340 Chapter 11
anode current resistance,Ra , which is measured by the current fl ow for a given applied
voltage— as shown in the equation
RVIaaadd/.Because the anode is bombarded by electrons accelerated toward it by the applied anode
voltage, when they collide with the anode their kinetic energy is converted into heat,
which raises the anode temperature. This heat evolution is normally unimportant, except
in the case of power rectifi ers or power output valves, when care should be taken to
ensure that the makers ’ current and voltage ratings are not exceeded. In particular, there is
an inherent problem that if the anode becomes too hot, any gases that have been trapped
in pores within its structure will be released, and this will impair the vacuum within the
valve, which can lead to other problems.
11.1.3 The Control Grid
If the cathode is surrounded by a wire grid or mesh—in practice, this will usually take the
form of a spiral coil, spot welded between two stiff supporting wires, of the form shown
in Figure 11.2 —the current fl ow from the cathode to the anode can be controlled by the
voltage applied to the grid, such that if the grid is made positive, more negatively charged
electrons will be attracted away from the cathode and encouraged to continue on their
way to the anode. However, if the grid is made negative, it will repel the electrons emitted
by the cathode and reduce the current fl ow to the anode.
Support rods Support rods
Figure 11.2 : Control grid construction.