I just made up a spice 3f4 model for the 12K5
space charge tetrode tube. These used to used in
car radios to drive the output transistor, back in the
late 1950's. I used two of these in push pull in
an AA5 radio with all the usual tubes swapped out with
the equivalent space charge versions. Produces about the same audio
volume as that from a transistor portable radio.

Various tube models:
http://home.netcom.com/~wb2jia/tubes/spice.txt
http://home.netcom.com/~wb2jia/tubes/spice2.txt
http://home.netcom.com/~wb2jia/tubes/spice3.txt

The 12K5 is at the bottom of:
http://home.netcom.com/~wb2jia/tubes/spice4.txt