Behold, Prune signalled from keyed 4-1000A filament:

Also, can this tube be used in class A? On Eimac's site, it says it's
designed for class B, but does that mean that it merely won't be as
efficient in class A, or it will blow up, or what?

The 3CX800A7 has been used sucessfully in class-A single-ended audiophile
amps for a while now.

--
Gregg t3h g33k
"Ratings are for transistors....tubes have guidelines"
http://geek.scorpiorising.ca