Power Drive

Author: N.H. Crowhurst

Driving grid positive increases available power

One more thing can be done to increase the power available from a given pair of tubes. If the control grid is made positive, the plate current will rise still farther, whether a tube is operated as a triode or pentode. In the case of pentode operation, the load value must be changed, otherwise additional grid swing into the positive cannot extend the length of the load line. This would still result in clipping that no amount of grid current could remedy.

Applying positive drive on the grid means that the grid itself will draw current when it becomes positive with respect to the cathode. (Some of the electrons will be attracted to the grid itself, instead of passing through it to the other electrodes.) This means that the grid is no longer just a voltage-operated electrode in the tube. We now have to make a more complicated design of the drive stage, as it is called, to provide the necessary grid current (when it is drawn) accompanying the correct voltage waveform, to give the right output.