Vacuum Tube Break-in

But tubes get hot and metals change when they glow.  I do believe tube burnin is significant...it isn't always positive.  

Agree with @carlsbad and I'll add that the circuit and the entire chain may determine if you hear these changes. I use all NOS or "tests as NOS" and do hear changes in tube sonics, I trust my ears. 

I did try new Linlai 6SN7's which have a 200 hr. burnin (according to manufacturer). I did hear many changes from bad to worse to better. After the 200 hr. mark, I didn't like their sonics even though they had improved. 

     This is a verbatim quote, from a letter Brent Jessee sends, whenever one buys valves from him:

                   "CARE AND FEEDING OF NEW TUBES: BREAK-IN
     New tubes need a break-in period before they can fully 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 in a no-signal condition for 2 days, or you can just enjoy the tubes for several hours each evening and they will be broken in after several weeks.

     New tubes, especially NOS vintage tubes, may sound a bit edgy at first, but after the break-in period will mellow out and sound wonderful."

IMO everything has a break-in period but results heard vary depending on your listening skills and the component.

I acquired a quad of Genlex KT88s for my VAC amp at one time to replace the factory tubes.  When I initially started using them I disliked the sound immediately and for the next 20 hours compared to the factory quad.  I ultimately swapped them out and put them away for a few months.  When I tried them again they were still not what I hoped for but suddenly over the next 10 hours they opened up and smoothed out. I thought I had given them enough time the first trial, but I eventually grew to really enjoy them.  Break in?  I don’t know, but many times there seemed to be improvement to be had in the first 20-40 hours of both power tubes and small signal tubes in my systems