(R): Repeller (Zurueckwerf) electrode
(H): Cavity
(f): Screws, in order to adjust the size of the cavity (frequency change)

From the Katode withdrawing electrons become of the cavity angozogon, troten however grosson the Geschwindigkoit wogon by the holes of the cavity into the area to the Repeller electrode. Because this is negative, the electrons are again back-thrown. Forms an electron cloud, which swings in the cavity back and forth. The size of the cavity determines thereby the oscillation duration and thus the frequency of the Oscillators.

Another kind klystron has a mechanically adjustable Repeller and can thereby the frequency change.

A third kind cannot be adjusted mechanically, but can be adjusted the tubing-like transverse electromagnetic wave by the klystron mechanically in the length.