The Ever Ready A80A is a Mullard FC4 made under contract. The FC4 was the first British produced Octode.
The A80A is a self oscillating mixer for frequency changing in radio superhet receivers. The oscillator has to generate 12 Volts peak to peak for correct operation. It would typically have used a 90 Volt oscillator anode voltage and up to 2.0 mA anode current.
The oscillator would use g1 as control grid and g2 as anode. Of the six grids the final one was a suppressor, g3 and g5 would be screen grids, and the signal would be applied to g4, hence its position as top cap connection.
The reverse is plain gold coloured zinc paint.
Through the dirt the top mica can be seen. This holds the electrodes firmly to the envelope. There is a second mica below that supports the tops of the electrodes.
The classic envelope is 45 mm in diameter and, excluding the B7 base pins, is 113 mm tall.