Behold, J M Noeding signalled from keyed 4-1000A filament:

They might be useful in the range 20-200kHz with 50 ohm, see the notes
on http://www.noding.com/pictures/la8ak/n12.htm.......

Yay! Another tubehead! :-)

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca