[BC] Current flow in old radios

[Jim Tonne]

> > On the Hallicrafters AC/DC s-38D receiver the dial light  
> > when you first turned the set on glowed birghtly for about 
> > a half a second until the other tubes started to draw current.

The tube filaments are all in series so they all "draw current" 
and in fact the same current.

The fact is the pilot light was low voltage, 6 volts as I recall.  It
was placed across a tap on the rectifier filament.  That filament
had a tap on it just for a pilot lamp, a voltage divider.    But when
first turned on, the filament string was cold and had lower 
resistance than when hot.  So there was an inrush of current into
this series-connected string.  Hence the tap on the filament of the
rectifier tube (35Z5, etc) delivered more output voltage.  As the
filament string warmed up the current in the string went down and
Presto! the voltage to the pilot lamp also went down.

- JimT who unfortunately is old nuff to remember this bit of trivia

(and not alone I suspect . . .)