High-power gridded tubes—1972

The technology described in this paper is intended to update the reader about the last decade of research and development in the field of high-power gridded tubes. Lower power level tubes are also discussed to serve as a foundation for the high-power tubes because they are generally used for drive stages. Special emphasis is placed on the development of a new generation of high-power gain-high frequency linear tubes for the broadcast industry. The new generation of linear tetrodes described operates at CW power levels up to 25 kW and meet two-tone single-sideband limits of better than -40 dB for third-order distortion products and better than -45 dB for fifth-order distortion products. These tetrodes perform in the 225-400-MHz military band and because of the good linearity are used in the new generation VHF television transmitters being introduced in the USA. Methods of designing circuits are discussed. Recently developed tetrodes operating at power levels up to 500 kW at power gains of 25-27 dB at 1 MHz are also described. Likewise, circuit approaches are detailed. The magnetically beamed triode tubes introduced in 1964 for modulator and low-frequency applications are described. These tubes provided a threefold breakthrough in high power, high power gain, and good tube efficiency. This approach reduced the conventional triode grid current by a factor of 10-15. Introduction to distributed amplifiers (DA) and the development of a 75-kW tube operating in a 1.6-MW distributed amplifier at 8-40 MHz is described. Included is the scaling of a 500-W distributed amplifier tube to a 75-kW tube. Development of gridded tubes for multi-megawatt phased array applications is included. An amplifier chain having a gain of 40 dB would cost approximately 4750 dollars as detailed in a 1967 study. A mean time to failure (MTTF) of 40 000 h is demonstrated. Gridded tubes and circuits operating at peak power levels up to 5 MW at frequencies up to 450 MHz are described. The major advance in this area has been the application and refinement of circuits used with high-power gridded tubes. The life of these tubes in high-power RF service continues to improve. Typical life is in the order of 10 000 to 40 000 h. Projections for the life of gridded tubes using various cathodes is made. In a well-designed stable system, the gridded tube life (and possibly other tubes) is approximately one half the theoretical life of the cathode. In the case of thoriated tungsten, the theoretical life approaches 80 000 h and likewise the MTTF approaches 40 000 h. The development of a 1.25-MW Coaxitron is discussed. The Coaxitron features the integration of the broad-band circuit components along with the gridded optics into one common vacuum envelope. The tube weighs approximately 135 lb, operates in the 400-450- MHz range, has 0.15°/1-percent RF phase sensitivity to plate voltage change, has 1.3°/1-dB RF phase sensitivity to RF drive power change, has a power gain of approximately 17 (without integral drive stage), can stand shock of 15g for an 11-ms pulse and is capable of vibration up to 500 Hz at approximately 2g.