Dennis
To answer your questions, no.
The filament in a tube heats up because of the current passing through the
resistance of the filament wire. The resistance may not be perfectly uniform
throughout the length of the filament, causing some locations on the wire to be
reach higher temperatures than others (called "hot spots").
Pure tungsten is not commonly used because of it's low emission efficiency
(meaning you have to get it VERY HOT to effectively emit). However, it is used
in some very high power tubes because of its ability to operate under adverse
conditions, such as found in very high voltage tubes. Thoriated Tungsten is
more commonly used in large tubes and oxide-coated emitters (usually indirectly
heated) are used in small receiving tubes. Tungstem emitters need to operate
at about 600 degrees C hotter than Thoriated Tungstem to achieve the same
emission efficiency!
If your tube has an indirectly heated cathode like the 8877, then there is no
reason for RF to be on the cathode. Common practice is to add a .01 uF
capacitor across the two two heater terminals so the only voltage across the
heater
is the heater voltage (either low frequency AC or DC).
In the case of a heater-cathode (like a 3-500), it is again common practice
to shunt the high frequency voltage across both terminals of hte
heater-cathode. Again, the only voltage across the heater is the applied
heater voltage (no
RF).
For these reasons, I would expect that RF emissions from the cathode would
not lead to hot spotting, unless there was some inconsistency in the thoriated
tungsten itself. So, hot spotting probably means you have a non-uniform
filament, not that you have a circuit problem.
I have not heard of DC causing a problem in hot spots as a result of "grain
growth" and even if the grain grew, I am not sure why it would not grow
uniformly. I have never heard of this.
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