Yes they do! Good Burn-in will assure maximum lifetime and develop the final sound. These tubes are burned-in initially. This burn-in process will be extended in the first 50-100 hours under normal (home) use conditions. This means you must switch off the amplifier after each use, and in the beginning not use the tubes longer than 4 hours at one time. Many short use periods have a better result than few long periods. The tubes need the „cold“ periods in between for best formatting of the filament. Some occasional blue glow effects will disappear during burn-in, or may stay. This is not gas, and it is normal. Use different loudness levels from the beginning, and increase the maximum loudness gradually. If tubes with very little use were switched off longer than 12 months, is necessary to repeat the burn in."