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
