Spice simulation for hard-tube modulators design

In general, high voltage (HV) power modulators use vacuum tube (tetrode) operating as an on-off switch to discharge a capacitor bank through the load. To ease pulser construction a special circuit layout with tube cathode near ground is used, whose switching control is made by a solid-state switch in series with the cathode electrode. This avoids isolation problems between tetrode deck and grounded panel if conventional configuration with HV floating potentials on cathode is used. However, in this arrangement the circuit design becomes very critical as tube performance depends strongly on grids potentials. Then SPICE circuit simulators with vacuum-tube models are useful tools to assess the modulator design. In particular, we tested the Circuit-Maker simulator with the tetrode model 6L6GC adapted to the electrical characteristics of the switch tube (Amperex-4PR1000A) used in the design.