04 Tube Operation.pdf - Kambing UI

for a tetrode or pentode is obtained through a dropping resistor
and grid leak bias is used (see Figure 39A). In this system a high
transconductance, low-µ triode is connected between screen and
cathode, and the controlling bias for the small triode is taken from
Figure 39A. Screen voltage control circuit for exciter keying or
protection against loss of excitation when supplying
screen from high voltage source.
the tetrode or pentode bias developed in the grid leak resistor.
When normal excitation is present on the r-f amplifier and grid
bias is developed, the triode control tube is cut off and the screen
voltage circuit operates normally. If excitation voltage is removed
from the tetrode or pentode power amplifier, the bias voltage
developed in the resistor drops to zero and the control triode
becomes conducting. The current drawn by the triode control tube
will increase the voltage drop across the series screen resistor and
lower the screen voltage to a very low value.
There is still some voltage on the tetrode or pentode screen and a
small static anode current flows. Usually this remaining current is
not enough for the anode dissipation rating to be exceeded. This
value can be reduced further by the use of a second control triode
in parallel with the first (this redundancy is also insurance against
failure of the single triode), by putting a gas regulator tube in series
with the lead to the screen before the screen r-f by-pass capacitor,
or by introducing a small amount of fixed bias on the power amplifier
between the grid resistor and the power tube grid.
Figure 39B shows a method of keying a tetrode or pentode r-f
power amplifier where the low voltage power supply for the screen
of the power tube and for the anode and screen of the driver stage
is keyed directly. This permits keying in a relatively low voltage, low
current circuit. The key click filter capacitor, resistor, and choke are
64