Development of the Pentode

Author: N.H. Crowhurst

The next circuit improvement comes from the design of the tube itself. We already mentioned the use of a pentode for voltage amplification. Tube development, however, was originally carried out to improve power output. The effect of plate voltage swing in reducing plate current swing in a triode tube, cuts down its power-handling capacity by an even greater amount than it does its voltage-handling capacity.

Typical tetrode curves

The first development from the triode was the tetrode, made by inserting only the second (screen) grid. This was found to result in an improvement in power output, although it was limited by secondary emission. When the plate potential is lower than that on the screen grid, electrons bounced off by the impact of arriving electrons at the plate will be attracted to the screen, resulting in plate current curves that give a curious kink and limit the range over which the tube can be successfully used for normal amplifying purposes.

The pentode overcomes this difficulty by using the third (suppressor) grid which prevents any electrons bounced off the plate from traveling back to the screen or second grid under any circumstances. All such electrons are repelled by the suppressor grid and sent back to the plate.