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

Yes, of course. Everything goes through cycles of burn-in and warmup. 

Here are just a few sentences (verbatim), from a letter that Brent Jesse includes, when you receive tubes purchased from him:  "New tubes need a break-in period before they can exhibit their true sonic character when used in audio circuits.       Preamp tubes especially benefit from a good break-in.       This period may vary widely, but NOS vintage tubes usually need at least 48 hours of use, sometimes up to 100 hours.       New current production tubes need 24-48 hours typically.       Good break-in can be accomplished by either leaving your tube unit turned on for 2 days in a no signal condition, or you can just enjoy the tubes for several hours each evening and they will be broken in after several weeks."         An opinion from another highly regarded (personally, at least) source, on power tube burn-in: https://tctubes.com/power-tube-testing.aspx        Upscale burns theirs in for 72 hours, before testing:  https://upscaleaudio.com/pages/test-equipment       interesting:     https://www.youtube.com/watch?reload=9&v=jwmDf5bSRMQ