Abstract: Two embodiments are disclosed of a system for the fast tuning of high power amplifier circuits. In one embodiment a strip line resonating circuit is selectively tuned by the actuation of selected ones of a plurality of PIN diodes which interconnect a relatively high impedance of strip line circuit with a strip line circuit having a lower characteristic impedance. In the second embodiment a coaxial cavity is terminated in an output transmission line. The cavity and the output transmission line are separated by an array of shorting stubs, each of which is selectively energizable by a suitably located PIN diode.