Originally Posted by cotdt
that doesn't explain why some tubes flash while other tubes of the same make and model do not. seems to be "normal" in the same way that diseases and illnesses are normal in humans.
Because they are NOT the same make and model. There is a incredibly huge amount of variation between tubes which are nominally the "same make & model", for instance a Philips 12AX7. The first issue is which year, many of these tubes had decades long production runs and the construction methods, materials, and actual physical construction changed over the years. Now that you have the production date picked out you need to find the factory codes, if any, and look very carefully at the physical structure of the tube. Two Philips tubes of the same year & model could've been made in separate factories, the machinery & construction methods will vary slightly and the tubes won't be exactly the same. One might have two spot welds holding the getter in place while the other has only one, and so on and so forth.
Now if you have tubes from the same production facility made at the same time and the physical structure looks identical, I'd be very surprised if some of them flash while others do not. I have large factory cartons of various tubes, if one of them flashes it's almost guaranteed that the entire lot will flash.
Going back to why they flash. For whatever reason, and there's been a great deal of speculation on this, some tubes were built with a thin spot on the tube filament wires, usually just below where the wires enter the cathode sleeve. When the filament is cold its resistance is very low, and on powerup it sucks down a large current which is forced to flow past the thin spot. The thin spot heats rapidly and glows orange to white for a few moments (this is the flash), after which its resistance and the resistance of the warming filament drops the current to its nominal values, ending the flash.