A Power Amplifier for Ultra-High Frequencies

A consideration of the special problems encountered at ultra-high frequencies has led to the design of a push-pull power pentode, useful as an amplifier, frequency multiplier, and modulator at frequencies of 300 megacycles per second and below. Unusual construction features include the mounting of two pentodes in the same envelope with interconnected screen and suppressor grids, complete shielding between the input and output circuits with no common leads, and provision for cooling all grids while maintaining extremely small interelectrode spacings. The electrical characteristics depart from the conventional mainly in the low value of lead inductances and the high value of the grid input resistance at ultra-high frequencies. The second part of the paper describes a single stage amplifier unit built for testing the tube at frequencies between eighty and 300 megacycles, and the associated apparatus for measuring input impedance, gain, and harmonic distortion. The results given indicate that by using this new tube it is possible to construct stable amplifiers at ultra-high frequencies up to 300 megacycles, having gains of twelve to twenty-five decibels per stage and delivering several watts of useful power. Stability and distortion compare favorably with those obtained from conventional tubes at much lower frequencies.