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