tube burn mcintosh pre-ampIs

IME any tubes MUST burn in for at least 24 hours before you can be sure that what you're hearing is a fair representation of what the tubes' sound will be. For small tubes much change beyond 24 hours will be small (unless you're super anal) power tubes can take a few more days.  FWIW.

I just like to play FM in the background when I burn a new piece in. For me it takes the placebo effect out of the equation because I’m not being critical about what I hear and it lets me acclimate to the possible change in sound.   

     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."

     interesting: https://www.youtube.com/watch?reload=9&v=jwmDf5bSRMQ