> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
> At what emission level does an indirectly heated cathode become an
> impractical emitter?
It's mostly a function of surface area with indirectly heated tubes,
so they can have very high peak emissions. There are no hard and fast
rules for indirectly heated tubes.
They are permanently damaged (they lose emission) by either excessive
high voltage, or excess peak emission demands. Running low filament
voltage, operating the tube without enough warm up time, or drawing
too much current will ruin the emission.
> Or in other words, why thoriated tungsten for tubes
> like a 3-500Z?
Thoriated tungsten tubes saturate at about 125 mA or so per watt of
filament power. No matter what you do with other voltages, you can't
get more than that out of the cathode.
> So why thoriated? Is it
> much cheaper or what?
It is cheaper AND the tube is more forgiving. The grids are not gold
plated, so that source of trouble is gone. You can often heat the
grids until they glow without grid damage. The tube is not sensitive
to warm up time, you can apply full power and drive even before the
filament is hot without damage.
Not only are warranty expenses reduced, the production costs are
less.
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|