Nonlinear memory effects in a klystron and their impact on digital communications are investigated using a time-domain physics-based model. The simulation results are compared to an idealized block model based on the frequency response and amplitude/phase drive curves typically used in system design with vacuum electronic amplifiers. Significant departures in transient behavior are noted in the physics-based model in comparison to the block model when the klystron is at or near saturation, provided that the signal bandwidth is simultaneously large ( ges ~65% of the klystron output cavity bandwidth). Such nonlinear memory effects exist for pure amplitude, pure phase, and mixed transients of both the step and ramp variety. The effects of these nonlinear phenomena on 16-state phase-shift keying (16-PSK) digital communications waveforms (with preequalized symbols) are examined using symbol constellation diagrams and symbol error rate (SER) plots. Operation at saturation with signal bandwidths of 32%, 65%, and 93% of the klystron output cavity bandwidth, for a bit-energy-based signal-to-noise ratio of 18 dB, yields SER values of 2.0 times 10-5, 2.3 times 10-4, and 2.7 times 10-3 , respectively, in comparison to the ideal 16-PSK value of 1.1 times 10-5.