3 Sep
2008
3 Sep
'08
11:45 p.m.
Chuck Guzis wrote:
On 3 Sep 2008 at 14:58, Brent Hilpert wrote:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Well, that was the hitch. I was looking at using a 6X5 (ind. heated)
but it was insufficient due to the fact that there is a polarization to
the H-C V limits (weird, but it's there, something to do with the oxide
formation I'm guessing).
On the other hand, for the ABC ASM reconstruction
I was working on I
was targetting using only period-appropriate components. I'm still
debating whether to cheat and use a silicon diode for the C- bias
rectifier (it could perhaps have been a selenium) or go to the bother
of throwing in another rectifier tube, which will also require the
hassle of an additional isolated filament supply.
In the old days, the most common bias supply in many applications was
a simple dry battery (e.g. the 4.5v "C" battery). If you want to use
an AC-powered bias supply, there's no particular reason that you need
to use a separate filament transformer--use a rectifier with an
indirectly-heated cathode
(and sufficient heater-to-cathode breakdown voltage). Since bias supplies don't generally supply lots
of
current, even a small-signal diode, such as a 6AL5 should work fine
(heater-cathode max voltage is 330).
I don't think the 6AL5 was around in 1939/40, but in the same spirit
I was considering a 6H6. The problem there (even though the H-C V permits
it to be used as a low current C- rect) was that the C- (and B+) for
the logic need to be regulated and by the time you account for the extra
current for a shunt regulator (gas tube) and some more for a resistor
divider, it was on the margins or exceeded what the little guys
could handle.
It was one of those annoying design problems where every optimisation
you try leads to some minor hitch and you have to revert to what seems
like an overblown solution.
You could, of course, use selenium or copper-oxide
rectifiers--Cougar
Electronics (supplies the elevator industry) still makes the selenium
jobs (you can't simply drop in a silicon diode in a lot of old
equipment without redesigning it). I don't know if any manufacturers
of copper-oxide rectifiers still exist.
Didn't know anybody was making them still. I could try scavenging a
selenium rect at the radio museum, but the old ones I run across are
usually in questionable condition.