[ARC5] Filament Question

[Robert Eleazer]

Thanks for the replies.  I found them learned and logical, although not necessarily both simultaneously.

I had assumed that, like light bulbs, the filaments in the RT-70 tubes would tend to fail on start up.  I have only had an incandescent bulb fail other than during start up twice - both times it was a 7.2V Krypton bulb in a lantern I was using in a hot attic. I guess the combination of the heat generated by the bulb itself and the ambient temperature was too much; I fixed the problem by using an 8.4V Krypton bulb.

But Ken says that the instant on aspect is a design feature and presumably believes that keeping the filaments hot (or as hot as they get) will cause an early decrease in emissivity - and that will be the failure mechanism rather than a thermally overstressed filament.  That's the kind of opinion I needed.

And as I understand it, the RT-70 and RT-68 were operated mainly in tanks (M47 Patton in the 50's according to a book I have on the Patton tank series).  In that installation they used power supplies powered off the tank's main power, so keeping the tubes hot would not have been difficult.  I don't know what tubes the RT-68 and R-110 used, but I could look it up if I needed to.

I suppose that I could add a bypass resistor and keep the filaments a little warm, but that would still appear to be counterproductive given that a turn-on failure mode seems unlikely.

Thanks!

Wayne