The EM83 is an unusual dual magic eye valve. Electrically it is similar to the EMM801, but the fluorescent target consists of two metal cowls inside the tube as opposed to the fluorescent screen area deposited on the inner surface of the glass envelope employed in the EMM801.
Although made by Mullard this exhibit carries the Bentley Acoustic Corporation lettering.
The two sections of the valve are separated from each other by a screen which is connected to pin six. The lower portion of the valve has a black coating to obscure a direct view of the electrodes. The display takes the form of a luminous band which rises and falls from the bottom of each cowl.
The chief use of the EM83 is as a level indicator in studio recording equipment and as a stereo indicator in various stereo tape recorders.
The heater requirement is 6.3 Volts but a 100 mA UM83 was also made. Both have a B9A base.
The two cowls seen face on with the screen between them.
Looking down into the valve we can see the control grid and the oxide coated cathode tube.
The side window envelope is 22 mm in diameter, and excluding the base pins is 59 mm tall.
References: Data-sheet, & 3002. Type EM83 was first introduced in 1958.