Radio broadcast .. (1922-30)

334 RADIO BROADCAST ADVERTISER SUPER-EMISSION RADIO TUBES Excel in All Characteristics Standard Types Power Amplifiers Type 200 Power Detector JOBBERS DEALERS A few territories open for exclusive distribution Write ^ov for ^Details CABLE SUPPLY CO. INC. Sales Offices 907 Broadway New York The Radio Broadcast SHEETS JNQUIRIES sent to the Questions and Answers department of RADIO BROADCAST have until recently •*• been answered either by letter or in " The Grid." The latter department las now been discontinued, and all questions addressed to our technical service department are now answered by mail. In place of " The Grid, " we present herewith a series of Laboratory Information Sheets. These sheets contain much the same type of information as has appeared in " The Grid," but we believe that the change in the method of presentation and the wider scope of the information in the sheets, will make this section of RADIO BROADCAST of much greater interest to our readers. The Laboratory Information Sheets cover a wide range of information of value to the experimenter, and they are so arranged that they may hi out from the magazine and preserved for constant reference. We suggest that the series of Sheets appearing in each issue be cut out with a ra^or blade and pasted on filing cards, or in a note book. The cards should be arranged in numerical order. Several times during the year, an index to all sheets previously printed will appear in this department. Those who wish to avail themselves of the service formerly supplied by "The Grid," are requested to send their questions to the Technical Information Service of the Laboratory, using the coupon which appears elsewhere in this issue. The June and July issues of RADIO BROADCAST, in which appeared the first two sets of Laboratory Sheets, may still be obtained from the Subscription Department of Doubleday, ~' Company. No. 1 7 RADIO BROADCAST Laboratoi Inductance of Single-La CALCULATION FORMULA JT IS possible to obtain quite a close approxima1 tion of the inductance of a solenoid coil by the use of the Bureau of Standards formula, which is as follows: 1 o a!n! y Information Sheet AugUSt, 1926 yer Solenoid Coils I \ I in which — s \ a = Radius of coil in inches, measured from the center of the coil to the center of any wire. °8 b = Length of coil, in inches, n = Number of turns. \ \ \ K A constant, depending upon the ratio of — . *c 7 b The constant K in the formula can be obtained from the accompanying curve. This formula can be used very well in determining the approximate inductance of any particular coil, or can be used to determine the number of turns 05 necessary in order to give certain inductances. It does not take into account the shape or size of the wire, nor does it consider the effect of the capacity of the coil. However, since the coil capac °4 ity is usually negligible in comparison with the capacity of the tuning condenser, it is not especially important insofar as the tuning range of the coil is o -i concerned. *s X X x •^ • — , 02 04 0.6 08 1.0 1.2 14 1.6 1.8 2.0 22 2.4 2.6 2 IA No. 18 RADIO BROADCAST Laboratory Information Sheet Volume Control August, 1926 CORRECT METHOD TO USE A GREAT many of the present receivers now in ^* use are not equipped with any really satisfactory means of volume control. The most common method used on these receivers is to control volume by means of one or more filament rheostats. Usually these rheostats control the audio frequency tubes and, when such is the case, the quality is sure to suffer when the volume is reduced by lowering the filament current. Under such conditions, the quality will be impaired due to the two following causes. In the first place, lowering the filament temperature will increase the plate impedance. Now the frequency characteristic curve of any audio transformer depends to a great extent upon the impedance of the plate circuit. If this impedance is high, the quality will be poorer than if the impedance was low and, for best results, the impedance of the transformer primary should be at least three times the impedance of the plate circuit. Lowering the filament temperature will destroy this ratio, and the quality thereby becomes poorer. In the second place, lowering the filament temperature has the same effect as increasing the negative grid bias. If the temperature is lowered to any great extent, the tube will operate on the lower bend of itscharacteristic curve and distortion of the signals will then take place because a certain amount of detection will occur. Detection should only take place in the detector circuit and, if it occurs at any other point, it will invariably cause distortion. If volume control is at present being accomplished by filament rheostats in the audio amplifier, it will be wise to revise the set so as to permit the use of some other system. Volume can be controlled quite satisfactorily by means of a potentiometer across the secondary of the first audio transformer. This resistance will, actually, somewhat better the quality, since, if a rather poor transformer is being used, it will smooth out the amplification curve and make it quite flat. This unit should have a maximum resistance of about 5(X),{XX) ohms, and should always be placed across the first audio transformer. It is then possible, on strong signals, to cut down the volume and incidentally prevent overloading of the audio tubes. However, if the resistance were connected across the second transformer, it would not he possible to prevent overloading of the first tube. Connection across the first transformer is, therefore, abvisable. It is also possible to control volume very nicely by means of the filament rheostat controlling the radio frequency tube, without getting into any of the difficulties that occur if filament variation of the audio tubes is used to control volume. This is due to the fact that variations in the plate impedance of the radio frequency tube merely tend to cut down the overall amplification, but there is no possibility of frequency distortion since we are working with what is practically a single frequency. Tested and approved by RADIO BROADCAST