The EFM1 is an oddity. It combines a tuning indicator with an audio variable μ pentode. The attached pdf gives a very interesting description of the valve and its use. The indicator is based on the EM1 tuning indicator. This valve could save on valves in a receiver but the design was a compromise and did not replace the more conventional indicators.
The pentode below the display bowl. At the top can be seen the control and screen grids.
The display bowl with a thin blue coating inside the glass.
A closer view. The helix around the cathode prolongs the life of the cathode.
The classic envelope is 36 mm in diameter, and excluding the Ct8 base pins is 78 mm tall.
References: Data-sheet & 3002. Type EFM1 was first introduced in 1938. See also1938 adverts.
