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
Vacuum Tube Break-in
Is there such a thing as tube break-in? I been laying with many tubes in about 20 tube amps for 40 years and never heard a difference in a new tube vs same tube with 50 hours use. I always bought and used NOS tubes, not used. I believe if a difference is heard with break-in, it is the placebo effect.