Hi all,
Just got back from holiday, which is why I haven't commented before. One problem
you can get with DC on filaments is metal migration. The filaments used in
battery tubes such as the 1L4, were just about dull red heat; thoriated
filaments are another matter. Metal migration occurs when current densities are
high, as in a thin filament, and increases very rapidly as temperature goes up.
As an example, an aluminium-copper-titanium metal, as used in ICs, is limited to
about 2 to 3 mA per square micron of conductor section. Because AC reverses the
field frequently, you don't see the problems. (It's been a problem with power
IC's in the past)
Heaters are fairly thick, and run at lower temperatures, so you don't see the
problem with them on DC. Whether or not it is a problem for transmitting tubes
is in reality a moot point. You have to to exceed the limits by quite a lot to
pull the life down to the 1000 hours or less - and radios built for WW2
airplanes didn't usually get that much use, or that long a life on DC before
being removed from service - like being shot down....The PT15 used in the RAF's
T1154 was designed for a short life of a few thousand hours, according to G6JP -
who designed valves. That had a thoriated filament run on DC...
There's a chance, therefore, that especially in amateur service, you might
reduce the life of a filament tube by getting an open circuit filament before
you run out of emission.
Of course when you're swapping your 211s at a few hundred hours, you're not
going to notice it, are you?
73
Peter G3RZP
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