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

If you reduce B+ and screen voltages proportional to the fil. voltage
reduction, you negate the problem of depleting the space charge and
stress-cracking the oxide coating (assuming there is a decent amount
of emission left, of course). Yes, output is reduced.
But I think most of us know the difference between
100 watts and 70 watts isn't worth mentioning.

73 Dave S.