The ECC84 is a VHF double triode, and apart from the heater is electrically equivalent to the PCC84. The ECC84 was designed as a cascode low noise RF amplifier.
The 6.3 Volt heater suggests more of a radio focus than television and Band II FM radio reception would seem the target market. probably for quality Band II radio receivers.
One physical feature of this valve is the very small anode cavities.
Seen from the side, the two triodes are separated by a dividing screen that is shaped Z fashion to give maximum isolation. The small box anodes are open at the sides.
Face view of one triode with the screen behind. The anode is formed from two plates stitched together. The back plate is flat.
The heater is coiled and forms a single loop in the cathode.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 48 mm tall.