I found vacuum tubes to be the most forgiving devices ever created.
I have sat and watched a horizontal output tube, when it had lost the oscillator, begin to glow red hot until the glass envelope started to collapse.
I shut it down, repaired the oscillator, and fired it back up after letting the output tube cool off a bit - it ran like a champ.
I have sat and watched a horizontal output transistor fail in a millisecond when it lost the appropriate drive.
I have seen good tough amplifier circuits "improved" over the years to become fragile finicky circuits.