Mullard flash....or not???

[GYMusic]

Flashover arcing is usually evident in RF linear amplifier circuits. The reason is usually caused by a parasitic oscillation - sets the tube off. In an audio circuit it could be due to the sudden filament inrush current. Not sure the jury is out on that one, but a very interesting topic of discussion.

Search - AG6K, Richard Measures, inrush filament current,

 

[JamesG]

I've seen this flash from three different radio tubes, each in a different radio.
I recall two of them are little 6AL5's.