My Mullard / Amperex 12AX7 ECC83 makes a brilliant flash when the amp is turned on - is this a bad tube?

Nope - the "flash" is perfectly normal and it's a trademark characteristic of Mullard / Amperex tubes. There are a couple other brands that have a similar flash but I'll discuss Mullard / Amperex tubes specifically since they are my store specialty.

The flash occurs on one side of the filament wires between the bottom plate and the inside bottom of the tube and this phenomenon only occurs when powering up from a cold start.   The sudden influx of current on the cold heater filament encounters very little resistance along this wire (hence the sudden burst of light).  So in effect one side of the tube always warms up 1st,  the second takes a while to catch up but before long the heat is evenly distributed (and your amp comes to life).

Turn off your amp - and restart it again. You won't see the same flash if the tubes are still hot - it only happens on cold startup.

Does the flash damage the tube?

Highly doubtful - my inventory of vintage Mullard / Amperex 12AX7 are 40-45 years old and while they don't test NOS they usually show ~ 80-90% of new on my calibrated B&K tester (which holds 1600/1600 as the value for a NOS 12AX7).  It's the hours of preamp overdrive that kill a 12AX7: not a momentary startup flash but the latter can be startling if you don't know what's happening.  The first time it happened to me I thought I fried a tube and went looking for a replacement.

US made 12AX7 (GE - General Electric, Sylvania etc) have a slightly different build inside and the warmup is slower & more controlled.  Therefore you won't see the brilliant flash you'd normally get from a vintage Mullard or Amperex tube.   You don't get the tone either in my opinion - but that's a review for another day.

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