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 direct equivalence of the CV2135 with the 6BR7 is demonstrated by the faint etched Brimar and 6BR7 on the glass and the white paint of the CV designation.
Extra mica sheets and significant internal screening are features of the design.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
References: 1040 & datasheet.