On 6/14/2012 11:59 PM, Charles Harpole wrote:
> to check on a supposition............
>
>
>
> It appears possible that the sudden application of voltage to a tube filament
> causes degradation of the filament.
>
>
There are others on here who can probably explain this better than I, but...
This is typically a problem with filaments that draw a lot of current.
At turn on the resistance is a fraction of that once the filament is up
to temperature. If the current is not limited the sudden rise in
*current* well beyond the operating current produces a very strong
magnetic field that interacts with the current flowing in the filament
which can cause the filament to move mechanically.
Take a 4CX-3000. The filament runs 9V at 41.5 Amps which is just shy of
375 watts, but the cold, inrush current can be several times higher if
the transformer can provide it. That would cause the filament to
"jump". A number of the larger QRO amps run much higher current than
the 4CX3000. This is the reason for "step start" or ramping up the
filament voltage so there is no sudden very high current pulse at turn on.
73
Roger (K8RI)
>
> 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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>
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