Bipotential cathodes for reduction of grid current

The useful operating range of triodes may be extended into the positive-grid region by preventing electrons from reaching the grid. To do so, electron emission must be suppressed from the cathode area under the grid, and the emission that does take place must be guided through the grid openings. Both of these requirements are met by the use of passive-base emitting areas and by clean metal nonemitting areas of high work function. Calculation of electron trajectories in the absence of space charge shows that the two potentials on the cathode surface provide transverse fields which focus the electrons between the grid wires. For best focusing, there is an optimum height of the nonemitting stripe relative to the emitting surface. Measurements on large scale models of planar triodes show that these results can be achieved with the use of appropriate materials. Striking reductions in electron current to the positive grid are obtained with titanium or platinum nonemitting stripes. Pinhole camera studies of the current distribution and transverse velocities at the anode plane reveal the nature of the focusing action and conditions in the grid plane. Additional benefits of this construction are reduction of transverse velocity spread and more uniform electron transit times. Bipotential cathodes have been incorporated in practical size metal-ceramic triodes with the expected improvements in performance.