Two reflex klystrons attached by waveguide to microwave cavities, from an ad by Varian, Inc. in an electronics magazine. At top is the Varian VA-1281 cavity with the VA-201 klystron, at bottom is the VA-1282 cavity and X-26 klystron. The reflex klystron is an obsolete vacuum tube used around World War 2 to produce microwaves. The microwave cavities, consisting of closed cylindrical metal containers, serve as tuned circuits, determining the resonant frequency.of the oscillators. Cavity resonators are used instead of tuned circuits at microwave frequencies because the size of discrete inductors and capacitors required would be very small. They can have much higher Q_factor than LC circuits, approaching the frequency stability of quartz crystals.