A resonant cavity (10) arranged for an electron beam to pass therethrough, is made of two spaced apart parallel substantially planar members (11,12) movable with respect to each other in a direction perpendicul­ar to their major surfaces, at least one of said members having a delay line structure (13A-13B) on its inner face within the gap between said two members. Means (e.g. 14) are arranged for moving the members relative to each other so as to vary the spacing between said members, whereby the oscillation fre­quency is tuned easily. The resonant cavity can be used to realize wide-band extended interaction oscil­lators and klystron amplifiers.