Abstract: A resonant-cavity type electronic tube having mechanically-operable tuning vanes associated with its resonant-cavity structure and disposed within an evacuable envelope. A support member is provided within the envelope for adjusting the tuning vanes relative to the resonant-cavity structure, so as to tune the frequency of resonance of the structure step-wise in accordance with a predetermined program. The tuning is carried out with a step motor, having a stator and armature with the envelope having a portion extending between the stator and armature for enclosing the armature within the evacuable space containing the tuning vanes and the resonant cavity structure. The armature is coupled to the tuning vanes for altering the position thereof relative to the resonant-cavity structure in fixed relation to the displacement of the armature relative to the stator. Electrical signals are coupled to the stator for establishing a programmable series of displacements of the armature relative to the stator.