There can be some inrush protection inherent in the separate
transformer. The resistance of a tungsten filament is 12 to 15 times
higher at operating temperature than at room temperature. This change is
roughly proportional to temperature so any low heat of the filament will
reduce its current when the step start on a single transformer kicks in
to the high voltage. The inherent current limits of a separate filament
transformer will limit the filament surge current though not as
effectively as a step start or a true current limiting transformer which
is required for higher power tubes.
There is a lot of change of resistance before the tungsten filament is
hot enough to glow.
If anything the transformer filament winding should be on the wimpy side
to act as current limiter. E.g. to have a bit poor voltage regulation.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson. Reproduction by
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