>
>
>
>What is the expected hour lifetime of a typical thoriated tungsten
>filament tube like the 3-500Z? Assuming no operator
>abuse/mistakes/parasitics, etc,
Very roughly 20k hours if one occasionally oils the fan motor and
operates the filament no higher than is needed to develop maximum PEP.
For operation at excess filament volts, emissive life decreases according
to (e2/e1)^23.4. [sic]
> is there a mechanism for the tube going flat (showing reduced output) simply
>from old age/use?
not if the tube has good seals.
>If there is an operational old age for cathode
>emission, does the clock tick if the tube is lit, but no current flows?
>
It matters not a jot if cathode current flows. Cathode temperature
controls evaporation of tungsten dicarbide -- the major emitter of
electrons from the cathode. Normal thoriated tungsten cathode temp. is
around 1830 deg. Kelvin. Since the electric mains potential typically
rises about 5% during standby or receive, incorporating a reed relay to
short an appropriate resistance in the primary of the filament
transformer causes the filament potential to decrease roughly 5% during
receive. When the R is right, no change in filament voltage will be
detected between Rx and Tx. This improvement more than Triples useful
emissive life of the cathode. The Ugly 8169 amplifier in the photos on
my Web site incorporates this circuit. .
>
cheers, Pat
- Rich..., 805.386.3734, www.vcnet.com/measures.
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