[R-390] RT301 current regulator tube

[Tisha Hayes]

The current regulator tube is a 3FT7. It is an iron-wire filament in a pure
hydrogen-gas atmosphere and since iron goes up in resistance as it gets
hotter and the hydrogen gas will not oxidize the iron, the filament never
burns away but the thermal conductivity remains pretty constant. The 3FT7
could maintain regulation to within about 1% of its rated value.

The downside of a hydrogen atmosphere; it is a very permeable gas and even
glass is ever so slightly leaky at the lead-seals on the tube pins.
Eventually the hydrogen gas will escape (over decades) and regulation may
get sloppy. Or the radio gets moved around; the iron filament is fragile
and it eventually breaks.

I bought a couple of the plug-in regulators that were made by one of the
R-390A enthusiasts (who was at least monitoring this list several years
ago). They were out of Germany and the ballast tubes (and balanced antenna
baluns) they made were top-notch. I do not know if they are still around or
making these devices; It was clearly a labor of love.

You could make your own regulator; I would go with the LM317 regulator, a
small heat-sink for the regulator, and run a ground wire to an unused pin
on the tube socket where the 3FT7 plugs in (there are lots of pins on that
socket that the 3FT7 does not use). You could add a couple of caps for a
soft-er start on the tube filaments and even a trimpot if you want to tweak
the regulation voltage. The heat-sink is needed because the regular is a
linear device and voltage drop * current = watts(heat) that needs to go
somewhere.


*Ms. Tisha Hayes*

*AA4HA*