Thats fine John but very few of us are running tubes with 5-7 figures of
plate dissipation!
Those that do can go to the CB forums for Care and Feeding data!
Carl
KM1H
----- Original Message -----
From: "John Lyles" <jtml@losalamos.com>
To: <amps@contesting.com>
Sent: Friday, June 15, 2012 3:12 PM
Subject: [Amps] filament instant heating query
> The mechanical expansion of a cold filament is the function that weakens
> them. I suppose this means molecular level... Some examples:
> As Jim Thompson said, thoriated tungsten cold R is much lower than hot.
> So the inrush current, unless limited, will cause rapid movement of the
> filament. Eimac and the others give recommended peak current limit in
> the datasheets. Usually for big tubes it is no more than 2 x the nominal
> operating current, sometimes less. A transformer without current
> limiting may exceed this on turnon with a cold filament. Step start or
> ramp up is a way to current limit open-loop and as the filament heats,
> resistance stabilizes at the operating level. When a filament basket
> expands rapidly, fractures can form along grains in the surface of the
> carburized layer on thoriated tungsten. Repeating this action time after
> time can eventually cause defects and other mechanical destruction on a
> filament. This may reduce the active emitting area, just like ion
> bombardment (poisoning) can do. This translates to low output, low gain,
> low efficiency, symptoms of a weak tube due to low cathode emission.
>
> Another worse problem, especially on big filament baskets of KW ratings
> and up, is the longitudinal and transverse expansion different and some
> tubes have spring mechanisms ("pantographs") to maintain tension in the
> length direction (up and down). If heated too rapidly, the transverse
> expansion may exceed tolerences and cause a grid to filament to touch,
> shorting out momentarily. If heated slowly, the longitudinal expansion
> is controlled, excess taken up by the mechanisms, and there is less risk
> of touching.
>
> Cathode type types (oxide coating) still have heaters albeit at lower
> temperature. They can be damaged similarly by too rapid a turn on.
> Emission of the oxide layer is facilitated by the indirect heating, so
> naturally it takes time for this to start happening in a cold tube.
>
> In all cases there are tubes which might survive many more hits of high
> starting current than others. Manufacturers apply statistical analysis
> to determine what is safe for a large family of tubes.
>
> Once the price of a tube exceeds a few thousand dollars, it behooves the
> user to follow the guidance and slow heat because the cost of failure is
> high, and warranties will not cover a transmitter user whose circuitry
> or operator disregards their recommended operating steps.
>
>
>
>
>
>> Message: 1
>> Date: Fri, 15 Jun 2012 03:59:12 +0000
>> From: Charles Harpole<k4vud@hotmail.com>
>> Subject: [Amps] filament instant heating query
>> to check on a supposition............
>> It appears possible that the sudden application of voltage to a tube
>> filament causes degradation of the filament.
>> What seems to happen? Is it possible that the very rapid heating of a
>> filament could cause so much quick molecular action (rapid increased
>> bouncing around of the molecules) that the shape of the filament could be
>> changed? A deformed filament could then be too close to other components
>> to cause an arc, perhaps?
>> Or is the action on the atomic level...i.e., does heat change the actions
>> of electrons in an atom?
>> But, maybe the filament rapid heating works just fine, and all of the
>> "slow start" "inrush protection" circuits are a waste?
>> Another inquiring mind wants to know.
>> 73,
>> Charles Harpole
>> k4vud@hotmail.com
>
>
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