Triode Connection of Pentodes

Author: N.H. Crowhurst

Connecting a pentode to work as a triode

Any pentode-type tube can be made to work as a triode-type tube. This is done by connecting the second grid directly to the plate, so that both swing together at the same voltage. Because the screen grid, in combination with the control grid, is principally responsible for controlling the plate current, the presence of the suppressor grid between the screen grid and plate does not materially alter the tube's performance from that of a triode.

Using the best possible load resistance with triode-connected tubes, the voltage fluctuation between B+ and plate reaches little more than half the B+ supply voltage. Changing the method of connection to pentode alters the curve so that the zero grid-voltage curve is pushed out into a "knee." This extends, very considerably, both the voltage and current fluctuation available in the plate circuit. This, in turn, triples or quadruples the power that any pair of tubes will give.