Figure 2 shows the relationship between output power,oscillation frequency and reflector voltage.The values are typical for a 2K25 klystron.It can be seen that oscillation will occur at many different reflector voltages(corresponding to different transit times).The klystron is said to oscillate in different"modes". The frequency is primarily determined by the dimensions of the resonator cavity. So by variation of the volume of the resonator mechanical tuning of the klystron is possible.