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

i would never leave a tube POWER amp on and unattended for an extended stretch of time... definite potential for hazard

most power tubes are run close or at their spec limits, they can go bad all of a sudden, with attendant pyrotechnics in your listening room

my audio research and vac amps have both blown power tubes with a memorable light show in years past... 

A User of Valves in a Circuit being a Audio Design, does have to have a keenness to monitor Valves during Power On. 

Valve Types can offer 'peace of mind' 

Factory Spec' NOS Vintage Mullards are a 10K Hour Valve. 

Modern equivalents do give up, but hours of usage varies  substantially

From the Avatar manual:

- A unique circuit has been fitted individually to each of the output EL34 tubes. In the event that an EL34 enters a ’run away’ condition and begins to draw excessive current, the Sentry circuit will engage and limit the current that the tube can draw to approximately 30 milliamperes ( about 60%  of normal )-

Is it enough to prevent big bang in the listening room ?

I once had SED tube red-plated on me right after I turned the amp on.

The Mullards XF2 that I use keep the perfect bias, as far as I can see, and they appear well-matched, in pairs and all four.

@inna 

A tube can red plate for a variety of reasons, none of them having to do with vintage or new production. 

Bad connection on the tube socket, damaged grid stopping resistor, open bias circuit, failed connection in the base of the tube itself, internal short or leakage in the tube.