The N78 is a 1953 audio output pentode with a high mutual conductance, suggesting that full output can be obtained with a low input signal.
In class AB1 push pull a pair of valves can deliver 9 Watts at 4.5% distortion. If connected as triodes the pair can deliver 6.3 Watts at 1.6% distortion.
M-OV produced these valves and branded them with paper labels: GEC for professional sales and commercially, Emitron, Marconi and Osram.
The Marconi label.
The three wire grids have the oval shape of the anode. The cathode has flat sides.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 60 mm tall.
References: Data-sheet & 1040. Type N78 was first introduced in 1952. See also1952 adverts.