Radio Broadcast (May 1928-Apr 1929)

A Simple A,C» Operated Tube Tester By THE LABORATORY STAFF THE following description of a simple tube tester will be interesting to service men and others who have occasion to doubt the efficiency of a tube. The tester determines the mutual conductance of the tube, that is, the amount of plate current variation with /4 a given change in grid bias, but will not indicate whether the tube is microphonic, noisy, gassy or, suffering from ■ other common ailments. The tester may & be plugged into any a.c. socket, provided the voltage and frequency have the values for which the transformer mmm is designed. The circuit diagram shows a transformer which has all standard filament voltages, including 7.5 for power tubes. In the Laboratory it is seldom necessary to test tubes of this sort so a socket for them was not included in the set-up shown in Fig. 2. A resistor in the center-tap circuit (and the cathode of heater-type tube) provides a C bias for the tube. A switch short circuits part of this resistor in order to change the C bias and thus to change the plate current. The mutual conductance is the ratio between the corresponding plate current and grid voltage changes. The circuit diagram and the picture show clearly how the parts of the tester are connected together. The list of apparatus includes only standard material, but any other similar units may be used, of course. In operation, the tube is placed in the socket and the tester is plugged into a lamp socket. This lights the filament and puts a voltage on the plate. Current flowing in the plate circuit is read on the meter, M, and in returning to the center of the filament — or the heater — passes through the biasing resistors, Ri and R2, which are connected in series. The values of these resistors are known ; in this case the total resistance is 4000 ohms. Then the resistor Ri is shorted and the plate current increases because the bias on the tube is reduced. The difference between the two plate currents as read by the meter, divided by the difference between the two grid-bias potentials gives an estimate of the mutual conductance. The bias on the tube is calculated by multiplying the plate current by the biasing Lamp VIEW OF A.C. TUBE TESTER CT^HE tube tester described on this •*■ may be used for checking the mutual conductance of any standard a.c. or d.c. tube. The tester is plugged into a 1 10-volt light socket and no batteries of any kind are required. Incidentally, the mutual conductance is the only important characteristic of a tube which it is necessary for a service man to check. — The Editor resistance in the circuit. As an example let us give the values obtained in the Laboratory when a 20iA-type tube was tested. With a C-bias resistance of 4000 ohms (3500 plus 500) the plate current was 1 milliampere. When the 3500-ohm resistor was shorted the plate current increased to 3.1 milliamperes. The two biases were, then, 4000X0.001 = 4 volt and 500 X 0.0031 or 1.55 volts. The mutual conductance is Gm — li .003 1 1-55 .0021 2.45 = 860 micromhos ACCURACY OF TESTER ^PHE value of mutual conductance obtained *■ by this tester is not very accurate because each change in grid-bias resistance changes the plate voltage as well as the plate current and the definition of Gm involves holding the plate voltage constant. In the Laboratory, however, several tubes were tested and the values of mutual conductances compared with values obtained on a bridge when the tube was operated at standard values of bias and plate voltage. The results outlined below show that the accuracy is all that is desired for practical purposes. Tube type No. tested % accurate scale reading is 1 5 milliamperes. A switch removes this shunt when desired so that lower currents may be read more accurately. The switch has a spring in it so that the meter is always shunted until the operator deliberately removes the resistor, R3. The value of the resistor varies with the meter, of course, but a rheostat may be placed across it and adjusted until any desired multiplying factor is secured. Then the wire of the rheostat used may be cut off and fixed in position. Between 3 and 4 ohms were required with the Weston meter. A small flash lamp is connected as a fuse in series with the a.c. line to the plate of the tube to prevent an accident in case of a short circuit. Incidentally, the meter is in a very dangerous position in the circuit and it might be wise to place a short-circuiting strip of wire across its terminals. Of course, after the tester has been connected properly the strip should be removed. However, such a complication was not considered necessary in the Laboratory and the mortality of meters has been nil. The operator will learn very quickly from experience the proper value of Gm for all standard types of tubes. Then, when tubes fall below this value they should be rejected or rejuvenated. He might, as an example, take a new tube, a very bad tube, and an old tube which still gives good signal strength. He can test them on this device and record their mutual conductances. Any tube which approaches the very bad tube should be rejuvenated or thrown away. LIST OF PARTS HP HE parts actually used in the construction * of the tester are as follows: Ti One Silver-Marshall filament transformer, type 325; Ri One Ward-Leonard fixed resistor, 3500-ohm, type 507-56; R2 One Ward-Leonard fixed resistor, 500-ohm, type 507-17; Ri One Frost resistor, 4-ohm; R4 One General Radio center-tapped resistor; type 439; Swi One Frost filament switch; Sw2 One jack switch, s.p.d.t.; One dial light and socket; Two Frost sockets, ux-type; One Benjamin socket, UY-type; One wooden baseboard. 226 201a 112a 171a 93% 78% 90% 89.5" FIG. I The meter used was a Model 301 Weston, 0-5 milliamperes. A shunt, R3, is provided to reduce the sensitivity of the meter by a factor of three, so that its full FIG. 2 '73