For those who are interested:
1.) The filament starting current can easily be 30 or 40
amperes RMS (50 amperes peak) for thousands of turn on
filament cycles and not cause damage. As a matter of fact
when Eimac tried to simulate inrush damage to 3-500Z's they
clamped a 200-amp transformer right to the pins of tubes and
cycled them over and over again without any sign of damage.
2.) The filament current is equal all through the filament.
It does not taper or have peaks or minimums, so the entire
structure contributes to the MMF.
3.) With a "oscillation", the peak current with even the
worse imaginary oscillation we could have is still limited
to the peak emission current of the filament. That emission
current is about 11 amperes PEAK. That 11 amperes is
distributed over the area of the filament. It has the
highest value at each END of the filament, and each end only
has the possibility of about 5.5 amperes or so PEAK. The
emission current at the very center of the filament is
actually at a null point. It isn't hard to picture this if
you draw it on paper.
We are supposed to believe a filament that easily takes
hundreds or thousands of 40 ampere turn-on surges that we
know cause high current across the entire structure won't
cause harm will suddenly be harmed by an imaginary saturated
emission of 11 amperes (that would take a thousand volts of
grid swing) that has maximum current only at the ends (where
each end has about 6 amperes peak).
If common sense fails us and we still intuitively want to
believe that a few amperes of emission current can bend
things that 50 amps of peak filament supply current can't
bend, we can fall back on simple formulas. It's easy to
calculate the MM force between conductors. All we have to do
is use Biot's law. We don't need to make wild guesses based
on comparisons between starting currents of multi-horsepower
motors over many feet of suspended cables that have nothing
to do with a few inches of imaginary 6-ampere currents in a
vacuum tube.
Try this link:
http://library.thinkquest.org/16600/advanced/currentmagneticfields.shtml
In a 3-500Z, it comes out to a few grams of force that peaks
about 1/5th and 4/5th of the distance from the ends, and is
almost zero at the middle. The force caused by normal
filament current is several times larger.
73 Tom
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