Putting a series resistor would degrade the power supply regulation
performance. This may or may not be important, depending on application.

Yes, but it will degrade it to the same level which using the original
rectifier tube would.
--scott

A vacuum rectifier shows a non linear behaviour, whereby the voltage drop vs. current tends to saturate (until one exceeds the rectifier rating). So, using a resistor to simulate the vacuum rectifier voltage drop, would cause a worse regulation around the rectifier operating point.

But even disregarding the above consideration, putting a resistor instead of a zener diode would mean losing a chance to improve things.

73

Tony I0JX