> The anode at the top may be a flat surface, at the 'dead
> head'
> region, like the inside of a metal can. At the bottom of
> the tube,
> there are edges of the anode and the fil/grid connections
> may have
> other attachments and sharp edges (like a getter assembly
> on
> filament). I see no reason to question why it wouldn't arc
> here just
> as easily as up at the top where the anode/grid may have
> smoother
> contours.
I thought most people knew that John, but perhaps it was
good to mention it.
Sharp points are a big problem. It's all a matter of voltage
gradient or charge distribution. The shape can be more
important than the distance.
This is why air variables and bandswitches, even when the
look physically robust with large air gaps, can have less
voltage breakdown than similar smaller items that have much
closer spacing. All it takes is a sharp point to ruin hold
off voltage.
73 Tom
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