The CV138 UK prototype was the 6F12 and subsequent commercial designation was the 6AM6, however the most usual Type designation for this valve is the EF91. It was (almost) the universal amplifier pentode of the early post war era, many manufacturers made versions of this standard valve. The version shown here was made by Brimar and is labelled for service use.
The original CV138s were made in the US in the mid 1940s. UK production was all post war.
The valve is a high gain, high impedance, variable mu, screened pentode. It could be used from AF to VHF up to about 200 MHz and was found in large numbers in most types of electronic equipment. In radio it was used for RF and IF amplification, for oscillators and frequency multipliers.
The electrodes fill the width of the valve. The anode is formed by two rectangular plates at either side of the valve. The other two sides of the box being completely open. All three grids can be clearly seen and all three grids have separate pins, unlike so many valves that strap the suppressor grid to cathode internally. The cathode is a flattened tube, g1 is closely wound and very near the cathode. The screen grid, again of thin wire, is a flat sided helix. The final grid is placed halfway between cathode and anode and is of a much looser pitch than the other grids.
Physically it is a miniature valve with the envelope having a diameter of 17 mm. Excluding the B7G base pins the valve stands 47 mm tall.
References: Data-sheet & 1040 Type CV138 was first introduced in 1947.