standard of audio amplification. For economical, simple power operation of the set the American Transformer Company is now offering two units of the finest type — especially adapted to the -use of the new 7J volt power tubes in the last audio stage. These are the AmerTran Power Transformer and the Amer Choke, both ideally constructed for the type of audio amplifier required. The Power Transf9rmer also has filament supply windings for the power tube, and supplies sufficient plate current, after rectification, for the operation of the set. AmerTran De Luxe, 1st Stage . $10.00 AmerTran De Luxe, 2nd Stage . 10.00 AmerTran AF-7 (' J-l) 5.00 AmerTran AF-6 (5-1) 5.00 AmerTran Power Trans. PF-52 18.00 AmerChoke Type 854 6.00 AmerTran Resiator Type 400 . 7.50 AmerTran Heater Transformer Type H-28 (for A. C. Tubes) . 10.00 Write to-day for interesting free booklet "Improving the Audio Amplifier" and other data on the subject of better radio. AMERICAN TRANSFORMER Co. 178 Emmet Street Newark, N. J. AmerTran Products Are Sold Only at Authorized AmerTran Dealers Transformer Builders for Over Twenty-Five Years No. 27 RADIO BROADCAST Laboratory Information Sheet September, 1926 A Voltmeter Made From a Milliammeter CALCULATING THE NECESSARY RESISTANCE TN THE course of their experiments, most home -1 constructors acquire one or more d. c. milliammeters for use in measuring the plate current of tubes. These instruments are comparatively cheap and are essential in making general tests on radio parts. Another very useful instrument is the d. c. voltmeter for use in measuring the voltage of all kinds of batteries and line supply devices. It is possible to make up a very useful and fairly accurate voltmeter using a milliammeter and a good fixed resistance, and thereby make unnecessary the purchase of a voltmeter. Actually, a voltmeter consists of a sensitive milliammeter in series with a high resistance. In calibrating, such a meter, in series with the resistance, is placed across known voltages, and its scale marked off in volts instead of milliamperes. Suppose we have a meter with a full-scale reading of 2 milliamperes (.002 amperes), and we want to use it as a voltmeter for use on line supply devices which supply voltages up to 200. To determine the required resistance necessary in series with the meter, we divide 200 by .002, and the quotient, 100,000, is the required resistance in ohms. If we place the milliammeter in series with the 100,000ohm resistance across an unknown voltage, as shown in the diagram, the needle will deflect an amount proportional to the voltage. We have made our voltmeter so that if the meter reads 2 milliami>eres the voltage is 200. Now, if the meter reads 1 i milliamperes, the voltage is 150; if it reads 1 milliampere, the voltage is 100, etc. It is not always possible to obtain accurate resistance units so that it is, in general, wise to calibrate the voltmeter so as to allow for errors in the fixed resistance. On Sheet No. 28 is given information regarding the calibration of a home-made 100,000 ofim resistant i WWVWV Unknown Voltage voltmeter, and if the calibration is done carefully, it should be possible to obtain readings which will be accurate within a few per cent. For rough measurements, no calibration is necessary since, if good fixed resistances capable of passing several milliamperes are purchased, their marked resistance value can lie depended upon within about ten per cent., and usually the per centage error will be even less than this. No. 28 RADIO BROADCAST Laboratory Information Sheet September, 1926 Calibrating a Home-Made Voltmeter PLOTTING THE CURVE (~)N LABORATORY Sheet No. 27 were given ^ data regarding the construction of a simple voltmeter from a milliammeter. Information is given here for the calibration of such a meter. First determine the required resistance in series with the milliammeter by the following formula: R = EX 1000 where E is the maximum voltage it is desired to read, I is the full scale reading of the meter in milliamperes, and R the unknown resistance. Examples: 1. It is desired to read 500 volts using a 10milliampere meter. Then R = 5°° *01000 = 50,000 ohms. The calibration is performed by placing the fixed resistance and meter across different known voltages and plotting a curve showing the deflection of the meter for different values of voltage. By making such a calibration, it will be possible to compensate for any inaccuracy in the fixed resistance. If no voltmeter is available whereby the applied voltages for calibrating purposes can be measured, it will be possible to use new B Batteries, since the marked voltages will then be quite dependable. First 22 J volts could be placed across the combination and the meter reading taken, then 45 volts, etc., until several points are obtained. It will not be necessary to calibrate the full scale of the meter since the calibration will be a straight line. The abscissa, or horizontal axis, of the curve should be plotted in milliampere deflections, and the ordinate, or vertical axis, should be plotted in the corresponding voltages. Such a curve is illustrated on this Sheet. 500 400 200 100 7 4567 MILLIAMPERES No. 29 RADIO BROADCAST Laboratory Information Sheet September, 1926 Tubes: Miscellaneous ux-213 uv-876 This is a full-wave rectifier for use with line supply devices. Its filament voltage is 5, and it takes a filament current of 2 amperes at this voltage. The maximum value of the a.c. input voltage is 220 volts (effective value), and the maximum rectified current the tube can deliver is 65 milliamperes. UX-216-B This tube is a half-wave rectifier for use in line supply circuits. Its filament voltage is 7.5, and current is 1 .25 amperes. The maximum value ot the a.c. input voltage is 550 volts (effective vr.lue). The maximum rectified current is 65 milliami>ercs. UX-874 This tube is used as a voltage regulator and, when correctly connected in a circuit, it functions to maintain a constant voltage. The voltage drop is W) volts d.c., and the starting voltage is 125 volts d.c. The maximum current is 50 milliamperrs d c The positive lead is connected to the rod and the negative lead connects to the cylinder. This tube is usi rl in the line supply device manufactured by the Radio Corjx)ration of America and also in their Model 10-1 loud speaker. This is a ballast tube and when correctly connected in a circuit it functions to maintain constant current. It has a current rating of 1.7 amperes, and the voltage drop is -10 to 60 volts. This tube is designed for use on units using 105 to 125 volts supply at from 50 to 75 cycles. uv-886 This tube is practically the same as the model uv-876 except that it is for use on from 40 to 45cycle current. It has a current rating of 2.05 amperes, and the voltage drop is from 40 to 60 volts. uv-877 This is a protective tube, and is used in the B battery circuits of i re-rivers to prevent damage to tulx's or wiring, if the 1J batteries are accidentally hhort-circiiited. The volume drop across half the filament is 2.5 volls at U> milhainperes d.c., and nsrs to -15 volts at {>() milliamperes d.c. Across the entire filament, the voltage drop at 20 milliamperes is ,r> volts. With 90 milliamperes flowing through it. the voltage drop is <M) volls. From llu-M figuirs it is evident lhat, if the B batteries are accxlrnialiy short-circuited, practically all the voltage will be consumed by this lube, and the current definitely limited to a safe value. Toted and approved by RADIO BROADCAST