Randall said:
>I'm beginning to think about regulated DC filament supply possibilites for my
project
>amp(s). One of my local 'Elmers' claimed that for 813s, it's a great idea but
one
>needs to periodically reverse the polarity in such a configuration.
I've heard this too. One story is some form of electrolysis. I think that a more
likely problem would be caused by metal migration. Metal migration in ICs halves
for every 12 degree decrease in temperature. It's an exponential function, so
it's not so easy to extrapolate from 150 deg C IC temp to the 1600 deg C or so
of a thoriated tungsten filament. However, Typical IC metal is rated at about
2mA per micron width: assume 2 micron thick metal, so that's about 2 mA/square
micron at 150 deg C. That means 2500 sq microns at the 5Amps of an 813.
Guessing at an 813 filament being around .3mm in diameter, that's an area of
around 70,000 sq microns. The temperature is more than 10 times as high, so it
suggests that metal migration could be the cause of a problem long term. For the
military applications where DC was used on the filaments, 500 to 1000 hour
operational life frequently exceeded the expected life of the platform on which
the radio was fitted......
Dull emitter battery tubes would have been much lower in temperature - 600 deg C
- so wouldn't suffer anywhere near as much.
Anyone any further comments on this?
As far as rectifiers and VOMs are concerned: I don't think anyone makes a single
cell that goes above 1200piv. Above that, as far as I'm aware, they are batch
matched (at least from the reputable suppliers) cells in series, all in the one
package. So the forward drop is 0.7v/cell, and the VOM really needs to be able
to source about 1 volt/cell to turn it on. Some of the older VOMs used up to 15
volts on the highest resistance ranges, which was fine.
73
Peter G3RZP
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