The AC/ME is an example of an early British tuning indicator from 1937. It would be mounted horizontally so that the domed end window was visible through the front panel of the radio. In use the radio would be tuned and when the green display had spread to its maximum the correct tuning had be achieved.
In 1937 both AC and DC mains were in use in Britain, the AC/ME was for AC and the ME920 was for DC mains.
The round screen has a centre electrode. When on a small V of bright green would be seen. In terms of a clock face the V would be from 11 to 1. With a maximum signal the bright green display would extend from 7 to 5.
The valve is built in two sections on a common cathode. The bright tube is the voltage amplifier triode and the bowl above is the display.
The insulated heater is shown on the left passing into the triode. The display bowl is on the right.
The glass pinch shows the multi stage lead. The red section is designed to have an expansion that matches that of the glass and is part of the vacuum seal whereas the tops are stout support rods to support the electrodes and the glass provides mechanical anchor points. In the open end of the pinch tube will be thin wires that connect to the base pins.
The green lettering of the Mazda logo and type designation.
The classic envelope is 38 mm in diameter and, excluding the B7 base pins, is 87 mm tall.
References: Data-sheet & 1040. Type AC/ME was first introduced in 1937.