> 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
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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