'Hot' valves?

[G8HQP Dave]

I've just stumbled across a list (in Dutch, which I don't understand) which gives Nato Stock Numbers and mentions some apparently ordinary valves which are radioactive. I knew that some cold cathode voltage regulators include a little radioactivity so that they start reliably in the dark - the military were very keen on this. However, the list mentions 6AU6WC with tritium (H3), 12AT7WC and 5687WB with Re-187 (a beta emitter?).
See http://mpbundels.mindef.nl/overig/Ba...960-00-500.htm

Can anyone say why a radioactive triode or pentode might be useful? Was it perhaps just luminous paint on the outside?

[Oldtimer74]

I doubt that it was luminous paint. The W after the valve number indicates that it was special ruggedised quality, clearly for rough military use. I tritium is mentioned then it could only be in gaseous form, presumably inside the envelope, but what for I have no idea.
As an aside the old barretters were filled with hydrogen to improve the thermal conductivity and enable the heat to get away from the very fine 'lamp type' filament by conduction, rather than relying largely on radiation.
Pat G3IKR

[G8HQP Dave]

Hydrogen is very good at transferring heat. When I joined the power industry many years ago I was surprised to find that hydrogen is used as the coolant in generators. A major issue is designing bearings which keep the hydrogen in place, as it is very good at leaking through the smallest gap. It also loves forming explosive mixtures in air.

I'm still puzzled by the radioactive valves. Is it possible that they became radioactive because of the equipment they were used in? Yet this would not imply a separate NSN.

[jjl]

Dave

Thorium as used in thoriated tungsten filaments in high power valves is mildly radioactive. The list you have linked does indeed refer to the 811A and other types that have thoriated filaments.

The most likely use of Rhenium in an indirectly heated valve like the 5687WB would appear to be to increase ductility of tungsten or molybdenum parts such as the heater, see here

http://en.wikipedia.org/wiki/Rhenium

Perhaps this addition somehow helps to improve the mechanical shock resistance of the valve?
Or perhaps Rhenium is added to the cathode coating in order to improve emission efficiency as with thoriated tungsten

John

[kalee20]

Thorium is the most likely explanation for radioactive valves. It's not used because it's radioactive, it's used because it's a good thermionic emitter at sensible temperatures. The fact that it happens to be radioactive is a nuisance!

This is in sharp contrast to radioactivity deliberately added to gas-filled cold-cathode tubes, as in Dave G8HQP's post above.

[G8HQP Dave]

The most likely use of Rhenium in an indirectly heated valve like the 5687WB would appear to be to increase ductility of tungsten or molybdenum parts such as the heater

Thanks, I think that explains the Rhenium.