2-2
Because the operating capabilities and design features of a tube are demonstrated by its electrical
characteristics, a tube is tested by measuring those characteristics and comparing them with representative
values established for that type of tube. Tubes that read abnormally high or low with respect to the
standard are suspect. Practical considerations, which take into account the limitations of the tube test in
predicting actual tube performance in a particular circuit, make it unnecessary to use complex and costly
test equipment with laboratory accuracy. For most applications, testing of a single tube characteristic is
good enough to determine tube performance. Some of the more important factors affecting the life
expectancy of an electron tube are listed below:
The circuit function of the tube
Deterioration of the cathode coating
A decrease in emission of impregnated emitters in aging filament-type tubes
Defective seals that permit air to leak into the envelope and oxidize the emitting surface
Internal short circuits and open circuits caused by vibration or excessive voltage
If the average receiving tube is not overdriven or operated continuously at maximum rating, it can
have a life of at least 2,000 hours before the filament opens. Because of the expansion and contraction of
tube elements during the process of heating and cooling, electrodes may lean or sag, which causes
excessive noise or microphonics to develop. Other electron-tube defects are cathode-to-heater leakage and
nonuniform electron emission of the cathode. These common tube defects contribute to about 50% of all
electronic equipment failures. For this reason you should immediately eliminate any tube known to be
faulty. However, avoid blind or random replacement of good tubes with fresh spares. The most common
cause of tube failure is open filaments. Evidence of a tube defect is often obvious when the filament is
open in glass-envelope tubes. You will also notice the brighter-than-normal cherry-red glow of the plate
when the plate current is excessive. Also, when the tube becomes gassy or when arcing occurs between
electrodes, you will probably have visual indication. Metal-encased tubes can be felt for warmth to
determine if the heater is operating. You can tap a tube while it is operating in a circuit to reveal an aural
indication of loose elements within the tube or microphonics, which are produced by loose elements.
Most tubes are extremely fragile and subject to damage during shipment. When you replace a tube,
never make the assumption that the new tube is good because it’s new. You should always test tubes
before installing them.
Q-2.
What is the most common cause of electron tube failure?
SUBSTITUTION METHODS
Substituting with a tube known to be in good condition is a simple method of testing a questionable
tube. However, in high-frequency circuits tube substitution should be carried out in a logical sequence.
Replace tubes one at a time so that you can observe the effect of differences in interelectrode capacitance
in the substituted tubes on tuned circuits. The tube substitution test method cannot be used to advantage in
locating more than one faulty tube in a single circuit for two reasons: (1) If both an rf amplifier tube and
IF amplifier tube are defective in a receiver, replacing either one will not correct the trouble; and (2) if all
the tubes are replaced, there is no way for you to know what tubes were defective. Under these
conditions, using test equipment designed for testing the quality of a tube saves you valuable time.
Q-3.
What is the most accurate method of determining the condition of an electron tube?
