Radio Broadcast (May 1929-Apr 1930)

low as 50 and 100 milliwatts as standard volume levels. He does not take into account the fact that these are levels desirable for laboratory measurement and that people in general in this country desire much more power output than these figures. The tendency, if anything, is to about double the power requirements that he mentions as desirable in England. At least 1.5 watts is now considered as necessary for average homes. '" "' Anyone who has mea Hurnps in a.f. sured the voltage across Transformer a good a.f. transformer Characteristics when various input fre i iiiiHiiimi Jin quencies at the same \oltage are put into its primary winding will remember the hump that occasionally takes place somewhere between 4000 and 10,000 cycles. Why is this hump? A transformer may be considered as Fig. 1, in which the primary and secondary leakage inductances are represented in series with the primary and secondary resistances and the previous tube resistance, and across this circuit is the mutual inductance between primary and secondary— which should be high — shunted by the capacity of the windings, the leads, and the tube input, and followed by a perfect transformer. Now all of this is a series circuit which may become resonant to some audio frequency. If so, the voltage across C will rise, and the transformer characteristic will show a hump in the neighborhood of this resonant frequency. If the "Q", L-j/R, of the circuit is high the hump may become high enough for the entire system to sing. If the Q is low, which may be due to high-resistance windings or a high-resistance tube, the hump may be quite small. Thus, a good transformer, which will be stable when worked out of a 201atype tube, may sing when worked out of a 112-type tube. Increasing the plate resistance, then, cuts down the peak. Putting an inductance in series with the primary lowers the frequency of the peak. Increasing the capacity across the secondary lowers the frequency of the peak. Putting a resistance in series with the grid lead cuts down the hump. At low frequencies this series leakage reactance is small compared to the mutual reactance. The response at low frequencies, then, is a function of how great this impedance is compared to the tube resistance. (See Fig. 2) If the mutual inductance of the transformer is high compared to the plate resistance of the tube, the lowfrequency response will be good. If a poor transformer with low mutual is used, or if the tube resistance is high, the response at low frequencies may become quite bad , and , in fact, may become markedly peaked in the neighborhood of 1000 cycles. If a good transformer, the Samson Symphonic for example, is worked out of a screen-grid tube some such curve as that in Fig. 4 will result. The series resonance hump at high frequencies can be cut down by a resistance near the grid of the tube. If carried far enough, the high-frequency response will begin to droop. If the resistance is in the filament side, as for C-bias purposes, the high frequencies will be reduced because the capacity current, Fig. 3, must flow through this resistance and sets up a voltage across it which is out of phase with the desired voltage across the grid-filament input. 2000 1500 Short Wave Transmitting Schedules < 1000 500 100, Wellxng T O N W. Muir, of Lockport, N.Y.,who probably has more "dx" records than any other consistent listener in this country, submits the following data on short-wave ° §> broadcasting. The Chief Engineer at Bandoeng, Java, will be pleased to receive reports from those hearing the following phone transmitters: ple, 15.74 meters, plf, 17.40 meters, and plg, 18.88 meters, daily from 7 to 11 a.m. (E.S.T.); plr, 27.80 meters, daily from 11 a.m. to 2 p. M. ; plg and ple, Wednesdays from 5:30 to 7:00 a. m. The Societe Francaise Radio-Electrique, 79 Boulevard Haussmann, Paris, will be pleased to receive reports from those hear O .A/WWNA Rp Fig. 2 ing the following phone transmitters: fw4, 24.50 meters; fw, 15.55 meters; and Buenos Aires, 15.02 meters. The radio station at Matala, Sweden, will be pleased to receive reports from those hearing their short-wave station on 98.90 meters, daily from 11 a. m. to 5 p. m. Pcjj, 31.30 meters, Eindhoven, Holland, will be pleased to receive reports. This station operates on the following schedule: Thursday 1 to 3 p. m. and 6 to 10 p. m., Friday 1 to 3 Fig. 3 p. m. and 8 to 12 p. m., and Saturday 12:01 to 1 a. m. All of the hours given above are in Eastern Standard Time." ' A WELL-KNOWN RE Resourcefulness ceiver manufacturer in of a Consulting the Middle West de Radio Engineer. cided to gO in for the in""""" 250-type of amplifier tube but discovered that no greater volume could be obtained from the set than with a 171-type power tube. The trouble was lack of plate voltage. The manufacturer called in a consulting radio engineer and the following is the result. The receiver had eight tubes. The engineer went over the entire receiver and finally secured better results from seven tubes. This left one extra socket on the chassis. He put a 226type tube in this socket, made it into a two-element rectifier, and supplied the bias for the 250 from it. This added 60 volts to the plate of the 250. The result was a seven-tube receiver with the performance of the eight-tube set, a 250-type tube with 300 volts on its plate, and plenty of volume. """"" The inability of Dr. Dr. Goldsmith Alfred N. Goldsmith to Resigns as serve longer as Editor of /. R. E. Editor the Proceedings of the < """""" "' Institute of Radio Engineers is a matter of regret which all members of the Institute share. Despite the difficulty of obtaining a constant supply of good material in the midst of intense rivalry, and considerable secrecy, among radio laboratories and manufacturers, and the labor involved in presenting this material in the proper form. Dr. Goldsmith has served for sixteen years as Editor of the Proceedings. A cursory glance through papers presented during those sixteen years indicates why the bound copies of the Proceedings have come to be the cover and contents of every radio engineer's library. These papers are signed by such names as Marconi, Armstrong, Poulson, Hazeltine, Pickard, Cohn, Stone, Zenneek, Austin. Morecroft, Pierce, and others, including Dr. Goldsmith. ' 1 m"" In January "Strays" Empirical we gave some empirical Coil formulas whereby the Formulas inductance of m u 1 1 i i i H i mn layered coils, solenoids. and helical coils could be calculated. The dimensions were given in inches. When the dimensions are given in centimeters the follow ing formulas will give the inductance. We are indebted to Harold F. Sc.HWEDE,*of Chicago for them: 0.315 a=n2 6a + 9b + 10c [l h; Ls 0.394 Lh = 0.391 a-'n8u + 11c 9a + 10b U. h SJLah; Where Lm = Inductance of multilayer coil Ls = " solenoid 11 Lh = " helical In the above formulas a, b, c, and N are as shown on page 170, January Radio Broadcast. — Keith Henney 102 • • JUNE • 1 9 29 •