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[AMPS] Filament breakage

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Subject: [AMPS] Filament breakage
From: (
Date: Mon, 12 May 1997 11:44:58 +0000
> Instead of wandering all around the subject how about sticking to one
> very basic tube like the 811A. Now explain the cause of sudden arcing,
> bangs, flashes, etc with the end result being a broken filament and even
> other circuit components.

There certainly are multiple causes Carl, but it is quite obvious 
from the emission limitations of the tube and the actual mechanical 
force available it can not be the direct result of an oscillation.

It's my opinion that whenever someone does not understand a problem, 
he falls back on something he can understand. This is the point 
where science departs from rumor and wive's tales.
> I'll even give you a hint...from my perspective; magnetic mumbo jumbo has
> nothing to do with it. 

The magnetic issue is indeed mumbo jumbo. It can be clearly 
demonstrated the amount of current required to mechanically 
stress the tube are unavailable from either inrush or oscillations. 

Look at a real world example of how trivial the magnetic force 
really is. A small induction type meter, typically used for a car 
ammeter, has to use very strong magnets, light weight pointers, and 
steel magnetic paths to cause a meter pointer to deflect with 
dozens of amperes of current flowing through a wire a fraction of an 
inch away.

It is the uneven HEATING of the filament that causes mechanical 
stresses, not magnetic flux.  

Two destructive mechanisms are at play in healthy vacuum 
tubes Carl.  I*I R losses producing heat, and kinetic energy from 
electrons heating the element they strike. Any heat problem is a 
function of BOTH power dissipation and time. 

Kinetic heating is closely defined by the accelerating voltage 
times the current. We do it every day. The anode dissipation is 
simply the cathode to anode voltage times the anode current, at any 
given instant of time. This is true under ANY operating condition.

Peak voltage is determined by the Q of the anode load and conduction 
angle of the tube. 

In normal operation a tube that comes up and idles fine for hours and 
hours on end can arc and fail when RF drive is applied. The reason is 
the anode voltage easily exceeds the quiescent voltage.

Outgassing also is a common occurrence. If the elements are heated, 
gas molecules can be brought to the surface and released. Graphite 
anode tubes are very susceptible to this problem, because the 
porous graphite traps gas. It takes longer to pump a graphite anode 
tube down, because the thicker porous anode material often traps more 

So if you fire up a tube, even if that tube behaved normally at lower 
voltages, and that tube suddenly arcs, it could be from several 
causes. The anode voltage could have reached a higher peak value 
and exceeded the breakdown voltage due to a load fault or exciter 
transient, the tube could have outgassed from additional heat 
(any of the elements inside can release gas, not just the anode), or 
a poor weld or weak area could fail from the thermal stress.

Any of these things can cause a tube to suddenly arc.

There is no single answer to WHY a tube arcs, as much as people 
who sell bottled cures would like there to be. The likelihood it is 
a parasitic is far down the list (actually very near the bottom), 
since system Q and drive power (and hence anode voltage) is MUCH 
higher at the operating frequency.

People are free to believe what they like, no one can prove  
UFO's don't land and abduct people for sinister sexual experiments. 
Just like with these UFO landings, the proponents of parasitics point 
to other abductees or enlist UFO believers as evidence the exist.

When asked to show the math proving the alleged magnetic destruction 
occurs, they fall back on more "people stories". 

You asked why the filaments failed. I have no idea, except it must 
have been a mechanical or thermal failure. The anode current is 
limited by a minimum total ESR of about ten ohms in that PA, and that 
limits discharge current to 170 amperes or less. A 170 ampere arc 
could certainly burn through a filament, but in order to arc the tube 
would have to have been gassy or defective anyway.

A good 811 tube will hi-pot above 8 kV, there is no likelihood at 
all the anode voltage could exceed 8 kV, since other parallel 
components are rated at much less breakdown voltage.   

In any event, virtually all 811 tube failures are from holes 
burned in the anodes, not filament death. Filament failures are far 
below glass failures in frequency, while anode failures (accompanied 
by holes melted in the anode) are relatively common.

73, Tom W8JI 

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