The HBC90 is a double diode triode for receiver applications. It is of medium impedance and very low current.
The cathode tube extends above the triode structure and two bright metal straps can be seen in the picture, these are placed either side of the cathode and form the diode anodes. The cathode also connects to the getter support rod seen on the left. The purpose is to connect the cathode to the screening tray that sits above the top mica.
A face on view of one of the diode anodes. The triode anode is the dark cylinder in the centre.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
References: Data-sheet & 1040. Type HBC90 was first introduced in 1950.