The M-OV type R5V was a landmark valve in that it was the first British high-vacuum receiving valve to incorporate a sprung 'inverted V' filament of the type which later became universal. The inverted V sprung filament had been used in transmitting valves as had the flattened anode.
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).
The paper strip label on this exhibit gives the filament current as 0.7A. In later versions of this valve the current was reduced very slightly to 0.66A in order to match that of the Type R. Type R5V was successful both technically and commercially. Like the type R it remained listed until the outbreak of WWII and many samples survive.
The R5V from Marconi-Osram was a general purpose valve, similar to the original R-type, and made from the early 1920s in considerable quantity. The differences from the R-type were a lower anode resistance and the 5.0 V filament but it is still a bright emitter.
The envelope clearly shows the name Osram, and above that are the words Marconi Valve. Within the envelope the bright nickel oval anode is clearly seen.
The construction has altered from the basic design of the R-type.
The balloon envelope is 42 mm in diameter and, excluding the B4 base pins, is 95 mm tall.
References: 1005. Type R5V was first introduced in 1922.