Radio broadcast .. (1922-30)

.RADIO HROADCAST- A REFLEX VACUUM-TUBE VOLTMETER P~T~^HE ORDINARY vacuum-tube voltmeter • has the disadvantage that its range is I quite limited, generally being not more than about 3:1 in voltage. By the use of an arrangement whereby the plate current is caused to flow through a resistance which functions to increase the bias on the grid of a tube, it is possible to increase the range of the instrument almost indefinitely. Such a device is called a "reflex" volt- meter. The first reflex voltmeter which came to the attention of the writer was one described by W. B. Medlam and U. A. Oschwald in the Novem- IXT. 1926, Experimental Wireless and Wireless Engineer (England). A de- scription of a reflex volt- meter formed part of an excellent series of articles by these two engineers. In Fig. 1 is given a calibra- tion curve of a reflex volt- meter taken from the pre- viously mentioned article. ()n the curve is shown the circuit arrangement used. It should be noted that the plate current, in order to get to the filament, must flow through the resistance R, and that increases in the current through R will also increase the bias on the grid of the tube. The calibration curve of a re- flex voltmeter is quite linear, in this particular case we find that from about 3 volts to 20 volts each increase of 10 micro- amperes in plate current corresponds to an increase of 2 volts on the input. The steady bias, E c , is used so that the plate current will not be ex- cessive when the a.c. input voltage is zero. In Fig. 2 is given the circuit diagram of a reflex voltmeter using a 227-type tube. This should make a very useful instrument. It should be possible to supply the volt- meter from a rectifier and filter system so that the instrument could be entirely self-contained and light-socket operated. plate current. The value of resistor R 2 is not critical; its value should simply be much greater than the resistance of the galvanometer so that the entire increment in plate current will flow through the galvanometer and not through Rs. Vacuum-tube voltmeters are gener- ally calibrated by connecting known a.c. voltages across their input, noting the corresponding plate current, and finally plotting the calibration curve showing the variations in plate current with input voltage. It is, however, possible to use a static characteristic curve to calculate 20 18 16 14 io 8 4 Ep=60 Volts_ R = 160,000 ohms Ee = -18 Volts 10 20 30 40 50 60 70 80 90 100 PLATE CURRENT mA. Figl the calibration curve of a simple voltmeter by any one of several methods. Medlam and Oschwald give a simple method in their article. To determine the calibration from the static characteristic curve, we must know the plate current correspond- ing to: EC the steady bias on the grid EF the steady bias plus the peak value of the a.c. voltage Ef the steady value minus the peak value of the a.c. voltage Knowing these three quantities it is pos- sible to calculate within two or three per cent, the mean plate current as read on the plate current meter, by means of the following simple relationship. Im • IF +If where Im = reading of the plate meter Ic = plate current corresponding to EC IF = plate current corresponding to EF If = plate current corresponding bo Ef Consider the following example. We desire to determine what would be the reading of the plate meter if a peak a.c. potential of 1 volt were applied to the grid. Assume that the steady bias on the tube is minus 3 volts. Therefore, EC -- 3 EF = — 3 -f 1 or — 2 volts Ef = — 3 — 1 or — 4 volts From an accurately measured static characteristic curve, we determine that I c is 10 microamperes, IF is 34.1 micro- amperes and If is 1.7 microamperes. Sub- stituting in the formula, we have To balance out the steady plate current flowing through the plate microammeter, when there is no input voltage, connection is made through resistance Ri to the plate side of the meter and the potential Eb is adjusted so that the current through G is exactly equal and opposite to the steady 10 Im - 34.1 + 1.7 ~~~~ Im = 13.95 By actually applying an a.c. voltage hav- ing a peak value of 1 volt to this tube it was found that the plate meter read 14 microamperes, which agrees very closely with the calculated result. Just Out • • • Keith Henney 9 s Book on Radio Principles of Radio By KEITH HENNEY Director of the Laboratory K<u/i» Broadcast Magazine Readers of Radio Broadcast, long familiar with the work of Keith Henney in his capacity as Director of the Magazine's Laboratory, will be eager to secure his first book, just released from the press. This book brings together within one cover the kind of information on radio which will appeal to the practical interest of every radio experimenter, technician, engineer, and fan. It contains the latest data and the most modern methods. It treats in a thoroughly practical way everything from the production of radio currents to their reception and transmission. Many problems, examples, illustrations, experi- ments, are here presented in book form for the first time. Keith Henney, by reason of his wide experience as an operator, en- gineer, and writer, has the gift of making technical information read- ily understood by the reader. See a Copy on Ten Days' Approval Price $3.50 A Wiley Book ON APPROVAL COUPON JOHN WILEY & SONS, INC. RB i-so 440 Fourth Ave., New York City Gentlemen: Kindly send me on approval Hen- ney's "Principles of Radio." I o^ree to remit the price ($3.50) within ten days after its receipt or return the book postpaid. Name .... Address .. Reference JANUARY 1930 • 173