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.
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 picture shows the grid radiators at the top and the ribs pressed into the anode for greater rigidity.
The side view of the anode seam.
A close-up of the radiating fins attached to the control grid support rods. At the centre can be seen the top of the rectangular cathode tube.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 68 mm tall.
References: Data-sheet & 1040. Type N709 was first introduced in 1953.