This invention provides a klystron amplifier arrangement having means for producing a radial component of magnetic field tending to direct beam electrons out of their path from the electron gun to the collector electrode of the kylstron. By suitably choosing the strength of the radial component of magnetic field generated undesired slow electrons in the beam will tend to be deflected, while the relatively fast electrons will be substantially unaffected.The radial component of magnetic field may be axially asymmetrical or symmetrical and may be generated in a variety of ways, for example, by means of magnetic material introduced into the field region of the normally provided main solenoid or by the use of an auxiliary winding. The magnetic material may form at least part of a cylinder or cylinders surrounding the beam path, for example, if provided as at least part of a drift tube or a water jacket. Any auxiliary solenoid may for example, be wound around a cooling jacket surrounding the beam path.