MULLARD EFMI TUNING INDICATOR or MAGIC EYE, 1930's

Based on the EM1 Item A0683 this indicator also contains an AF amplifier facility, not actually two valves in one but with amplification incorporated with an amplification factor of 60 maximum. This can reduce the number of valves in a circuit by one in many applications, making this valve unique. The tube shown has a concave disc inside that is illuminated on the sides of the bottom edge as the signal is increased the two illuminated edges move towards each other in an arc until they touch at full signal.

A0686