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.
tube burn mcintosh pre-ampIs
Is there a rule for burn in on a mcintosh c-12000. This is a tube and solid state pre-amp. From what I hear it would be days or not at all. Please chime in.