  This paper reviews the main types of radio-frequency power amplifiers which
are, or may be, used for high-power hadron accelerators. It covers tetrodes,
inductive output tubes, klystrons and magnetrons with power outputs greater
than 10 kW continuous wave or 100 kW pulsed at frequencies from 50 MHz to 30
GHz. Factors affecting the satisfactory operation of amplifiers include
cooling, matching and protection circuits are discussed. The paper concludes
with a summary of the state of the art for the different technologies.
