The N709 output pentode is the M-OV offering of the 12 Watt dissipation audio output valve standard. The Mullard EL84 has the same design specification and both designations are present on the glass.
The performance figures given here refer to class A operation. When used as a pair in push-pull with 43% distributed loading in the output transformer to feed the screen grid (g2) the output could reach 10 W with 0.9% total distortion. The effect of distributed loading was to introduce negative feedback directly within the output stage.
The discolouration inside the glass indicates that this has been worked hard.
The radiating fins on the control grid and the discolouration where electrons have bombarded the envelope.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 70 mm tall.
References: Data-sheet & 1040. Type N709 was first introduced in 1953.