John Gibson contacted us to say his Eddystone 840C worked nicely as purchased except that the magic eye was ineffective even on strong signals. He said that examination of the DM70 data sheet reveals that for the tube to be operational, its plate voltage needs to be held at about 60 volts, half the high tension level. Any higher and the tube is de-sensitised and barely works. The solution is to wire a 10K 1 W across the existing DM70 plate resistor and another 10K 1 W from the plate to ground (0 V). The plate of the DM70 is now clamped at 60 V, and the magic eye works fine.

(EUG can take no responsibility for the accuracy, safety or consequences resulting from this advice from John but offered in the spirit of the EUG group)