Abstract: A circuit for applying a compensating ripple signal at a low voltage point, such as the collector electrode of a travelling wave tube to compensate for pulse-top ripple extant on a high voltage pulse applied at the travelling wave tube cathode, to provide a substantially constant voltage across the travelling wave tube during pulsing to preserve its radio frequency phase and amplitude characteristics. The portion of the pulse-top including the undesired ripple is compared against the applied compensating collector signal and the result is digitized and stored discretely in a plurality of sample bins over the duration of the modulation pulse. The stored digital values are updated over a plurality of successive modulation pulses until an equalibrium is reached, a random access memory being arranged to provide a readout over one half of each of the sample bins and to receive updating in the remaining halves of the corresponding sample bins.