R. Jepsen wrote:
I'm going to site ohm's law. If the filament is abnormally bright, this would indicate excessive current due to decreased resistance. My best guess is that the 12J5 filament has a partial short and will probably burn out soon.
Kirchoffs Law. It is a series circuit. The current is the same for all of the filaments in the string.
That is, if the current is high in one section of one filament, the current through all of the series
filaments must also be the same.
Cold filaments have less resistance and draw more current, the exposed filament leads will heat
much faster than those bundled in the cathode assembly. That "flash" isn't unusual since the
the voltage on those small exposed filament sections will be abnormally high until the filaments
bundled in the cathodes also reach normal operating temperatures. And those "flash points"
are typically where a tube will fail on turn on. A properly sized PTC device will help extend tube
life but few restorers bother using them.