Antonio Vernucci wrote:

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.

Ahh! That makes perfect sense! I had not thought about that... I guess
I always just thought about the diode conductance as being a fixed value.

Hmm.... I just got out the old RCA Receiving Tube handbook and the only
rectifier I can find with actual curves is the 5Y3... the curve is pretty
flat, but you're right that it isn't linear. No knee on it, though.

Is this typical? Are the gas-filled types significantly different (other
than the turn-on discontinuity)?

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

True.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."