The YL1550 is a power tetrode that was used in the RF power amplifier stages of the military Clansman tranceiver. As yet we have no operating data.
One of the numbers on the anode is 038-0786. The anode has no integral cooling fins and probably was designed to be conduction cooled with an external block. The Y-799 shows a similar valve with a built-in conduction block.
The anode also carries the GEC logo.
The grid ring is designed to be bolted in place.
The base looks to be close to the B8F used for this type of power tetrode but the pins have solder on them and the shell has tabs. It was probable that this valve was permanently mounted and the whole PA section of two valves, heatsinks etc. came as an interchangeable module.
Another view of the base. The base shell is not connected to the central spigot. The grid ring is separated from the base shell by a thin ceramic disc.
The wide ceramic tube envelope is 35 mm in diameter, and excluding the base pins is 49 mm tall.