Instruction for the Tube Tester Type W 19

grid circuit; therefore the negative bias voltage is decreased and the plate current increases. The rise of plate current by inserting a resistor in the grid circuit is therefore a measure of the degree of vacuum.

However, there are two exceptions: The same may happen if there is leakage between control grid and other electrodes; but this does not matter because for the tube performance it does not make any difference whether distortion is caused by bad vacuum or by leakage; the tube cannot be used anyway. The other exception is thermionic grid emission. If a tube has high mutual conductance the space between cathode and control grid is very small and therefore small amounts of the active emitting substance may be spread to the grid. When the tube is heated the grid becomes hot too and acts in the same way as the cathode and the same grid current flows as with low vacuum in spite of the fact that the tube is all right. If the tube has the highest possible mutual conductance of 14,000 µmhos the 1 Megohm resistor may increase the plate current by as much as 20%.

The vacuum test is made in the following way: After the tube has been tested for control effect in position 13 the test switch is turned to position 14. If then the plate current rises only a little or not at all the tube has a good vacuum, i. e. it is all right. But if the rise is larger one may assume that a bad vacuum Is the reason, i. e. that the tube would cause distortion in a receiver. In this case a further test has to be made in the receiver itself for it is impossible to give a fixed limit between a good and a distorting feature because it is dependent on too many factors. However, if the deflection of the pointer is approximately the same as in position 12 it is sure that the vacuum is a bad and the tube cannot be used. With tubes with large mutual conductance (7000 to 14,000 µmhos) a rise of plate current a by 10—20%) may be caused by thermionic grid emission and the tube may be all right nevertheless.

If the plate current decreased in switch position 14, it means that there are oscillations, but that the tube itself is all right.. This would not happen in a receiver since there is a 1000 ohm resistor in series with the control grid or a 100 ohm resistor in series with the screen grid which prevents oscillation. That cannot be done in a tube tester because each tube holder has to be used for many different sockets and the position of the prongs connectet whith the control and screen grid is not always the same.