The ECC84 is a VHF double triode. Apart from the heater it is electrically equivalent to the PCC84. The ECC84 was designed as a cascode low noise RF amplifier.
One physical feature of this valve is the very small anode cavities. The image also shows the screen between the triodes.
One of the triodes in close-up. In addition to copper grid supports the spiral wound heater can be seen with and without its insulation.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 48 mm tall.