Users of vacuum tube-based equipment know that when a failure occurs, the tubes employed in the equipment should be among the first things checked. Since tube manufacturers have published typical operating conditions for most tube types, suitable tests can be made which will help determine whether a tube is functioning satisfactorily. These tests are created with data compiled from comparing the manufacturer's specified conditions for the tube against observations of the tube’s performance in various types of equipment designed for it.
The tests are performed using a device known as, not surprisingly, a tube tester (sometimes called a tube checker). Tube testers normally perform one or more of the three major types of tests to which a tube may be subjected. Those tests typically are:
The Short Circuit Test. In this test, a low-current voltage is applied between the internal elements of the tube with an indicator bulb wired in series with the elements. Any two elements that are touching will light the bulb and indicate an internal short circuit. The current must be kept low so that high resistance shorts (where the elements are just barely touching) will not cause the bulb to light and thus indicate a short that may not adversely affect tube performance. Shorted tubes should be discarded without further testing. If a tube passes the short test, though, it still may fail other tests.
The Emission Test. A vacuum tube operates by emitting electrons from its cathode. As a tube ages, cathode emission tends to decrease until no further emission is possible. An emission tester is configured to tie together and apply a direct current (DC) voltage to all elements of the tube except the plate or anode. Next, a positive voltage is applied to the plate. Emission quality is then read from a meter that indicates total plate current. If the reading is below the average published value for the tube type, the tube can be tossed out immediately.
The Transconductance Test. This test’s purpose is to measure the ability of the tube to amplify a given signal. The short and emission tests are static tests, whereas the transconductance test is a dynamic one in that it measures the effect of a changing voltage on the tube. This type of test is considered a more accurate indicator of a tube’s worth and gives a clue as to how the tube will perform under varying conditions.
In this test, separate DC voltages are applied to the tube elements, with an alternating current (AC) voltage applied to the grid or control electrode. This causes an AC component to appear as part of the total plate current and that component is read on an output meter. Dividing the indicated AC plate current by the voltage applied to the grid gives the transconductance of the tube. If this value is below the published value for the tube, the tube is a candidate for the discard pile. However, the tube may still work fine in less demanding circuits.
These tests, whose accuracy is only as good as the correlated data from manufacturer’s specifications and actual observation, are usually sufficient for general use. For more demanding situations, there are laboratory-grade tube testers that are capable of analyzing all aspects of a tube’s performance. The final test of a tube’s worth, once the tube tester has passed it, is whether the tube will perform in circuits designed for it.
RCA Staff. RCA Receiving Tube Manual, technical series RC-22
. Harrison, New Jersey
: Radio Corporation of America
ARRL Staff. The Radio Amateur’s Handbook
, 46th edition. Newington, Connecticut
: The American Radio Relay League