An electron beam tube apparatus comprises a plurality of electron beam tubes having a common output cavity. Power is coupled to the common cavity from the resonant cavities of the beam tubes, and is then fed to an output line. This arrangement permits the outputs of two or more beam tubes to be combined in a compact arrangement with little rf power loss. Previously, the signals from the output lines of respective beam tubes were combined. The output line may be transmission line or waveguide. The means for coupling signals between the cavities may comprise loops or irises, both of which may be selectively adjustable.