Type 6BW6 is essentially the once-ubiquitous Octal type 6V6G squeezed into a B9A envelope. The nominal ratings of the two types are identical. Not surprisingly, 6BW6s usually run (very) hot and require good ventilation if they are not to 'die' prematurely.
Since the whole idea of making B9A versions of standard Octal radio valves was to enable sets to be made smaller, overheating was common. Note that the once white markings on the envelope are beginning to 'toast'. Of course, the manufacturers shed crocodile tears all the way to the bank as they struggled to satisfy the demand for spares.
The 6BW6 was most common in radio transmitters for the audio power stages of the amplitude modulator. In the latter application, with 315 Volts on the anodes, a pair would produce 30 Watts in class AB1.
The 6BW6 can also generate RF power at a few tens of megahertz.
In the 1960s this valve was still current production for its manufacturers Brimar.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 58 mm tall.
References: Datasheet & 1040. Type 6BW6 was first introduced in 1950.