[AMPS] series filaments

[Peter Chadwick]

Tom says
>How do you get the tubes to share equal hum-free emission currents?


That's where the advantage of a separate winding for each tube, centre
tapped comes in, with the centre taps connected together - or even with
their own cathode metering resistor if you want. The downside is an
extra filament choke, the upside is 14 amp wiring instead of 28 amp
wiring.

The modulation of the emission by the filament voltage is obviously much
more of a problem with zero bias (or nearly) grounded grid tubes than it
is with grid driven tetrodes. This leads me to what must be naive
questions:

At what emission level does an indirectly heated cathode become an
impractical emitter? Or in other words, why thoriated tungsten for tubes
like a 3-500Z? is it something to do with the plate voltage? (although I
don't see that - magnetrons, klystrons and TWT's running very high power
and very high voltage use indirectly heated cathodes)  According to the
Radio Engineers Reference book, the amp/sq cm are 0.5 to 2.5 for oxide
coated, and 0.5 to 3.0 for thoriated tungsten. So why thoriated? Is it
much cheaper or what?

73

Peter G3RZP

--
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