The Mullard ME1402 is a subminiature electrometer tetrode with a very high input impedance for sensitive measuring instruments such as pH meters and nuclear radiation dosemeters.
The grid current of the ME1402 is 3 x 10 to the minus 15 Amp when operated in complete darkness and as Mullard instruct. All quoted operating voltages are measured from the negative end of the filament. The first grid is operated as an accelerator grid at about 3 Volts. The second grid is then the control grid and is maintained at about -3.2 Volts. The anode voltage would be 4.5 Volts. The current to grid 1 would be about 250 μA and the anode current 20 μA. The first grid is being used as a space charge grid and in a sense this valve is operated like the bi-grid valves of the 1920s.
To avoid excessive drift of characteristics the filament voltage has to be applied before the anode voltage.
The flying leads need to be left at least 13 mm long before soldering into circuit and must not be bent closer than 1.5 mm from the seal.
The red spot marks the anode lead (pin 4).
The valve comes in a clear wallet with warnings not to touch the base area as touching the base will contaminate it and prevent the design grid current from being obtained.
The thin glass tube envelope is 9 mm in diameter and, excluding the base leads is 44 mm tall.
Reference: Data-sheet. Type ME1402 was first introduced in 1950.