Tubes with pure tungsten filaments don't have to be preheated. They
don't have any coating or other material alloyed with it to damage.
Tungsten filament tubes can be run in emission-limited mode with no
problem; some are even made to be used this way. Examples are the 1236
and 2AS15. Tungsten filaments are also used in vacuum gauge tubes
because they aren't hurt by being exposed to air (when cold).

Pure tungsten filaments are used in very few tubes because they are
inefficient and have short life because of their high operating
temperature (they glow bright white).

Tubes that we are likely to encounter with pure tungsten filaments are
old ones like the 01. The inefficiency of these filaments can be seen
from the data sheets for these tubes. They take 5V at 1A whereas the
thoriated tungsten version, 01A, is 5V at 0.25A. Oxide coated filaments
are even more efficient; the 01B is 5V at 0.125A.

Heinz Schmitz wrote:
John Byrns wrote:

So does anyone have a comprehensive explanation of this cathode warm up issue,
for both low power and high power tubes?

My present understandig:
When the anode voltage is applied to the cold tube, the cathode during
heating up is operated in the saturation region. i.e. it cannot emit
as many electrons as could be drawn off.
It is said, that this can damage the cathode.

Why else would we find many devices where a relay does just that:
Switch on anode voltage only after the cathode is properly heated?
Just because of nice to have or to avoid start-up hum?

Regards,
H.




--
Jim Mueller

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