The M-OV type R5V was the first British high-vacuum receiving valve to incorporate a sprung 'inverted V' filament. As far as we know all R5Vs were gettered to ensure a high vacuum and allow use with up to 120 V on the anode. This, and its (relatively) low Ra value (30,000 ohms) enabled the R5V to provide sufficient audio power to operate a horn loudspeaker at (just) acceptable volume (tens of milliwatts).
Type bright emitter R5V was successful both technically and commercially. Like the type R it remained listed until the outbreak of WWII and many samples survive this exhibit of ours has a working filament.
This is a later version than R5V and has a Bakelite base. It has a faint etched BBC stamp of type 2 indicating that it is Entirely British Manufacture and thus dates to between September 1924 and 1927.
The type R5 was fitted with an acorn like cap (base) but the R5V was different in that it had the standard commercial B4 base. The inverted V filament was introduced by Captain Round for military valves and had also been used by some American valves. The R5V was the first M-OV valve designed for commercial rather than military purposes.
The red insulation tape is present as the cement seal between base and glass has failed. The etched logo proclaims that it is a Marconi Valve.
The anode and silvered gettering behind.
The spring holding the top of the inverted V filamnet can be seen. The grid is wound onto a rigid support that loops above the anode.
The balloon envelope is 44 mm in diameter and, excluding the B4 base pins, is 84 mm tall.
References: 1005 & 1004. Type R5V was first introduced in 1922.