Preamp tube. Bugera valves are subjected to rigorous testing to ensure you'll get great tone right out of the box. First, this 12AX7B preamp tube must pass a 48-hour burn-in where it's pushed to the limit. Next, a vibration table test ensures the structural integrity of each 12AX7B will stand up to your gig schedule. After its been warmed up and shaken around, each tube goes through computer-controlled testing to evaluate 12 more operating parameters, so you know when your new Bugera 12AX7B preamp tube arrives, it'll be in perfect operating condition.