(4) Therefore, instead of the spark gap, modern radar sets use the
HYDROGEN THYRATRON as an electronic switch because it is small, light, and
highly efficient. The THYRATRON can be triggered accurately by low-voltage
pulses on the grid, and it can operate over a wide range of plate voltages.
The HYDROGEN THYRATRON.
a. Figure 57 shows the internal structure of a type 5C22 HYDROGEN
THYRATRON. First, look at the anode lead. Notice that it is enclosed in a
glass sleeve to prevent interaction between the anode-lead wire and the
outside of the grid structure.
Note, too, that the grid structure
completely surrounds the anode.
This keeps the perpendicular distance
between grid and anode constant.
Figure 57. Construction of 5C22 thyratron.
b. HYDROGEN THYRATRONS differ from the more common thyratron types you
studied in that the HYDROGEN THYRATRON has a positive control-grid
characteristic. This means the tube won't ionize with zero bias. Instead,
it must be triggered by a positive voltage high enough to draw grid current.
To have a positive control-grid characteristic, the cathode is completely
shielded from the anode (Figure 57). Besides the cathode-shield structure,
there are two baffles just below the perforated grid section located between
the cathode and anode. These baffles prevent the anode field from extending
into the cathode region and affecting the electrons emitted from the
The THYRATRON as a switch.