Type 6BR7 was carefully designed for low internal noise and, especially, for low internal hum pick-up from the 6.3V AC heater, thus making it unnecessary to provide a DC source of heater current.
These features made it the standard 1960s choice for the input stage of sensitive hi-fi systems, displacing type EF37A which was the previous (British) favourite for this application. As such type 6BR7 remains in demand today for replacement purposes.
The main disadvantage was that it is not much good reducing hum and noise inside the valve if ignorance and clumsy wiring at the valve socket put back what has been so painstakingly minimized. Type EF37A was metal-sprayed and had its signal grid taken out to a top cap. The input signal could, therefore, be taken to the top cap on a screened lead routed inches away from the heater wiring. Type 6BR7 is single-ended with closely-spaced pins and a certain amount of 'know-how' is necessary to keep grid and heater wiring apart effectively, especially where one side of the heater supply and the 'return' of the grid input signal are both 'earthed' to chassis. The professionals knew how to do this but many DIY hi-fi systems using 6BR7s were far from hum-free. Needless to say, EF37As are also still in demand...!
See also EF86.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 45 mm tall.
References: 1040 & data-sheet.