Do vintage output power tubes - Mullard, Amperex, Telefunken etc - ever red-plate ?

I understand, most often red-plating happens right after you turn the amp on, though it can happen at any moment.

Very recently my friend of 30+ Years, who is also an EE of longer years and is my  Tube EE, was doing a spruce up to a Amp' brought in for a little TLC rejuvenation.

The Amp' is quite well known in the UK and has no horror stories associated with it. This same Amp' ignited when undergoing added parts reliability testing.

My EE has always strongly suggested the Tube Amp or Tube device are always with a eyes on them when the power is Power On.

I have followed this advice for 30+ Years and have no horror stories to tell.

I have had a few Horror Valves that proved to be totally awful. 

When red-plating happens, how many seconds do you usually have to turn the power off before meltdown event ?

Many moons ago I purchased a tube amp that to bias you placed a very tiny screwdriver deep in a very small hole in the case and turn the slotted screw until two red leds glowed at the same brightness.  It was very difficult to see the screw as it was recessed.  Basically, you gently turned the screwdriver until you felt it go into the slotted screw head and then turn to adjust the bias. 

I did not know that the previous owner had changed one of the two bias pots, so instead of being a slotted screw per the instructions it had a phillips head.

Long story short, I apparently broke the bias pot that fell into the circuit and then proceeded to cause the amplifier to smoke and the tubes were glowing like the 4th of July.  I reached around back and switched the amp off which did absolutely nothing to stop the fireworks.  Unplugging it did.

I thought the amp was toast, but was amazed at how inexpensive it was, relatively speaking, for my then tech to repair.  But that was a small town superb tech who worked out of his house, but later decided to only work on commercial equipment.  

Never had another problem with that amp or any of my other EL34 amps.

It's not the Tube that is the concern, it is the Output Tranx's that are the major concern, I am reliably informed these can perish from a Red Metal Valve .

My own ones on the 845's are hand wound by an individual known to have similar skills to Mr TDP himself and both my Tranx Winder and Mr TDP are passed.

The custom Tranx's would not be obtainable again, hence the Amp's could be brought on with new Tranx's but all that care for the production and matching for the circuit is lost.