Since all tube diodes follow a 3/2 power curve, any rectifier tube can be modeled by simply changing one term in the .model description.

Run the test circuit to see the model that I fitted to the data from the following link.

http://www.wooaudio.com/docs/tube_data/5AR4.pdf

All that was necessary was to play with the value pointed to by the text in the test circuit until I got 750 mA at 33 V.

After thinking about it, it is easy to calculate the value.

N = I / ( V ^ 1.5 )

For I = .75 and V = 33
N = 3.9563e-3

Dan Hopper K9WEK