Do tubes require burn in?

50 hours should do it with most tubes.

A major change usually takes place within the first 24 hours. After that its much more about system synergy but they do continue to change some for a while. Power tubes especially.

Yup 20-50 they BREAK IN.. LOL.  Pulls that are rolls (but broke in) give it a good listen 2 - 4 hours a couple of times before you say "this isn't working".  I seen a LOT of valves pulled WAY before they should have been..They have to build a little "SKIN".  If your a pin painter, a week, for the graphene to really work..40-100 hours or so..Be patience..

newbee is right on the mark. 24 hours... pretty close there, failures and microphonics, will show their nasty heads.

Regards

Yes, and, low voltage in a tuner, may take longer, like 100hrs or more... give it time. 

yes they do, depending on valve and circuit.
G