2-7
Figure 2-7.—Cutoff in a conventional tube.
In view B, a bias voltage of -6 volts is applied to the grid. As you can see, some of the electrostatic
lines of force are attracted to the negatively charged grid wires while the rest pass through the grid
spacings. Because there are fewer lines of force reaching the cathode, there are fewer paths for electrons
to use to reach the plate. As a result, conduction through the tube is decreased.
In view C, the negative potential of the grid has been raised to -20 volts, which drives the tube into
cutoff. All of the electrostatic lines of force terminate at the negatively charged grid, instead of continuing
on to the cathode. The electrons emitted by the cathode will not feel the electrostatic attractive force of
the positively charged plate. Under these conditions, current cannot flow through the tube.
Now look at figure 2-8. Here you see a diagram of a variable-mu, or remote-cutoff, tube. The only
difference between the remote-cutoff tube depicted and the sharp-cutoff tube is in the grid wire spacing.
In the conventional sharp-cutoff tube, the grid wires are evenly spaced, while in the remote-cutoff tube
the grid wires in the middle of the grid are placed relatively far apart. This is shown in view A.
