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[AMPS] How do filaments break?

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Subject: [AMPS] How do filaments break?
From: (Lee Buller)
Date: Thu, 08 May 1997 08:51:09 -0500
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?

Is there so much current that the filament wire separates or burns in two
pieces?  If that is the case, where does this current come from?  How much
more is this current that what is flowing normally through the tube?  On
most tubes, what is the tolerance of current (a percentage) for the filament
until it burns in two?

Is there some physical oscillations that the internal structure of the tube
cannot hold and the filament breaks?  What causes the physical shaking or
bending of the filament until it breaks?

This has never happened to me before on the amps I have built.  I had a pair
of 3-500z that oscillated quite easily when I first started them up, but
that was mostly to my layout and design mistakes.  The input and the output
where not shielded enough.  That was solve easily enough because the
oscillation  was not a VHF oscillation.  Once I had that tamed, I did have a
VHF oscillation which was tamed withe some globar resistors and some silver
plated wire.  I had about three turns of wire #16 in about an 1" or 1" and a
half around the globar resistors.  That did the trick.

During that time, the amp would go into oscillations, the power supply would
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 determined
what was going on.  And the minutes added into at least an hour or more.
Never damaged the tubes.  Once I got the thing tamed down, I could get 1300
watts out on 10 meters with those tubes.  I beat the tubes up pretty bad
getting them to work and then used them extensively after that.

Anyone have an answer?


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