[BC] Current flow in old radios

[Paul Smith W4KNX]

Typical inrush for a tungsten filament is 6X the steady state current,
unless some other resistance limits the current.

Paul Smith
W4KNX
Sarasota, FL

----- Original Message ----- 
From: "Jerry Mathis" <thebeaver32 at gmail.com>
To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
Sent: Monday, December 04, 2006 7:46 PM
Subject: Re: [BC] Current flow in old radios


> 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.
> >
> >
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