The ME1400 is an electrometer pentode for use in high resistance circuits for such applications as PH meters, photocell units and valve voltmeters.
The input current into g1 is of the order of 10 to the -11 Amps. The maximum anode voltage is 90 Volts and the maximum cathode current is 1.0 mA.
Gerry Horrox of Crowthorne Tubes has provided information that the special quality ME1400 was developed from the EF37A and in addition to its specific design use as an electrometer pentode it functions well in studio audio equipment where it would be running at 6.3 Volt on the heater and up to 250 Volt on the anode . Edition 20 of the AVO manual gives the heater as 6.3 Volt. The second set of operating conditions reflects this audio use the first set are from the original Mullard data.
We have another ME1400 that has lost its metal paint. Here we have a close-up view of the electrodes. The suppressor grid has a very open pitch.
Grid 3 is wound on notched copper rods and all grids extend beyond the anode cylinder. No attempt is made to darken the anode as this valve operates at very low power, almost an electrostatic device when one considers the very high input impedance.
Tilted slightly the winding arrangements of the grids are revealed.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 84 mm tall.
References: Datasheet & Gerry Horrox. Type ME1400 was first introduced in 1950.