[BC] Current flow in old radios

[Jerry Mathis]

No, it wasn't deliberate, it was a matter of physics.

Tube filaments are somewhat like light bulb filaments; when they are cold
their resistance is much lower than when they are at their normal operating
temperature. The tubes therefore pull more filament current at turn-on than
they do after they warm up. The tube filaments have more physical mass than
the light bulb filament, so it takes them longer to warm up. Therefore, when
the light bulb is already fully lit, the tubes are still drawing larger
amounts of current. That current causes a higher voltage than normal across
the low-voltage light bulb filament, and causes it to glow very brightly for
about 15 seconds, after which time the tube filaments are near normal
operating temperature, and are drawing the normal amount of current.

JM




On 12/4/06, Kevin Tekel <amstereoexp at yahoo.com> wrote:
>
> Dave Hultsman wrote:
> > The main reason was the reduction of parts.  No power transformer and
> > the filaments were series inline including the dial light.  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.
>
> I have an S-38C and its dial light does that.  Maybe it was intentional,
> to let people know that the radio is working.  Otherwise if you turn it on
> and see and hear nothing for 15-20 seconds (not even a pilot light) you
> might think it's dead.
>
> .
>
> In some other radios where the filaments in series didn't add up to the AC
> supply voltage, they used a "ballast tube" (a large wire-wound resistor
> plugged into a tube base) or even a special resistance line cord to make
> up the difference.
>
>