Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST ADVERTISER. CLARITY Noted for the Bell-Like Clarity of their Tone CECO's exquisite tone quality is not accidental. It is the result of deliberate effort in design, a higher quality of materials and special and exclusive processes which are employed in the rugged construction of these beautifully toned, longlasting tubes. Sold everywhere. Have you heard CeCo's delightfully entertaining radio program over the entire Columbia Broadcasting Chain of 20 cities— every Monday evening, 8:30 Eastern time, 7:30 Central time? CeCo MSg. Co., Inc. ■» Providence, R. I. Radio Tubes FOR EVERY RADIO NEED The Radio Broadcast LABORATORY INFORMATION SHEETS T^HE aim of the Radio Broadcast Laboratory Information Sheets is to present, in a convenient form, concise and accurate information in the field of radio and closely allied sciences. It is not the purpose of the Sheets to include only new information, but to present practical data, whether new or old, that may be of value to the experimenter, engineer or serviceman. In order to make the Sheets easier to refer to, they are arranged so that they may be cut from the magazine and preserved, either in a blank book or on 4" x 6" filing cards. The cards should be arranged in numerical order. Since they began, in June, 1926, the popularity of the Information Sheets has increased so greatly that it has been decided to reprint the first one hundred and ninety of them (June, 1926-May, 1928) in a single substantially bound volume. This volume, " Radio Broadcast's Data Sheets", may now be bought on the newsstands, or from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Inside each volume is a credit coupon which is worth $1.00 toward the subscription price of this magazine. In other words, a year's subscription to Radio Bboadcast, accompanied by this $1.00 credit coupon, gives you Radio Broadcast for one year for $3.00, instead of the usual subscription price of $4.00. — The Editor. No. 256 Radio Broadcast Laboratory Information Sheet February, 1929 Three Types of Graphs IF WE have before us a job of plotting a curve of an a.f. amplifier to show how the voltage gain varies with frequency, we must decide just how the curve is to be plotted. Curves may be plotted on several types ot cross-section paper which will be illustrated in a future Laboratory Sheet. The problem is this, should we plot the curve on ordinary cross-section paper or on log or log-log paper, and should we plot frequency against db or against voltage output. The essential purpose of a curve is to enable one to obtain a visual idea of the characteristics of the amplifier. Since the purpose of an a.f. amplifier is to amplify currents which will finally be converted into sound, it is preferable to plot the curve to such a scale that its final shape indicates as nearly as possible the variations in response as they would be audible to the ear. Now it has been determined that the ear hears variations in intensity in accordance with a logarithmic function. For this reason, if we are to plot fre quency against output voltage, it is advisable to plot the curve on log-log paper so that the variations will be indicated on the curve in their relative importance as heard by the ear. If we desire to plot frequency against db then the curve should be plotted on log paper. In such a case we would find that the shape of the resultant curve was the same as that of the preceding curve plotted ou log-log paper, for in converting from voltage to db we take into consideration the logarithmic function. In all cases the frequency scale should be plotted on a log scale so that each octave in the scale takes up an equal amount of space. Take a piece of crosssection paper with a log scale on it and measure the distance in inches between 10 cycles and 100 cycles, a change in frequency of 10 to 1. Then measure the distance between 100 and 1000 and between 1000 and 10,000. The distances are all equal and equal sections of the curve therefore receive an equal amount of space. No. 257 Radio Bboadcast Laboratory Information Sheet February, 1929 Heater Connections for A. C. Tubes A N EXAMINATION of the circuits of various -f* a.c. receivers using one or more 227-type tubes shows several different ways the heaters of these tubes may be connected into the circuit. In sketch A we show the heater of the tube quite indi-priidi'iil '>i the remainder <>l the circuit. In sketch B the center tap of a resistor connected across the heater is grounded and in sketch C the center tap of the resistor connected across the heater is connected to the plus 45-volt terminal. Of these three arrangements the one most commonly used is B in which the heater is grounded, since such an arrangement gives satisfactory oper ation in most cases. It is generally unwise to arrange the circuit as indicated at A, since the heater under such conditions is more or less floating and is liable to introduce hum into some part of the circuit. The reason for the use of the arrangement shown at C is somewhat complicated. When the heater of the tube becomes hot it, of course, emits some electrons and it is possible for some of these electrons to enter the plate circuit. Since the heater is operated on a.c. the emission from it is not uniform and, therefore, a hum will be produced if any appreciable number of electrons are drawn from the heater. • february, 1929 . . . page 272 •