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

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."

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)?

All vacuum rectifiers show a similar behaviour, corresponding to the so-called 2/3 law. The I-V curve is typically S-Shaped. At low currents, voltage drop grows with current. For higher currents, voltage drop does not grow too much with current.. If current exceeds tube ratings, voltage drop again strongly grows with current.

I could not find the curve for a mercury rectifier. An old RCA tube handbook reports:

- for model 83 (mercury rectifier): voltage drop: 15V
- for model 5U4 (vacuum rectifier): voltage drop: 44V at 225mA, 50V at 275mA, 54V at 300mA

73

Tony, I0JX