Rich Measures wrote:
>>
>>>>On the other hand, if the short is due to an arc *inside* the tube, the
>>>>arc could go right through the grid and hit the cathode.
>>>
>>>? Have you ever seen an 8877 grid?
>>>
>>Yes, there are some nice photographs in "Parasitics Revisited". The
>>point being...?
>
>Penetrating the grid seems quite unlikely.
Depends what you mean by "penetrating". It doesn't have to blow a
physical hole in the grid.
What could be happening is that the very local area of the grid where
the arc strikes is pulled strongly positive, which then produces an
extreme current density in the local section of the cathode. The self-
inductance and resistance in the grid and cathode structures could be
high enough to allow a local section to go positive for a very short
time, before the potentials equalize.
How else can you explain the burn marks on the cathode, and that each
one is opposite a burn mark on the grid?
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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