Tubes, being physical devices, can be manufactured with physical defects. Some may not be noticed until the tube has completely warmed up and can be overlooked when they are tested since testing doesn't usually allow sufficient warmup time.
That sounds like what happened with your tube, especially if it was only one. I had a tube do something similar one time - the grid was loose and when the tube warmed up it contacted the cathode, killing the bias causing it to overconduct and the plate to glow red. I caught it in time... luckily.
The model T has either 47 or 50 ohm 5 watt resistors between the plates of the power tube and the output transformer. The purpose of these resistors are to act like fuses by burning up if an output tube conducts too much current and protecting the output transformer. DO NOT replace or allow a tech to replace these resistors with something more than a 5 watt resistor or you may be buying a new output transformer!!
The same goes for the 1.5K resistors on the screens of the output tube.
Take it back to the tech. Have the tech watch it for at least 10 minutes and determine what the problem is - could be a bunch of different things.
