Grid Biasing Methods

Author: N.H. Crowhurst

We have seen how to achieve the correct supply voltage for the plate circuits, but to work in an amplifier correctly, each tube must also have the correct grid bias voltage and, if it is a pentode, the correct screen voltage. There are two ways of providing the grid bias voltage: one method uses a a separate supply of fixed voltage. This can be the same as any of the plate supply arrangements, except that the polarity is reversed and very little current is needed.

Negative grid bias supply

However, the most commonly used method works by making the cathode positive rather than the grid negative. As far as the tube is concerned, the effect is the same. A small resistor, from 100 to 5000 ohms, according to plate current and the bias voltage required, is connected between cathode and ground. This makes the cathode positive with respect to ground by an amount that varies with the plate current. Because one end of the grid resistor is connected to ground, the d-c or bias voltage on the grid is that of ground. For this reason, making the cathode positive with respect to ground will be the same as making the grid negative from cathode.

Use of a cathode bias resistor