Flash from power tube

If you buy a set of tubes that are new manufacture, its a good bet that they have not been pre-conditioned.

This is the process that essentially cures the cathode coatings in the tube.

Its done by running the filaments for several days and nights with no B+; if B+ is applied the process stops.

This can double the life of the power tube, and reduce premature arcing caused by flakes of the cathode coating (which is conductive) falling off the cathode and shorting to other elements in the tube. 

This is ruining my desire to obtain a set of "winged C" tubes, although there might be some fun in waiting for them to explode…move the furniture away from the amp and get the extinguisher ready.

I had some issues with Winged C EL-34 in my primaluna.

There is no guaranty of longevity unless there is one from wherever you buy tubes, and that applies to NOS and new tubes. The only tube I've had explode in recent years was a NOS Phillips 12AT7 in a previous amp (a Jolida actually) and luckily I was in the room with it (jumped up and turned the amp off) and "thetubestore" warranty was still in effect. The amp was fine (whew). I've used new KT120s, KT150s, Sovteks (which are no longer made I think), JJs, Mullards, etc., and Gold Lion pre and power tubes and liked them, and I've heard very little regarding a lack of reliability with any of those and none of mine have blown (I had a quad of 120s last a surprisingly long time). I currently use new and old stuff, but the old ones are always NOS (Mullard, Amperex, Sylvania), and I'm often surprised at how durable tubes are in general in my guitar amps and pre and power amps. Not sure if any tube gets punished like a tube in a combo guitar amp, poor things.

Thanks for the help guys. Ordered a replacement tube and i'm going to open the bottom cover and see if anything looks fried

For my last retube I used Winged C Sed 6550C tubes and had 1 of 16 fail after a couple of hundred hours at most.  Subsequently I have learned/been advised exactly what roberjerman has said "Current production tubes DO NOT have the longevity of past production"

Buy old production USA, UK, Holland, German, Japanese tubes whenever possible for longevity AND sound quality! Avoid the rest!

It is always good to have spare tubes on hand for quick replacement. Current production tubes DO NOT have the longevity of past production (pre 1970's)  Meaning that they (power tubes) cannot safely handle higher plate voltages. 

A shorted tube. Check the fuses! Hopefully they will blow before any major damage is done (fried resistors and blown caps). Open the bottom cover and look for damage (fried parts). If everything looks good and the fuses are intact you can replace the dead tube with a known good one and try turning on the amp. Be sure to have a load (speaker or power resistor) connected! If the fuses are blown you can try replacing them - but ONLY if there is NO sign of damage! If on turn on the fuses blow again you will have to bring the amp to a competent tech for further diagnosis (shorted power or output transformers).