Peter,
I don't think he said they absolutely couldn't happen. What I've read about
this was that the most vacuum that can be pulled is around 10 atmospheres.
Also, the innards and glass are baked to get rid of what gas they can before
they are evacuated and sealed. The problem is that after a while, from the
anode getting hot, some gas may still get loose in the tube to where enough can
cause a flash-over. The flash-over generally takes care of the small amount of
gas itself and the tube operates normally again after power is re-applied.
There's a difference though between a quick "flash-over" arc and one from a
badly gassy tube. A gassy tubes arc can be a long one with a large plasma
stream, and can last for some time (until the caps are discharged or something
opens up). It can be very destructive. Cathode to anode arcs are not uncommon
with a very gassy tube. These types of arcs can and do knock out the B+ power
supply. That's my opinion on this, and I think what was meant.
Best,
Will
*********** REPLY SEPARATOR ***********
On 7/24/06 at 7:40 AM Peter Chadwick wrote:
>Rich said:
>>I autopsy tubes, and I have no yet seen an arc mark in a kaput tube
>with a good vacuum.<
>It's sad that G6JP is no longer alive. He spent his entire working life
>doing tube manufacture and applications at the M-O valve company, and he
>said that flashovers weren't uncommon. There's also the articles in the
>Journal of the IEE. My father in law tells me that they had flashovers in
>continously pumped tubes at the QRO point to point stations he worked at
>-although the biggest problem was when someone with greasy fingers touched
>the grid during re-assembly. That meant that they had a lot of pumping to
>do before the valve really got hard.
>So Mr. Measures, the fact you've never seen what you take to be marks of a
>flashover in a tube with good vacuum doesn't mean that it doesn't occur.
>But, of course, proving (or disproving) a negative is not easy, so either
>party can sit back in the warm glow of satisfaction that really, they're
>the ones who are right!
>73
>Peter G3RZP
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