Model PB-5 | Why is the flat top pulse not symmetrical for minimum settings (rise time 50 ns  / fall time 500 ns)?

Symmetry would require a fast discharge of capacitors used to produce exponential tails. The critical issue in the PB-5 is to preserve exponential decay with a clean return to the baseline. The PB-5 meets the same fall time specification as the PB-4 with a cleaner signal as it decays to the baseline - allowing twice the rep rate (500 kHz) as the PB-4.