[Milsurplus] LV Tube operation.... was half-voltage dynos

[J Forster]

  Hi,

  There are some low power receiver applications that run the filaments
or
  heaters "starved" -half voltage or less. They also run at very low
plate
  voltages - maybe 8 volts or so. The writers have not said anything
about
  tube life except that the tubes loafing along and should last
"forever".
  I am going to try it eventually.

  There are different materials used for filaments and indirectly heated

  cathodes. Some of those are very sensitive to reduced voltages and
  especially when used at the full rated plate power. Some transmitting
  tubes come to mind. I have some old surplus radios and I don't think
the
  tubes would be particularly sensitive to reduced heater/filament
  voltages. Obviously they need enough heat to produce some emission. I
  have limited personal experience with this and I won't be able to try
it
  in a timely manner for this thread. Apparently other people have
  successfully done it beyond the extent proposed here.

The problem is related to cathode poisoning. There are apparently
several different things going on  cathodes, including diffusion of
Thorium from deep within the cathode to the surface, where the action
is. Such processes are temperature dependant.

Also, the heat loss from the cathode by radiation is a 4th power of the
cathode temperature, so the temperature is NOT in direct proportion to
the filament voltage (they will be hotter than 1/2 temperature at 1/2
voltage).

I think all bets are off when you run the tubes outside their design
envelopes.

FWIW,

-John