Re: Regulation for Filament Voltage on 833A Tubes

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

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

At 11:22 AM 5/21/2003 -0600, you wrote:
>Original poster: "Mccauley, Daniel H by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
>
>I finally got my 833A VTTC up and running and I noticed that the actual
>filament voltage coming from my
>10V, 10A filament transformers is about 11.6 VAC.
>
>The nominal spec for the 833A tube is 10V +/- 0.5V  (either AC or DC).
>
>Common sense always says you should abide by the datasheet and that
>exceeding this limit would either be destructive to the tube, or reduce
>optimal performance.  However, I have very limited experience with vacuum
>tubes so I don't know how critical
>this actually is.

Criticality: It depends... The actual critical thing is the temperature of 
the cathode. One can measure the filament temperature by knowing I and V, 
since the resistance is highly dependent on temperature. Since the anode 
radiates some power back to the cathode, the anode dissipation affects the 
cathode temperature, which in turn affects how much power you need from the 
filament.  There's also the tricky thing of "back heating" and the cooling 
effect of the electrons leaving the cathode (i.e. all other things being 
equal, when cutoff, the cathode gets hotter, because no energy is being 
lost by electrons being pulled away).

This tube has a thoriated tungsten filament, which are more picky about 
temperature
Too hot definitely shortens the life
Too cold also causes problems.

Eimac "Care and Feeding.." (p26) says that thoriated filaments should run 
at 1900K, normally corresponding to 5-10 mA of CW anode current per watt of 
filament power.

You should look at the care and feeding book, specifically page 159 
(section 6.11) which discusses how to set the filament current by looking 
at the performance.
http://www.cpii-dot-com/eimac/PDF/C&F6Web.pdf#page=1


Svetlana data sheet http://www.tubebuilder-dot-com/images/tubedata/833a.pdf
(be aware that Svetlana is actually a consortium of 3 manufacturing 
companies, although the data sheet says that the 833As are made in Ukraine)


>What are your thoughts?  Okay as is, or do I need to control this precisely
>via variac???
>
>Thanks
>
>The Captain
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