General Theory and Application of Dynamic Coupling in Power Tube Design

This paper presents a simplified method of driving a power tube without the need of coupling devices and grid-biasing means. The power section is one whose useful plate-current versus grid-voltage characteristic is realized only with positive values of grid voltage. Its low input impedance is in series with the cathode-ground circuit of the driver tube. This impedance, also, automatically provides a negative bias for the grid of the driver, thereby eliminating external biasing. Since the electronic coupling of the two tubes varies with signal excursions, this method of amplification is termed "dynamic coupling." Practical considerations show immediately that the driver must operate into an impedance which is considerably lower than its own plate impedance. It is shown that the distortion which is produced when working with such ratios is minimized partly by making the driver circuit degenerative in order to nullify the varying effect of the driver's mu. This is treated first with a pure resistive load. When the grid impedance is the load, a further reduction in distortion is shown, for then the ratio of plate-to-load impedance remains more nearly constant throughout a signal excursion. The remaining part of the paper deals with a commercial application of these principles. Certain design considerations for a tube embodying both driver and power sections are discussed. A current surge phenomenon caused by secondary emission is discussed and a practical means for its elimination is given. The delayed point at which the driver's grid begins drawing current is shown to be particularly advantageous.