>Tom says
>>How do you get the tubes to share equal hum-free emission currents?
>
- At 50 or 60 Hz, the emissivity of a 3-500Z filament is virtually
constant. . . . What could this possibly have to do with Mr. Rauch's
claim of inaccurate measurement of inrush-current in the series-connected
TL-922 filaments?
>
>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.
- However, unless the two tubes had different bias v. requirements,
there would seemingly be no advantage.
>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.
It is my opinion that:
A. Due to the mass thereof, emission-modulation is not likely to be a
problem even if a thoriated-tungsten filament were powered up by 40Hz
current.
B. Grid-driven tetrodes are affected no differently than are
cathode-driven triodes.
>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?
- There are a number of indirectly-heated cathode tubes that. are rated
for operation at 5000 volts.
>(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?
It is cheaper to build. Why use an uncheap tube that is rated for
"Amplifier and Oscillator Service" at up to 500 MHz if you are only
building an HF amplifier?
RE: ruggedness: - Because the thoriated tungsten cathode's emissive
layer is atomically-bonded, it is more resistant to being damaged by
electrons bouncing off the anode than is the case with an oxide-cathode
(whose emissive layer is mechanically-bonded). One can use thoriated
tungsten cathodes with anode supplies up to about 22kV. (4CX35,000C &
up). Other advantages of thoriated tungsten is that it can not be
permnently damaged by low filament voltage, and, when that rare one
suddenly appears out of nowhere, flick the switch and in one second 'we
have liftoff'. .
Rich---
R. L. Measures, 805-386-3734, AG6K
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