Radio broadcast .. (1922-30)

.RADIO BROADCAST. 3.3, 5, and 7.5 volts to supply the various filament voltages to the different types of tubes. The American Transformer Com- pany (Amertran) has manufactured such a transformer on special order and other transformer manufacturers are also equip- ped to do the same thing. In operation the variable resistance, RI, in the 110-volt lead is varied until the a.c. voltmeter, M 2 , across the transformer primary reads 100 volts with the tube inserted in the socket. This insures correct filament and plate voltage on the tube being tested at various a.c. line voltages. The shunting lamp, L», across the trans- former primary is to reduce the size of the series resistor required for some of the low-filament-consumption tubes and also serves as a pilot lamp to indicate that the set is turned on. An alternative method of filament light- ing is shown in Fig. 4. All filaments are connected in parallel across the secondary of a transformer having a 110-volt prim- ary and a 7.5-volt, 3-ampere secondary. In this case the primary voltmeter and resistance control change only the plate voltage to the tube, the filament voltage control being obtained from two series filament rheostats, R 4 and R 6 , and read on a two-scale voltmeter, M 3 . It is very important to adjust the rheostats R 4 and Rs to maximum resistance before inserting any tubes. Two rheostats are provided, one for low- and the other for high-current tubes. Because of the voltmeter current the 50-ohm rheostat, RI, provides all regula- tion necessary even for 60-mA. tubes. Operation of Tester The operation of this tester will then be as follows: In the case of a tapped transformer, the tube is inserted in the proper socket and the resistance R, is adjusted so that voltmeter, Mj, reads 100 volts. The plate current reading as shown on MI is then noted both before and after closing the key, KI. With the untapped transformer (alternative method), rheo- stats R 4 and R 6 are first set at maximum resistance value. The tube is then inserted in the proper socket and R 4 and R 6 are adjusted to give the correct filament voltage as shown on meter M 3 . Resistance RI is then changed to make meter Mj read 100 volts and from then on procedure is just as in the first case. It will be found that the tube readings are dependent of the way in which A and AI, the plate voltage supply connections, are made to the a.c. supply and filament transformers. These leads should be reversed until the highest readings are obtained, in order to obtain results comparable to the following table. The difference is especially notice- able in the case of tubes with a 7.5-volt filament. In testing the tubes with higher readings on the meter ]Vli it will be found that as the CX-299 CX-220 CX-112 CX-371 CX-U2A CX-340 CX-3T1A CX-300A CX-310 CX-322 .CX-301A CX-350 -«-VWM 0-50 OHMS (jAMR) 10 WATT LAMP Fig. 1 10-watt lamp becomes slightly heated, the meter reading will decrease slightly. This is entirely caused by the lamp and OHMS OHMS OHMS OHMS Fig. 3 should not be construed to mean any de- fect or abnormality in the tube being tested. A table of approximate changes in Cunningham tubes is given here, this table being by no means a criterion but merely to suggest the results to be ob- tained. Variations in the values of biasing resistor, transformers, meter calibration, etc., will cause some changes in the result. Tube oil cx-12 cx-326 cx-345 c-324 c-327 cx-299 cx-220 cx-322 cx-112 cx-301* cx-340 cx-371 cx-37lA CX-300A cx-310 cx-350 CX-112A . Average Plate Current Values for Tubes i Open 1-1.5 1-1.5 1.5 3 1 1.5-2 1.5 2.5-3 2 2 1.7 .7 3.5-^1 3.5-4 1.5 2 3 2 K, Closed 2-2.5 2-2.5 4 11 2.6 3-5.5 3 5.5-6 4-6 6.5-7.0 4.5-5.0 1.7 12-13 12-13 3.5 6 10.5 6.5-7.0 Fig. 4 For constructing the test set a sug- gested layout is drawn in perspective, For quick work and convenience it is es- sential to have one socket for each of the groups of tubes indicated on the diagram. It is seen that for the screen-grid tubes the screen grid is connected to the plate mak- ing it a three-element tube for purposes of this test. For this reason it is necessary to use a separate socket for screen-grid tubes since on them the control-grid connection is on top and the screen grid is connected to the usual grid terminal of the socket. The equipment listed in Table II was used in the experimental model but any equivalent apparatus will give satis- factory results. Table II Ti — one special filament transformer, 20-watt capacity and having a 7.5-volt secondary with taps at 1.1, 1.5, 2.5, 3.3, and 5.0 volts. Primary wound for a 100-voIts 60-cycle supply and secon- - dary delivering above voltages at rated primary voltage. (American Transformer Company.) Ri — One variable resistor, 50-ohm, 500-mA. • (General Radio 50-ohm rheostat type 214) Lr— One lamp, 25-watt, 110-volt Li — One lamp, 10-watt, 110-volt Mz — One Weston 476 a.c. voltmeter, 0-150 scale. Mi — One Weston model 301 milliammeter (d.c.) 0-15-mA. Six— Sockets, CX type Three — Sockets, five-prong Ki — One S.P.S.T. switch or key (push button will do) R2 — One Electrad resistance, type B, 2250-ohm Rs — One Electrad resistance, type B, 400-ohm Two — Clips for connection to screen-grid tubes K2 — One General Electric single-pole tumbler switch (Catalogue No. 269943) For the alternative method of filament supply, the following changes would be made in the ap- paratus requirements: T2 — One Thordarson type T2230 filament trans- former having a 7,5-volt, 3-ampere secondary and a 110-volt primary Ma — One Weston model 528 a.c. voltmeter with two scales of 4- and 8-volt ranges Ru — One General Radio type 214 rheostat, 7-ohm, 2 -ampere IU — One General Radio type 214 rheostat, 50-ohm, J -ampere The rectifier test consists in operation of (Concluded on page i7i) • JANUARY 1930 • 145