Radio age (Jan-Dec 1925)

RADIO AGE for July, 1925 ^Formulae and Tables for Testing and Plotting Charts for Vacuum Tubes Used in Radio Reception Learning Vacuum Tube Characteristics By H. FRANK HOPKINS The Magazine of the Hour 11 THERE are numerous characteristics of vacuum tubes used in radio reception and transmission that may be determined, but which have no value to the average radio fan. However, there are a few of vital importance to all users of vacuum tubes and it is the writer's intention to make clear such of these characteristics which are of importance to the fan so that he will be able to plot curves or charts and match his tubes as easily as he tunes his receiving set. The equipment required for this work is a good vacuum tube test set, such as described in the May issue of RADIO AGE. An instrument such as this may be built at a nominal cost or one similar may be purchased ready made at a good range of prices, from the simple one-meter affair to those having a complete set of meters. The One-Meter Tester '"PHE one meter tube testers are -* limited in their use, however, and outside of a plate current curve at a fixed grid bias, no other features may be determined. This type of test set will only give a fair idea of how a tube will act. They will sometimes show a good plate current curve, but fail to perform efficiently when in operation. Therefore, it is worth the difference in price to have a set that will show the filament current or voltage and the grid bias voltage in addition to the plate current. For this article, the RADIO AGE test set was used. It consists of three meters; a filament voltmeter, a grid volt meter and a plate milliammeter with the necessary resistances to vary the filament and plate voltage and grid bias voltage as desired. It was prepared for operation by connecting an "A" or filament battery of suitable voltage for the tube under test — to terminals (A BAT + ) and (A BAT -). A "B" or plate battery of ninety volts was connected across the binding posts (B — ) and (B90) with taps at 22 J^ volts, connected to binding post (B22), 45 volts to binding post (B4S), and 67}^ volts to binding post (B67), Two 7 Y2 volt "C" or grid batteries were connected to the "C" The milliameter shown above shows the plate current characteristic in milliamperes. battery terminals. The negative terminal of one connected to binding post (C — ) the positive terminal of this and the negative terminal of the second "C" battery connected to binding post (C-j ) and the positive terminal of the second "C" battery to binding post (C+). The filament rheostat (R) is moved to its off position, a tube placed in the socket (T) and the set is ready for operation. By moving the switch (BS) to point ( — ) and closing switch (GS) a negative grid bias voltage will be shown on the two-scale voltmeter (GM), this grid bias voltage may be varied at will from 0 volts to 7H volts by moving the potentiometer (GP) until the meter shows the desired voltage. By moving the switch (BS) to the ( + ) point, a positive grid bias will be shown on the meter (GM) and will be varied as above. The switch (PS) and the rheostat (BR) regulate the "B" or plate battery voltage. By moving the switch (PS) to terminal (1) and moving the rheostat (BR) a plate voltage range from Figs. 1 and 2. Fig. 1 shows the typical curve of plate current voltage, characteristics, the result of tests described in while Fig. 2 shows the typical curves of grid and plate the accompanying article.