Type E1138 is a WWII VHF/UHF power amplifier/oscillator. Essentially this valve is a development of type 6J5GT (q.v.) with grid and anode taken out through short and stubby low-inductance leads. To improve the operating frequency two top caps are used, one for anode and one for the grid.
In the UK the same design process resulted in the DET20 which was named type CV6 for government procurement. In both cases the need was for a VHF/UHF oscillator triode that became widely used in WWII radars. The μ value is 20, which is about favourite for RF oscillators.
The Type designation octagon - almost nothing remaining.
It took almost 30 minutes to read the designation originally, however, with the microscope and different lighting this image was achieved in a few minutes.
The anode connection side.
An enhanced image showing the anode connection and dirt on the envelope.
The heater connects at each end but is folded and passes through the cathode tube three times.
The grid connection end. The cathode tube is also connected this end. The anode looks very much like a 6J5. Additional rods on either side of the anode are mechanical supports.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 72 mm tall.