Hi Peter,
The problems associated with series filaments would escape anyone who
considers the accumulated damage to be a current-only time
independent problem.
With a one track "it's the magnetic field" view, you are right. The
damage would be the same to each series tube because the current
would be the same.
The majority of the filament stress is thermal in nature, since
the rapid heating warms some area (generally the weakest area) of the
filament first. When that area gets hot, it's resistance increases
and the I^2 R losses in that area go up...of course that makes that
area hotter still (while the rest of the filament warms more slowly).
With two filaments in series, the weakest and hottest filament
heats the fastest. It has move resistance, and more I^2 R power
dissipation, so it heats even more.
What happen during warm-up is the weakest filament, the one least
able to take the stress, get the most rapid and uneven thermal
cycling.
When in parallel, the lowest R (coldest and thickest) filament gets
the most power, so the tubes tend to heat evenly.
That's why the little tubes in series filament strings, and even
the bulbs in Christmas lamps, fail more often in series systems than
in parallel systems. That's why one of the tubes in a ALL American
Five radio always used to light like a lamp while the others slowly
reached operating temp.
While the "equalizing" winding tap does helps this, it only helps
when the tubes filaments are hot enough that the
accumulated resistance of that "equalizing" path is much less than
the tube's filament resistance.
None of this is good, and the combination of excessive voltage,
excessive current, and unequal heating puts an unnecessary stress on
the tubes.
73, Tom W8JI
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