[AMPS] Another arc question

Rich Measures measures@vc.net
Wed, 6 May 98 07:40:26 -0800

>Oh, dear Jon (how often does a gent get to begin a letter with "dear 
>Jon" eh?, our poor dear junior engineer.
>Some of your questions do embarrass those of us who think highly of 
>college education.  

Bill Clinton has a college education and he apparently still has problems 
with ordinary zippers.  

> One would have thought the immediate and volumnous 
>response you got from the first time would have answered your question, 
>but perhaps another try is worth it.
>>Thanks to those who responded to my seemingly dumb question about why no 
>>carbonization marks exist around tubes which supposedly had an arc-over 
>>in them.  Specifically this is in regards to the bent filament helix 
>>phenomena in the 3-500Z's.
>>So here's a follow up question:
>>Several people pointed out that you wouldn't get carbon scoring from an 
>>arc because the tube is evacuated and therefore there is nothing that 
>>would cause carbonization (OK, maybe that's not the scientific 
>>explanation, but you know what I mean).  However, if there were no gas of 
>>any kind inside the tube, then how did the arc occur?  
>Arcs do not depend on the existence of a gas.  Do you believe you could 
>never get an arc in outer space, for instance?  Please refer, if you 
>will, to your sophomore physics book.  In a vacuum, with adequate 

Like a bunch o' gigavolts

>between two conductors, the electromagnetic forces ultimately 
>cause particles to traverse the vacuum gap from one conductor to 
>another. >.......

>However, as you point out, in the presence of a gas, something different 
>occurs.  You ionize the gas, and as the gas de-ionizes, it produces 
>energy, usually visible light, thus the traditional purple glow in a 
>gassy tube.  Again, sophomore physics.  Taking one more lesson from 
>sophomore physics, the source of the gas is the metal in the tube 
>itself.  Metals are very good at absorbing gas - that's why most glass 
>tubes and some metal tubes have a "getter".  In the presence of gas, the 
>ionization of the gas can cause a plasma, which reduces the ultimate 
>breakdown between the two conductors - thus a "gas arc" which means that 
>the arc occurs at lower voltage than were the gas absent.  
>Unfortunately, metals are also porous (Rich, I know you'll argue this 

Not a good bet, Arlen.   

>but it is true - just as glass is a fluid, as demonstrated by the 
>shape of stained glass windows 150 years old that are thicker at the 
>bottom - just because it's a slow fluid or a slow porosity does not mean 
>they're not fluid or porous), 

Indeed.  .  As I recall, there is a word for a slow-fluid that ends in 

>and gas can seep in after years of non-use 

The problem at hand is that gas is Not being detected in 3-500Z tubes 
with bent filament helices.  

Until recently, I owned some new 4E27 pentodes that were manufactured in 
the 1940s.  Some of them still exhibited good vacuums, however other 
4E27s  from the same lot were quite gassy, presumably due to imperfect 
metal/glass seals.  

>of a tube (heat tends to force out gas from metals).  By the way, the 
>ionization of gas is not by any means the same as an arc.  But it does 
>provide a path making an arc easier to create.  

well put

>And yes, to get 
>carbonization in an arc, one must have carbon. 


> I know of no elemental 
>gasses (those are what a metal can absorb) which contain carbon.  

except for carbon gas existing in the fusion furnace up yonder.

>And, yes, powdered metal tends to be black, but usually because it is 
>oxidized, but sometimes merely because the tiny particles don't reflect 
>light well, not being polished.  But again, in a vacuum or in a very 
>low-pressure gas, an arc causes sputtering, which transfers metal in a 
>film-like manner, not as tiny particles.
>>Again, it may sound like I am asking old or dumb questions, but I am 
>>relatively new to this group (couple months) and haven't heard all the 
>>answers before.  And as I pointed out before, I am not an expert on tubes.
>Being new to the group may explain lack of expertise on tubes, but it 
>does not explain absence of knowledge of undergraduate physics.
>Sorry to be so rough, Jon, ........

I concur that Jon erred on the carbonization of arc-marks matter, Arlen, 
however, perhaps You would like to proffer a scientific explanation to 
the [AMPS] group on how/why the filament helices in a 3-500Z can be 
suddenly bent by a "gas discharge" or "barnacles" -- even though the grid 
is grounded -- even though no arc-marks can be found at autopsy -- even 
though the VHF suppressor resistor has more than doubled in resistance 
without exibiting cosmetic damage?   


R. L. Measures, 805-386-3734, AG6K   

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