[Amps] Arc distance

[Ian White GM3SEK]

Will Matney wrote:
>Ian,
>
>That was a good description, see the rest below;
>

Just to clear up a few details, then...


>>There are also gas molecules chemically bound onto the exposed surfaces
>>of metals, ceramic and glass. These are largely removed in the later
>>stages of vacuum pumping, by heating the whole tube far above its normal
>>operating temperature while continuing to pump. The better the
>>materials, and the longer the time for which the tube is pumped, the
>>better the vacuum will be... but they can't pump and bake forever, so
>>eventually the tube has to be sealed off.
>
>Agreed. They bake both the anode, and the glass itself so they will 
>liberate all the gas they
>can get them to, then evacuate the tube.
>

The bake-out can only be done *after* having pumped the tube down to 
quite a good vacuum. If it was done at atmospheric pressure, all the 
metals would oxidize!


>>Immediately after manufacture, the vacuum will probably be about 10^-8
>>mmHg, which is really quite good for a routine production-line process,
>>but no great shakes by the standards of a vacuum lab. At this standard
>>of vacuum, a typical tube may contain anything between a million and a
>>billion gas molecules! (PV=nRT... work it out)
>
>I had read earlier that a vacuum could be pulled up to 10 atmospheres. 
>That's not
>saying they do that on the tube, just that they can do it.
>

"Up to 10 atmospheres" doesn't make any sense when talking about vacuum. 
Normal air pressure is 1 atmosphere, and a perfect vacuum is exactly 
zero pressure, so the largest possible change is 1 atm.

The challenge in vacuum technology is how close you can get to zero.
A typical vacuum of 10^-8 mmHg (millimetres of mercury) means a pressure 
of 0.000000000013 atmospheres... and each one of those leading zeros is 
progressively harder and harder to achieve.

The process hits a plateau somewhere around the 10^-7 mmHg level, due to 
slow, continuous outgassing from internal glass and metal surfaces. At 
this stage you have to bake the tube to release the gas more quickly - 
pumping all the while - and then the vacuum will improve.

At atmospheric pressure, a small vacuum tube contains about 
100,000,000,000 million gas molecules. By the time this has been pumped 
down to 10^-8 mmHg, you have removed about 99,999,999,999 million... but 
the improvement is becoming slower and slower.

Fortunately, tubes work very well with this quality of vacuum, because 
the electron current completely dominates the small ion current in the 
opposite direction. So the tube is sealed of at about 10^-8 mmHg, and 
relies on the getter to hold the tube at about that level of vacuum for 
the rest of its life.


>>If the getter is active, it will mop up the impurities within typically
>>a few seconds (determined by the time it takes for the molecules to
>>bounce around until they strike the getter surface, and by the
>>probability that a molecule will hit a chemically active spot that can
>>form a strong enough bond to make it stick). But a few seconds is far
>>too slow to prevent an arc, which can strike within microseconds if all
>>the other conditions are right.
>
>Agreed. A getter can also hold just so much gas. Once that limit is reached,
>there's no hope for the tube if still gassy. Getters like zirconium liberate
>hydrogen gas when heated above 300 deg C, and start working around 700-800
>deg C to absorb O2, CO2, CO, etc. The optimum temperature is said to be 1400
>deg C. Tantalum though does not have the problem of emitting hydrogen gas
>and its optimum temperature is around 1000 deg C if I recall.
>
Overnight I remembered some older correspondence about this. Because 
getter materials mop up different gases at different
temperatures, some transmitting tubes have multiple getters at different 
locations, possibly using different materials as well. For example, the 
3-500Z has getters located at the base of the grid and the base of the 
filament, as well as the big one sprayed on the anode (information from 
an Eimac tube designer).


-- 
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek