We burn-in these valves for a minimum of 30-40 hours before matching. The reason we do this is to get rid of any faulty or weak valves and as a pre-grading process to allow the plate/grid mechanism to relax into it's rest position. Tiny differences in construction make for enormous differences in electrical characteristics, and thus if a tube is graded after it's been burned in then the measured characteristics hold true over its lifespan to a greater extent. This seems simple but it's amazing how many vendors ignore it. It's the reason so many people complain about poorly graded valves.