Re: Filament Issue

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

 >
 > Of course light bulbs aren't used for their emissive properties.
True, but the point is that running a pure tungsten filament at lower than
normal temperatures doesn't "break it"... the lightbulb (and the tube
filament) work just fine when brought back to their normal temp.

 >
 >  > I suppose one could have some circuit where the low emission, and
 >  > hence the low current through the tube, would result in some sort of
 >  > rise in the plate voltage (shunt regulator, where the tube is normally
 >  > almost all the way on?) that might cause a flashover, but it would be
 >  > pretty contrived...
 >
 > Normally? Class C oscillators with a tank in the anode circuit is a
 > classic case where plate voltages rise well above the supply as are
 > PP amplifiers.

I was trying to contrive a scenario where low emission would cause a
problem.... Tube biased almost all the way on all the time, high source
impedance, high voltages, tube running at ragged edge of plate voltage
limit.  Tube cuts off, voltage goes way up, over the limit and flashes over.

As you say, a Class C amp or oscillator with a high Q tank, where the tube
is cutoff most of the time, the plate voltage would be higher than supply,
but then, you'd expect that and design for it.  Low emission wouldn't be any
different than having the tube cutoff.