Type CV5086 (commercially the 6BS7) is essentially a type 6BR7 with the signal grid taken out to a top cap.
R J Sutherland has been looking at an old Brimar data book and found they recommend the 6BS7 for use in valve voltmeters, pH meters and other electrometer applications. In an example electrometer circuit with an input current of less than 5 pA. Such high input impedance would require very careful circuit design and layout and would pick up any extraneous signal. The 7851 is similar but was designed only for use as an electrometer valve. The 7851 has a 2.5 Volt heater and is designed for very low anode voltages.
BVA and Made in England on the glass. Within there is significant screening of the electrodes.
The notch reveals the suppressor grid helix of this pentode.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 51 mm tall.
