On offer today we have the famed CV138, the high grade miltary variant of the EF91 6AM6. These are Mullard/Thorn-EMI/GEC manufacture and carry factory production codes from 1952 - 1959. These are all new devices which were bulk packed for the military and hence come white boxed after their liberation by myself from a Carlisle storage cache.
These are an ideal valve for the famed Little Dot headphone amp range amongst other applications.
The EF91 was introduced in 1947 and in it's high quality military iteration, the CV138 was the amplifier pentode of choice the early post war era. The valve is a high gain, high impedance, variable mu, screened pentode which found uses as diverse as AF to VHF applications up to about 200 MHz as well as RF and IF amplification in radio oscillators and frequency multipliers.
Construction is interesting and a triumph of miniaturised thermionic engineering. 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 are upen such that 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.