The 3B28 is a half wave gas rectifier with low pressure xenon as the gas. The maximum Peak Inverse Voltage is rated at 5,000 Volts. Although the average rectified current is 500 mA the peak value can be as high as 2 Amps.
The screen is connected to one side of the filament and the anode is within and cannot be seen.
The large top cap has a ceramic ring on the outside of the envelope. Within the envelope the anode connection, also the anode support, is sheathed in glass as it passes through the screen.
The wide glass tube envelope is 51 mm in diameter and, excluding the UX4 base pins, is 138 mm tall.
Reference: Datasheet. Type 3B28 was first introduced in 1952.