Introduction The Model 111 Gm and Em Tube Tester introduced to the electronics industry the most complete tube evaluation available, other than by expensive and complicated laboratory Bridge Systems. The Model III makes the following general tests: Gm — Mutual Plate Transconductance. Em — Plate Emission. Filament Current. Leakage and Shorts. Gas. Noise. Life. 1- Gm Gm (Mutual Plate Transconductance) is the ability of the grid of the tube to control its plate current. Mathematically it is a small change in plate current divided by the change in grid voltage required to make the plate current change. The plate voltage is kept constant. From this it can be seen that the Gm is a most important measurement - especially in video, I.F. and power amplifiers. The Gm test is made by applying pulsating DC voltages as follows:- Positive to the plate and screen; negative to the grid. An AC signal voltage is also fed to the grid which causes an AC increment to appear in the plate current. It is the measurement of this increment which ascertains the Gm of the tube. These are actual average Gm measurements and are not hypothesized. 2- Em Em (Plate current measurement) of a tube is, as the name imnlies, measurement of the tube plate current. Although the voltages applied to the plate are swept, the measurement is still primarily static. It is an indication of the cathode current (which, of course, controls self bias) and also the plate resistance, etc. In this test separate voltages are again fed to the plate, screen and arid of the tube. The voltages are AC, the current is DC since the tube rectifies it. SELECTING THE PROPER TEST It may be said that the more tests made, the more correct the analysis of the condition of a tube. |t is necessary, however, to occasionally look for a faster means. With this idea in mind, PRECISE has * (Asterisked) the most important single test for each tube or section thereof. |f you do not wish to make both the Gm and Em tests, we recommend that you select the one with the * in back of its listing on the roll chart. TESTING A TUBE FOR Gm 1- Select the tube type under the heading marked 'TYPE' abovo the Roll Chart. Gm tests are indicated by 'Gm' in the 'NOTE' column. 2- Set the FILAMENT Switch to the same value as shown under the heading • FI L ’ - The top value listed is the Filament voltage. The value directly beneath it is the Filament current. Measuring the current is discussed in a later section.