12BH7, 12BH7A, CV5042, 13D6, OUTPUT, B9A

The 12BH7 double triode was first introduced in 1950 as a TV frame oscillator, vertical deflection amplifier or output valve demonstrating a medium-mu characteristic curve. However, it's pin compatibility with the ECC81 - 83 range of twin triodes along with its higher anode dissipation and large rigid anode construction makes it eminently suitable for high grade audio usage.

There is much folklore about the 12BH7 as opposed to the 12BH7A variants and the distinction between the two is simple with the 12A differing from its younger brother ONLY by incorporating a controlled filament heater which was introduced to stop flash-burnout on series connected heater chains in 1950s designs of television receivers.  This difference imparts no discernible sonic improvements nor detractions.  Accordingly, in my opinion both variants are interchangeable.