This document describes improvements to reflex klystrons. It discloses a reflex klystron design that avoids the formation of a virtual cathode in the reflector space by ensuring the ratio of the reflector surface area to the cathode surface area is more than twice the ratio of the length of the retarding path in front of the reflector to the length of the acceleration path in front of the cathode. The klystron is suitable for modulation purposes due to its relatively large bandwidth.