A klystron amplifier in which the drift tube is in the form of a casing and a central member disposed coaxially about the same centerline to form a coaxial space between the central member and the casing which runs parallel to the centerline, and through which the klystron's electron beam flows. The casing and central member are maintained at ground potential, which maintains the space charge of the electron beam at a low level, permitting the klystron to operate more efficiently and output higher power.