A coupled cavity circuit for a microwave electron tube comprises at least two resonant cavities adjacent to each other. An electron beam tunnel passes through the coupled cavity circuit to allow a beam of electrons to pass through and interact with the electromagnetic energy in the cavities. An iris connecting the adjacent cavities allows electromagnetic energy to flow from one cavity to the next. The iris is generally symmetrical about a perpendicular axis of the electron beam tunnel with the iris having flared ends and a central portion connecting the flared ends. The iris shape causes the iris mode passband to be lower in frequency than the cavity mode passband while still providing broadband frequency response.