3.6M north10.pdf - Dean-O's Toy Box

166 High-Power Microwave-Tube Transmitters
higher the p factor will be, the higher the cathode-current-density change will be
for a given grid-voltage change, and the greater the anode-voltage hold-off is
likely to be. The wire-to-wire spacing of the grid, however, will have to be
smaller, approximately equal to the spacing between grid and cathode. This is
done so the “screening” effect of the grid, or the amount of total grid area that is
physically blocked by the presence of grid wire, will be greater than that of a
lower-p grid spaced farther away from the cathode. The greater the screening
effect, the more likely the grid is to intercept electrons on their way from cathode
to anode when the grid is driven positive with respect to the cathode. This
screening effect is reduced if the diameter of the grid wire can be reduced. But
the diameter must still be large enough to dissipate heat, including the heating
caused by its proximity to the hot cathode and the interception of electrons.
Even more importantly the wire must have cross-sectional area (or AWG gauge)
sufficient to absorb the action content of an arc discharge without fusing. (As
will be discussed later, the transmitter designer is responsible for limiting the arc
action by employing surge-limiting resistance and, when needed, an electronic
crowbar charge diverter.) Even in high-p tubes, the grid screening factor is less
than 259’0, and it typically varies down to 109’o for low-p tubes. Grid current at
anode-saturation voltage is typically 1.5 times the screening factor, so that a 2570-
screened, high-p triode may intercept grid current that is 1.5 times 259’o, or 389’o,
of cathode current, compared with 1.5 times 107o, or 15?40,for a low+. triode.
Grids are almost always wound of tungsten or molybdenum wim that has
been coated with gold or platinum. This is done so they can operate at temperatures
higher than the cathode-even as high as 1400°C (compared to 1000”C for
the cathode)—without becoming primary emitters of electrons themselves. When
they are driven positive with respect to the cathode they will attract and intercept
electrons. The sum total of these electrons constitutes grid current, and the
product of this current and the peak-grid voltage is the peak-grid dissipation.
This current and power must be supplied by the external grid-drive circuitry and
must be dissipated by the grid as heat.
Because of an effect called secondary emission, not all electrons that collide
with the grid are counted as grid current, however. Primary electrons strike the
grid surface and knock off others, which continue onto the anode. So long as the
secondary emission is less than the primary interception, things look good for
both the tube and driver. On the other hand, if more secondary electrons are
emitted than primaries are collected, grid current is negative and the grid can run
away, unless it is externally swamped by shunt resistance. Pulse stretching is the
usual symptom of excessive secondary emission; cathode and plate currents continue
even after the grid drive pulse has stopped. Pulse stretching in some
thoriated-tungsten tubes can be corrected by a process called grid blackening. In
this process, filament voltage is increased to twice its normal value (so filament
power approximately trebles). Some of the filament carburization boils off and is
deposited on the grid, making its surface stickier and less prone to secondary
emission. Secondary emission is less likely from grid wires whose surfaces have
been made intentionally rough, but anode-voltage hold-off is degraded as a consequence.
Recently manufactures have made grids and screens of pyrolytic graphite.
This material does not emit secondary electrons, but tubes using it do not always