[R-390] 6080 in place of 6802 - RMS ???

Wed, 9 Jan 2002 18:37:29 -0500

Hi James,

The resistance of tungsten goes up with temperature.  The actual
temperature achieved would be slightly less than that predicted from
assuming a constant resistance.  However the effect is not nearly large
enough to protect the tube.

You would see more of an effect with direct filament tubes where the higher
temperature would mean more heat loss by radiation (visible light and
infrared).  However oxide coated cathodes would show little increase in
radiation heat transfer.  And remember that in vacuum, radiation and
conduction are the only means of heat transfer.

Since the thermal time constant is so much longer than the period of the
pulsations, the pulsations themselves would have no effect.

        73,  Barry     WA4VZQ     [email protected]

----- Original Message -----
From: "James Miller" <[email protected]>
To: "Barry L. Ornitz" <[email protected]>
Cc: <[email protected]>
Sent: Wednesday, January 09, 2002 05:49 PM
Subject: Re: [R-390] 6080 in place of 6802 - RMS ???

> And what about the changes in filament resistance as a function of
> temperature caused by pulsating filament current?  Would the filament be
> cooler on average, hence having a lower resistance on average, further
> compounding the problem?