New high efficiency 500 kW tetrodes for short wave broadcast

Two 500-kW tetrodes, developed for shortwave broadcast service using computer-aided design (CAD) techniques are presented. The two tubes are similar, one having pyrolytic graphite grids and the other Y-3 grids. The tubes are interchangeable and have similar operating parameters. Cooperation between transmitter designers and tube designers facilitated the fulfilling of requirements established by the Voice of America for it´s new generation of efficient shortwave international broadcast transmitters. Tubes are now operating in the model 420B transmitter at frequencies between 3.9 and 26.1 MHz at the Voice of America site in Greenville, North Carolina. The tubes operate interchangeably as the power amplifier tube, or the pulse duration modulator tube, and they have multiphase water-cooled anodes rated at 400 kW, low internal inductance and capacitance, mesh-thoriated tungsten cathodes, and two choices of grids, either platinum-zirconium-coated molybdenum wire or laser-machined pyrolytic graphite