On 2-Aug-2005, Don Bowey wrote:

I suspect it would. In the early days of radio, filament voltage
rheostats
were sometimes used to control volume. Also as tubes aged the voltage was
cranked up to keep it usable.

Forgive me if this comment is out of place, but I think I see a trap here.
It is very easy to use the terms "heater" and "filament" interchangeably.
They are two entirely different animals and should be treated accordingly.
The comments regarding reduced life at reduced voltage would probably apply
more to indirectly heated cathodes (heaters) rather than directly heated
cathodes (filaments).

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

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.