Paragraphs 3-1 to 3-13
3-2. This section contains operating instructions for
theModels 492A and 494A Traveling WaveTube Ampli-
fiers. Figure 3-1 gives basic operating instructions.
The remainder of this section supplements these in-
3-3. PRELIMINARY OPERATING PROCEDURE.
3-4. The front panel controls, indicator, and con-
nectors for the 492A and 494A are shown in figure 3-1.
This figure also shows the uses for the controls, indi-
cator, and connectors.
Whenever the twt is turned
on, use the front panel meter to measure the current
to each electrode in the traveling-wave tube with the
GRID BIAS control set for zero bias. The safe maxi-
mum current for each electrode is shown in table 3-1.
ormal cathode current for your instrument is indi-
cated on the plate attached to the meter face. Cur-
rents are usually a little high when the instrument is
first turned on, but decrease to normal during warm-
Allow IS minutes warmup before making final
The GRID BIAS control may be used to re-
duce the tube currents during warmup.
Table 3-1. Maximum Operating Currents for
Models 492A and 494A.
Adjusting the HELIX voltage control for maximum
gain at the upper frequency limit usually produces the
flattest frequency response over the band. Maximiz-
ing the gain at frequencies below the upper frequency
limit usually results in additional gain and power out-
put over that obtained when adjusted for flattest broad-
The final setting of the HELIX con-
trol is independent of the type of signal amplified.
3-8. SATURATION POWER OUTPUT.
3-9. Saturation power output is the maximum output
power obtainable with a given collector current
and optimized helix voltage.
As the input signal is
increased from the noise level, the output-vs-input
characteristic is linear until saturation is approached
and the gain begins to decrease. Eventually the output
reaches a peak which is the saturation power output
and any further increase in the input causes the
output to decrease.
the collector current is reduced,
the power output at which saturation occurs will be
The primary effects which lead to
the forces between the electrons
in the beam begin to limit the electron density in
the bunches 2) the energy transfer from the electron
beam to the helix causes the beam velocity to decrease
and gradually lose synchronism with the wave on the
These effects become more pronounced as
saturation is approached, and cause the gain of the
twt to decrease.
3-6. HELIX CONTROL.
3-7. The HELIX voltage control on the front panel
maximizes the gain and power output of the twt at a
selected frequency or optimizes the gain and power
output over the entire band; see figures 3-2Aand 3-2B.
following tube replacement, or for some
other reason, the cathode current can be increased
with the GRID BIAS control to slightly above the safe
maximum current (see table 3-1), readjust the anode
voltage control, paragraph 5-26.
The anode voltage
control is set to limit the twt to its normal cathode
current when the bias voltage on the twt is zero.
the cathode current is limited to its normal value,
but the current to another electrode is excessive, the
amplifier requires service or adjustment, see para-
graphs 5-21 or 5-30.
CAUTION: DO NOT EXCEED:
3-10. Operating the 492A or 494A near saturation
power output will produce second harmonic content in
the output. To obtain maximum power output from the
twt without excessive second harmonic content in the
output, increase the input signal level until satura-
tion is reached, then reduce the input until the out-
put decreases approximately 6 db (an input reduction
of 10 to 15 db).
second harmonic content is un-
important, operation at saturation is usually very
3-11. One advantage of operating the twt near satura-
tion is the constant output characteristic exhibited by
a twt at saturation.
At saturation the gain of a twt
varies inversely with the input level and input varia-
tions of 10 to 15 db cause the output to vary only
4 to 6 db.
a more nearly constant output is desired,
see paragraph 3-14.
3-12. BANDWIDTH CONSIDERATIONS.
3-13. The graphs in figures 3-2A and B show typical
gain vs frequency and saturation power output curves
for the 492A and the 494A amplifiers using two differ-
ent conditions of helix voltage.
One set of curves
shows the gain and saturation power output when the
helix voltage is optimized at each frequency. The other
set of curves shows the gain and saturation power out-
put when the HELIX control is set to the broadband