It would be nice if one of the proponents of "parasitics breaking
filaments" could mathematically demonstrate how current levels
intense enough to produce magnetic forces strong enough to break the
filament could possibly occur from the alleged oscillation.
It's a pretty straight forward calculation. The only parameters
needed to find the mechanical force are distance, current intensity,
and current distribution.
We can assume the current distribution would be equally spread
over the total area of the heated electron emitting surface. With a
single wire in a "V" shape, at half the distance from the upper end
of the filament we could have a maximum of half the potential
saturated emission current flowing past that point.
With a 811A or 572B, we could make a generous allowance using the
following data:
The filament power is about 25 watts. The best thoriated tungsten
materials produce less than 125 mA per watt of peak saturated
emission current. This potentially allows 3 amperes total emission
current flowing in the filament (if the grid was driven to hundreds
or thousands of volts positive), divided between four vertical wires.
(The filament looks like an "M".)
Emission current at the bottom of the middle wires center point
(this is the electrical center of the filament) would be zero, and
current at the two outer ends of the filament (at the point where the
filament connection exits the tube) would each be 1.5 amperes.
The total emission current flowing into the tube would be 3
amperes.
The primary physical force driving the wires would be between two
adjacent wires. This would be the additional force produced by the
field interaction of 1.25 amperes in one wire moving in a direction
opposite .375 amperes in the other wire.
This allegedly LARGE force (from one conductor carrying 1.25 amperes
while another carries .375 amperes) supposedly SNAPS the filament
like a twig, while the 4 amperes of normal filament current (evenly
distributed in the series wires, so each opposing wire has four
amperes creating a magnetic field) has no effect at all.
We are supposed to believe the wires are so close to breaking that an
additional few percent of magnetic induced force (a pull of a few
grams) breaks the wires, even though that small amount of additional
current would only occur if the oscillation was so severe it drove
the tube into complete saturation!!!
Anyone want to run 4 amperes through a linear wire and try to pick up
a steel pin 1/4 inch away?
I do have a personal opinion about this. Such obvious non-sense
theories are a waste of time and energy. They demonstrate how
quickly some people leap to conclusions without thinking about the
problem at hand, and give answers that can't be supported by physics.
73, Tom W8JI
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