04 Tube Operation.pdf - Kambing UI

Another method of assuring that the screen grid is at rf ground is
to use a quarter- wavelength coaxial stub, shorted at one end and
resonant at the operating frequency, connected from the screen
to ground. A blocking capacitor is required to prevent shorting
the screen dc supply voltage to ground. This is generally used
in an amplifier operating at a single fixed frequency at VHF. The
length of the stub becomes physically too long at lower operating
frequencies.
3.8 GROUNDED-GRID CIRCUITS
Zero-bias triodes employed in rf power amplifiers for general
purpose rf amplification, linear and even class-C FM service are
typically used only in the grounded-grid configuration. The control
grid is operated at rf ground, similar to the screen grid in a tetrode
or pentode; therefore, good isolation is obtained between the
input and output. The control grid may be bypassed directly at
the socket or grounded directly to the chassis. The dc-grounded
approach is favored, because no bypass capacitor is required and
stability is optimum. Figures 34 and 35 illustrate the two circuit
configurations.
Above 30 MHz the example in fig. 35 should be avoided because
of less than ideal rf grounding associated with this configuration. A
case can also be made wherein fig. 35 has less inherent reliability.
In the event of an internal or external arc from anode to grid, the grid
bypass capacitor and metering circuit could be destroyed, unless
the available energy is limited to less than a few joules. Protection
diodes connected across the meters are highly recommended in
both circuits.
Figure 34. A typical circuit using “zero-bias” triodes showing
metering circuits and method of grounding the grid
to r-f. The grid current is measured in the return lead
from ground to filament.
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