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This paper describes a theoretical and experimental investigation of a special form of klystron which seems to have many important uses as a frequency-modulated oscillator. Basically, it is a single-cavity, two-gap klystron with a floating drift tube between the two gaps. By applying a separate voltage to the floating drift tube it is possible to produce frequency modulation by varying the transit angle between the gaps in a similar manner to the frequency modulation produced in a reflex klystron. An examination of the theory indeed indicates that the theory of the floating-drifttube klystron (FDTK) and the reflex klystrons are essentially identical. One merely replaces the transit angle wherever it appears in reflex theory by the same number divided by a factor H, where H is the ratio of the effective voltages at the two gaps. However, in spite of this identity of theory, in practice the FDTK should have higher efficiency and power-handling capabilities merely because of its geometry. In addition, the absence of the reflection problem, i. e., of electrons transversing the same gap twice, eliminates all the difficulties connected with multiple transits which produce hysteresis, mode distortion, and the like, which are characteristic of many reflex klystrons. This prognosis fulfilled in the tube which was constructed.