Radio Broadcast (May 1928-Apr 1929)

170 RADIO BROADCAST ADVERTISER -for BETTER RADIO A manual of 36 pages and cover — with 88 illustrations and over 20.000 words of practical, concise, readily understood text — prepared by Austin C. Lescarboura in collaboration with our engineering staff. "The Gateway to Better Radio-1* tells what's what for bettering your radio receiver, amplifier or power unit! what's what in A-C tubes, short-wave reception, improved tone quality, added sensitivity and so on! and what's what in interpreting radio circuits and innovations for best results. Usable. Unselfish. Unbiased. Just the plain radio truth, such as you can put to work. All for 25 cents, to defray mechanical costs of publishing! Order your copy of this manual now — before it is out of print — from your local radio dealer, or direct from us by remitting 25c in stamps or coin American Mechanical Laboratories, Inc. Specialists in Variable Resistors 285-7 N. Sixth St. Brooklyn, N. Y. Solve All B Eliminator Problems! 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Radio Broadcast Laboratory Information Sheet Tuned Circuits July, 1928 CALCULATING EFFECTIVE RESISTANCE T ABORATORY Sheet No. 198 published in the June issue explained how to calculate the gain of a radio-frequency amplifier using a screengrid tube. In calculating the gain we had to make use of the factor Jt which denoted the effective resistance of a tuned circuit at resonance. In this Sheet we will explain how this effective resistance is calculated. A simple tuned circuit is indicated in the sketch and it can be proved mathematically that, at resonance, the circuit between points a and b acts like a high resistance with a value equal -to R = where R is the effective resistance of the circuit at resonance as measured between points a and b u is equal to 2ic times the frequency L is the inductance of the coil in henries 7 is the series resistance of the circuit. The value r is the series resistance of the tuned circuit when actually connected in a tube circuit. Example: What is the effective resistance of a tuned circuit whose resonant frequency is 1000 kc. (300 meters), the scries resistance of the circuit being 20 ohms and the inductance of the coil 0.25 millihenries (0.00025 henries) (2x X 1, OOO.OOO)2 (.00025)2 20 115,000 ohms, effective resistance If this circuit were to be used in conjunction with a screen-grid tube the gain, calculated using the formula given in Sheet No. 198 would be: Gain =Gm X R = 0.000350 X 115,000 = 40 Jo. Radio Broadcast Laboratory Information Sheet Line Voltage Variations July, 1928 EFFECT ON TUBE LIFE LETTERS from readers have been received by the Laboratory from time to time to the effect that the life of the 171 type tube used in their power unit was very short, sometimes lasting only about 100 hours. The normal life of a 171 type tube should be at least 1000 hours. The probable cause, in many cases, of such short life is excessive filament voltage. The transformer in a power unit is designed generally to operate with a line voltage of 110 volts a.c. With this voltage across the primary the voltage across the filament terminals of the 171 type power amplifier should be 5 volts. If the voltage across the primary is less than 110 volts, then the voltage across the filament of the tube is less than 5 volts and conversely, with input voltages higher than 110 volts the voltage across the filament of. the tube will be excessive, i.e., more than 5 volts. If the filament voltage drops very much, the electronic emission from the filament will decrease and distortion of the signal will result. If, on the other hand, the filament voltage is excessive, the output of the system is not audibly affected and so with no audible indication of the excessive voltage, it is likely that it will go by unnoticed. It is excessive filament voltage which must be guarded against however, if a normal length of life is to be obtained from any tube. The extent of the fluctuations in line voltage is, of course, different in different parts of the country — in large cities the voltage is generally quite constant, while in rural communities comparatively large variations in line voltage are probable. These problems, brought about by inconstancy of line voltage, are becoming more serious as the use of a.c. operated receivers becomes more popular. In such receivers, all of the tubes are operated directly from the power line and decreased tube life due to excessive filament voltage is to be carefully guarded against. The solution of these difficulties lies in the design of a device which will automatically control the voltage actually applied to a power unit. The type 886 tube is a device of the sort, designed to insure constant input to power operated radio receivers, despite fluctuations in line voltage. Several devices to accomplish regulation by other means are also being developed by other manufacturers and will probably be available shortly. No. 205 Radio Broadcast Laboratory Information Sheet Electrical Measuring Instruments July, 1928 THE GALVANOMETER 'TyHIS is the first of a series of Laboratory Information Sheets to be devoted to the subject of electrical measuring instruments. In this Laboratory Sheet we discuss what is probably the oldest instrument for measuring current and voltage. This instrument is the galvanometer, and most of our modern ammeters and voltmeters are merely adaptations in one form or another of the galvanometer. The galvanometer in its earliest form consisted of a compass needle suspended in the center of a coil of wire. When a current passed through the coil the compass needle was deflected from its normal position. It was termed a tangent galvanometer, for the current flowing in the coil is proportional to the tangent of the angle through which the needle is deflected. The tangent galvanometer is not very sensitive and, finding no practical use to-day, its major interest is historical. Sir William Thomson (Lord Kelvin) did considerable work to improve the galvanometer and succeeded in developing an instrument of high sensitivity. Instruments made in accordance with his recommendations are known as Thomson galvanometers. Thomson made use of two coils in his galvanometer arranged to neutralize each other and found it possible to make the needle of the instru ment move with only an exceedingly small current flowing in the coils. Galvanometers of this type have been made so sensitive that ^-billion of an ampere would cause the pointer to deflect. A Thomson galvanometer, although very sensitive, has the disadvantage that in its simplest form it does not return to the zero point very quickly when the current flow through the coil is stopped and also the pointer oscillates back and forth for quite a long period of time before it finally comes to rest at any position. Thomson galvanometers can be made more satisfactory by attaching a vane to the suspension so that the air resistance created as the vane turns tends to bring the galvanometer to rest more quickly. This mechanical type of "damping" is the only type that can be applied to the Thomson galvanometer and for this reason another form of the instrument has come into more general use, known after its inventor as the D'Arsonval galvanometer. In the Thomson galvanometer we had a stationary coil and a moving magnetic needle; in the D'Arsonval type we use a stationary magnet and a moving coil. The magnet is a very strong one and the coil moves in a small air gap in the magnetic circuit. The constructional features of such an instrument will be given in a Sheet to follow this.