The 7D8 is an audio output pentode from the mid 1930's. It has a 13 Volt heater at 0.65 Amps, neither clearly for parallel or series heater chains. The advert cites motor car sets as a use - this would make sense of the 13.0 Volt heater.
The 7D8 should operate with 8,500 Ω load and a 150 Ω cathode resistor. Under these conditions the audio output is 3.75 Watts.
The Brimar orange/yellow paint and the pinch.
The cathode is seen as a broad flat box. The inner grids pass flat across the cathode.
An enhanced view of the grids.
The classic envelope is 39 mm in diameter, and excluding the B7 base pins is 99 mm tall.
References: Data-sheet, 1040 & 1043. Type 7D8 was first introduced in 1935. See also1935 adverts.
