>
>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,
The grid is rather well grounded through the Eimac fixture. .
>which then produces an
>extreme current density in the local section of the cathode.
I do not see how such a scenario would be possible. None of the current
that supplies an anode/grid arc is emitted by the cathode. The current
carriers are either from a seal leak or from loose gold meltballs -- the
latter being more common in my experiences.
>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?
I have never found such a thing in a autopsied 8877. When an anode arc
takes place across the anode insulator, the gold meltballs appear to
remelt and recondense.
>
cheers, Ian
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|