Whether operating with reduced filament voltage is detrimental seems to
depend on whether the tube is an oxide-cathode type.

Pure tungsten or thoriated tungsten filaments do not suffer damage from
filament under-voltage, while oxide cathode types do.

Thanks to W8JI for the info:

http://www.w8ji.com/vacuum_tubes_and...e_failures.htm
Vacuum power tubes, using old valves, and vacuum tube failures


Chuck