A system is described whereby in a superheterodyne spectrometer a pulsed klystron, used to saturate electron spin resonance lines in a saturation-recovery method of observing relaxation times, is maintained at the same frequency as the monitor klystron, which in turn is locked to the cavity resonant frequency. The a.f.c. system makes use of the variation of frequency of the pulses appearing in the i.f. stages to control the height of the pulse applied to the reflector of the pulsed klystron.