Recently gained interest in Russian military surplus tubes, and inevitably the power tubes that tops the list include: GM70, 6C33 and GU50, etc. Here are some info on the 6C33 tube.
It appears there are two kinds of the 6C33; 6C33C and 6C33C-B. From reading various tech info, the 'B' designation seems to imply 'vibration proof, high reliability'. On the website above, the operation time (presume rated life) is shorter for the 'B' version at 750h rather than 1000h. The 'B' version seems to have lower internal resistance does have better acceleration and impact ratings. The curves for the two appear to be near identical.