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Fwd: [AMPS] How do filaments break?

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Subject: Fwd: [AMPS] How do filaments break?
From: (Rich Measures)
Date: Sat, 10 May 97 21:34:44 -0700
>Subject:     [AMPS] How do filaments break?
>Sent:        5/8/97 6:51 AM
>Received:    5/9/97 7:09 AM
>From:        Lee Buller,
>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. 
>Is there so much current that the filament wire separates or burns in two
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".]
 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.  
>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.  
>How much
>more is this current that what is flowing normally through the tube?  
In 3-500Z amplifiers, it is not uncommon to see 1A grid to gnd. chokes 
open during a push-push vhf parasitic.   As I recall, the fusing current 
rating of #27 gauge wire it is at least 15a.  That is about 100 times 
normal grid current.  

>Is there some physical oscillations that the internal structure of the 
>cannot hold and the filament breaks?  
IMO, there is a single pulse of EMF.  
>... the power supply 
>hum very loudly, the tubes glowed dull red, the meters would show current
>flow, but nothing ever flashed.  This went on for minutes while I 
>what was going on.  
You were apparently dealing with a push-pull parasitic oscillation.  The 
anodes get hot, but nothing arcs.  In an SB-220, placing the parasitic 
suppressors side by side will produce a push-pull oscillation at about 
>And the minutes added into at least an hour or more.
>Never damaged the tubes. ......
That's the difference between the push-pull and the push-push variety.  

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

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