[Amps] Glass envelope

[Ian White GM3SEK]

GGLL wrote:
>>>Does it still has nothing?. So it must be equal regardless inside
>>>volume.
>>>
>>
>>
>> Nothing?
>>
>> Tubes are typically pumped down to about 1E-8mmHg, which means that the
>> "vacuum" space of an amateur-sized transmitting tube will contain well
>> over a million free gas molecules.
>
>       Well, I ignored that and simplified things also; then, after 
>"vacuum
>process",  and to "kill" those over a million molecules tube's 
>manufacturer use the getter method?.
>

I don't have any specific information (so this message is also being 
blind-copied to a couple of people who might know better) but my 
impression is that the getter in transmitting tubes is mostly intended 
to *maintain* the quality of the vacuum, not to improve it.

That wouldn't be true of small glass tubes that use a barium getter  - 
the silvery-looking film that has been evaporated onto the surface of 
the glass. In those tubes the getter film is not created until *after* 
the tube has been pumped and sealed. In that case there would be a big 
improvement in the vacuum when the film is thrown onto the glass (it's a 
standard cleanup technique in high-vacuum lab work). But that is all 
part of the manufacturing process; afterwards, the getter would tend to 
*maintain* the vacuum in its post-manufactured state.

Transmitting tubes are different because the getter is already in place 
when the tubes are being pumped down, flash-heated way above the 
operating temperature to remove as much gas as possible, pumped some 
more and finally sealed. In that case, the getter surface would be 
pretty close to a chemical equilibrium with the residual gas pressure 
*before* the tube is sealed, so afterwards it would tend to *maintain* 
that condition.

We're also aware from a few weeks ago that the getters used in 
transmitting tubes need to be hot in order to work at optimum 
effectiveness, so the vacuum can deteriorate when the tube is left cold 
for long periods.

However, the basic principle is that electron tubes are quite happy to 
share the space with quite large numbers of gas molecules. That is 
because the electron current is usually much larger than the small 
reverse ion current... but if it isn't, things can go badly wrong.



-- 
73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek