[ARC5] [Milsurplus] Cold Filament Inrush Current

[Jack Antonio]

On 3/15/2018 4:28 PM, Michael Clarson wrote:
> Wasn't the original post about command set rcvrs? These were used in

I pulled a couple of 12SK7s off the shelf and measured them.

Setting the supply at 12.6, the first tube initially drew .4A and
took roughly 5 seconds to come down to .15A.

The second tube hit .5A, at which point my little
lab bench supply hits current limit, and came
down to .15A in 10 seconds.

I tried a 6SK7, but it severely went into current
limit at .5A, so would need a beefier supply to
check that. It came down to .3A in
about 10 seconds. Probably would have come down
faster had the supply not gone into current limit.

I have been poking around BC-348s lately, and the
current bench project involves in de-ham-modding
a BC-348-N.

One question that has come up, is that the designers
of the -348 deemed it necessary to put in dropping
resistors to bring the nominal 28 down to the
specified 25.2V the 4 series 6.3V tubes require.

Our beloved command sets, and many other receivers
of the era do not bother with that, and simply
accept the variations in the source voltage that
come with a generator/battery system.

I think the answer has been long lost to
history, but it would be interesting to know
why the designers of the -348 thought it was
necessary. It would have provided some protection
for inrush current, but don't know if that is
the reason or not.

Jack Antonio
WA7DIA