Abstract: Apparatus is disclosed including a compact thermionic generator formed with an outer cathode cylinder and an axially concentric inner anode cylinder. The cylinders are sealed and evacuated and provided with external electrical connections leading to the cathode and anode cylinders. A thermoelectric generator formed of a folded length of thermocouples is nested within the anode cylinder with respective ends thermically and electrically coupled to the cathode and anode cylinders. Sufficient heat applied to the outer cylinder causes the thermionic emission from the cathode to anode cylinders to be enhanced by the positive potential applied to the anode cylinder from the internal thermoelectric generator which is also responsive to the applied heat to generate the positive potential relative to the potential of the cathode cylinder. A generator system is also disclosed in which a pair of these generators is interconnected by a solid state switching circuit to an output load via intermediate charging capacitors.