Fwd: [AMPS] How do filaments break?

[Rich Measures]

>> >This is extremely interesting discussion.... but....   Could somebody
>> >explain to me the physics of how a parasitic oscillation can affect the
>> >filament in a tube enough to break it?
>
>>  Electromagnetic force occurs at a right angle to the flow of electric 
>> current.  This is why, when arc welding with high currents, the cables 
>> tend to twich when the arc is struck. 
>
>But the cable MELT long before they are torn apart by the tiny little 
>twitch produced by many HUNDREDS of amperes.
I do not understand this statement.  
...snip...
>Filament failures (inrush or otherwise) occur from heat related 
>fatigue, not magnetic force.  ...snip...
So why did the filament shatter in more than one 811a at the same 
instant? 
.......... 
>
>> The filament does not burn in two during an intermittent vhf parasitic 
>> oscillation.  The filament shatters in 811As and 572Bs.  In 3-500Zs the 
>> filament is pushed sideways at the unsupported center of the filament 
>> helices.  This decreases the clearance between the filament and the grid. 
>>  [9/90 QST magazine, "Parasitics Revisited".]'
>
>Can you explain how? 
It's in the article.
>The magnetic force is proportional to the 
>current. Where do all those hundreds or thousands of amperes come 
>from when the saturated emission current is a few amperes?
Who said anything about thousands of amperes?
>
>>  Today I tested 3, 3-500Zs that were removed from an LK-550 that had 
>> arc-damaged tune capacitor plates..  The grid/filament breakdown 
>> potentials were 2400v, 2800v and 5100v.  However, a normal 3-500Z has a 
>> grid/filament breakdown of 8kV to 9kV.  How about the vacuum?  Two of the 
>> tubes exhibited under 3uA of anode/grid leakage @8800v.  The third tube 
>> was 5uA.  In other words, the vacuums were good. 
>
>Many things OTHER than spacing control breakdown voltage. 
Take a look at the photograph on page 15 in the September 1990 issue of 
QST magazine and tell us that the filament helices are not bent toward 
the grid cage, Mr. Rauch.
>The problem 
>is voltage gradient, not simply overall spacing. What does an arced 
>capacitor have to do with a grid? 
Nothing.  There is no problem with the grid.
...snip...
>Many things could have caused the grid to overheat at the same time 
>the capacitor arced.   
The grid did not overheat.  The filament helices were bent out of line at 
the midpoint.  When viewed from the base end, an faint arc could be seen 
between the bow in the filament and the grid cage.  
>> >If that is the case, where does this current come from?  
>> The pulse of current comes from the cloud of electrons that surround the 
>> cathode.  In a high Mu triode, oscillation with no load results in high 
>> grid current.  
>
>Really? And how much saturated current is available? 
I don't know for sure, but I do know that the fusing current rating of 
the copper wire used in the 1A grid chokes exceeds 15 amperes.  Since the 
copper wire melts, I assume that such a current occured.  
>Does it seem 
>logical that the "oscillation" could produce hundreds of amperes of 
>current for enough time to overcome the mechanical or thermal  lag of 
>the filament?  
No.  The statement about hundreds of amperes of current is yours.  
>Factually, it is impossible to obtain ten or 12 amperes of cathode 
>current if the grid is driven to a few thousand volts positive. Where 
>does the emission current come from?
The current in question is a pulse.  
...snip...
>The normal cause of choke failure is a HV anode to grid fault. Either 
>a "barnacle" or gas in the tube. 
None of the three 3-500Zs in question exhibited anything close to 
abnormal gas. ........
...snip...
>> That's the difference between the push-pull and the push-push variety.  
>
>So if I run my PA push-pull, the tubes will last longer???? 
>That sounds very strange. What is the logic behind that statement?
>73, Tom W8JI 
Seemingly, None.  
Rich---

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


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