2-7
frequency, and output power with the transmitter’s built-in test equipment. You can check the output of
klystrons used as receiver local oscillators by measuring the current in the crystal mixer unit.
Klystron tubes that remain inoperative for more than 6 months may become gassy. This condition
occurs in klystrons installed in stored or spare equipment as well as in klystrons stored as stock supplies.
Operation of a gassy klystron at its rated voltages will ionize the gas molecules and may cause excessive
beam current to flow. This excessive beam current may shorten the life of the klystron or produce
immediate failure. You can detect gas in a klystron tube by setting the applied reflector voltage to zero
and slowly increasing the beam voltage while observing a meter that indicates the beam current -
excessive beam current for a specific value of voltage indicates that the tube is gassy.
A gassy klystron tube can usually be restored to serviceable condition if you temporarily operate it at
reduced beam voltage. Eight hours or more of reduced voltage operation may be required for klystrons
that have been inoperative for periods in excess of 6 months.
The beam current is also an indication of the power output of the klystron. As klystrons age they
normally draw less beam current; when this current decreases to a minimum value for a specific beam
voltage, the tube must be replaced. You can usually determine the power output of transmitter klystrons
by measuring the transmitter power output during equipment performance checks.
Q-5.
What should you do if a klystron becomes gassy?
Traveling-Wave Tube
You can usually test a traveling-wave tube (twt) in the equipment in which it is used. When the twt is
installed, you can usually measure the collector current and voltage and check the power output for
various inputs. Any deviation greater than 10% from normal specifications may be considered to be an
indication of a defective tube. Most amplifiers are supplied with built-in panel meters and selector
switches so that the cathode, anode, helix, focus, and collector currents may be measured. Thus,
continuous monitoring of amplifier operation and tube evaluation is possible. Adjustments usually are
provided for you to set the helix, grid bias, and collector voltages for optimum operation. If variation of
these controls will not produce normal currents and if all voltages are normal, you should consider the
tube to be defective and replace it with a new tube or one known to be in good operating condition. To
avoid needless replacement of tubes, however, you should make an additional check by measuring the
input power and output power and determining the tube gain. If, with normal operating conditions, the
gain level drops below the minimum indicated value in the equipment technical manual, the tube is
defective.
Q-6.
When used as an amplifier, what is the best indication that a twt is operating properly?
In the absence of special field-test sets, you may construct a laboratory test mock-up similar to that
shown in figure 2-3. Because of the variations in power and gain between tubes and the large frequency
ranges offered, we can illustrate only a general type of equipment. The equipment you select must have
the proper range, impedance, and attenuation to make the test for a specific type of twt. To make gain
measurements, you turn the switch shown in figure 2-3 to position 1 and set the precision attenuator to
provide a convenient level of detector output. Then turn the switch to position 2 and insert attenuation
until the detector output level is identical to that obtained without the twt in the circuit. The gain of the
traveling-wave tube is equal to the amount of added attenuation.
