How to determine whether tinpest layer round an end of the filament or a
physical break?, just durious as to failure mode. 200mm F type. Breaking
the glass is likely to disrupt the failure situation. Filament seems to
be in proper place , diagonal along the visible part of the tube and no
observable movement on normal handling. No circuit failure to have
caused long term aging stress failure of the filament at one end. I've
not tried 500V/1000V insulation test (could perhaps marginally "weld"
the failure) but DVM-R 30M