RCA took out an ad in the 1942 Radio Handbook. This, of course, was a
wartime publication.

The title to the article is "How to make transmitting tubes last longer."
They state:

"Long life - not maximum output - is the keynote of transmitting tube
operation today in many services where, because of war restrictions, it may
prove difficult to replace tubes that wear out. For tube types using pure
tungston, a 5% decrease in filament voltage will dobule tube life... Care
should always be taken in starting up tungsten filaments. Never should the
filament current exceed, even momentarily, a value of more than 150% of
normal.

For types using thoriated-tungsten filaments and oxide coated filaments, the
filament may be operated on the low side - as much as 5% below normal
voltage if the loading is light. The filament voltage should be increased
gradually to maintain output. Toward the end of life, additional service
may be obtained by operating the filament above its rated voltage."

The 813 has a thoriated-tungsten filament.

73, Colin K7FM