The 813 filament is constructed of thoriated tungsten. One of the
characteristics of this type filament is "instant" on.

See http://www.w8ji.com/vacuum_tubes_and...e_failures.htm and look
under thoriated filament.

You do want the filament to heat up and that probably means a second or two.
I remember Bill Orr wrote an article about failure of the 3-500 ( which is
considered instant on also). Some individual had a number of tubes fail and
kept sending them back under warranty. Finally Eimac called him and learned
that the chap was turning the tube filaments off between transmissions and
turning them on immediately before each transmission.

So, do not use either tube for break in cw by keying the filament.

73, Colin K7FM