becoming more sophisticated and wanted to squeeze as much performance as they could without compromising reliability.
      A look at tube ratings shows that there is about a 10% difference in maximum ratings between each rating system. For example, look at the following ratings:

     The 7189A came out just after the maximum rating system change-over -- its ratings are precisely 10% over the earlier design-center version. This suggests that there is no physical difference between the tubes, at least as far as the maximums go.
     The 6CB6, a TV IF amplifier tube which came out in the early 1950s, was originally rated in the design-center system. Later tube manuals changed the ratings to design-maximum, again precisely 10% higher.
     The difference between the 6L6 and the 1614 span two ratings systems: design-center to design-maximum and design-maximum to absolute maximum. If the tubes were really the same, the 1614's ratings should be 20% higher than the 6L6's. The power is only 10% higher, the plate voltage is 4% higher, but the screen dissipation is 40% higher. The ICAS ratings are quite a bit higher, except for the screen dissipation.

    The 1614 is basically a selected 6L6 intended for class-C RF amplifier use. Its CCS and ICAS ratings are based around this type of usage, hence its different-than-expected ratings. In a sense, the difference between a 6L6 and a 1614 is in the ratings, not the tube itself.
     The 6BG6G is an early TV sweep tube, and the 807 is a transmitting beam tetrode. Both have virtually identical structures - with the exception of the packaging, the same as a 6L6. The steady increase in maximum ratings as more stringent rating systems are used is clear. However, the high maximum plate voltage of the 6BG6G is due to its specified use as a horizontal output stage, which has a low duty-cycle (only turned-on for a very short percentage of time).
      So why all these comparisons? It is just to point out that essentially identical tubes can have different ratings, depending on the rating system used.