2-9
Q1.
What is the major difference in grid construction between power pentodes and conventional
pentodes?
Q2.
Beam-forming tubes and power tubes are similar except that power pentodes lack what element?
Q3.
What effect does the shielding of the screen grid by the control grid have on plate current in
beam-forming tetrodes?
Q4.
What effect does a large negative input signal applied to a variable-mu tube have on
a. conduction through the control grid, and
b. gain of the tube?
Q5.
Identify the type of electron tube(s) that would be most suitable for the following applications.
a. Power amplifier
b. Voltage amplifier with small signal inputs
c. Low distortion amplifiers for use with large signal inputs
SPECIAL UHF TUBES
In the earlier discussion of conventional-electron tubes, you learned some of the limitations of tubes.
One of these limitations was that the conventional tube was not able to operate (amplify) at extremely
high frequencies such as those used in radar equipment. Even at frequencies lower than those used in
radar equipment, problems occur. For example, at ultrahigh frequencies (300 MHz to 3000 MHz), transit
time effects make the operation of a conventional-electron tube impossible. For this reason, the special
ultrahigh frequency tubes were developed to operate within this frequency range.
Before we discuss the way in which special uhf tubes counter the effects of transit time, you should
understand the manner in which transit time affects conventional tubes.
LIMITATION OF TRANSIT TIME
We will explain the limitation of transit time by using figure 2-9. In view A, the positive-going
alternation of a uhf ac signal is applied to the grid of a conventional-triode tube. The first positive-going
alternation reduces the negative bias on the grid, and electrons start to move toward the grid. Since the
input is an ultrahigh frequency signal, the majority of the electrons cannot pass the grid before the input
signal progresses to the negative alternation. The electrons that have not yet passed the grid are either
stopped or repelled back toward the cathode. This is shown in view B. Before these electrons can move
very far, the second positive alteration reaches the grid, and causes even more electrons to move from the
cathode (view C). At the same time, the electrons that were repelled from the grid toward the cathode by
the first negative alternation feel the effect of the positive-going grid. These electrons reverse direction
and again move toward the grid. Because these electrons had to first reverse direction, they are now
moving slower than the electrons that are attracted from the cathode by the second positive alteration. The
result is that the electrons from the cathode catch up to the slower moving electrons and the two groups
combine (view C). This action is called BUNCHING.
