The 12B4A is a low μ triode that was designed for use as a TV video amplifier. The data sheet indicates that it was intended for use within a series heater chain but the heater centre tap allows for a heater voltage of 6.3 Volts to be used.
The rating given here is for use as a Class A amplifier. For video amplifier service the anode voltage would be much higher. The article on transformerless audio amplifiers shows how these valves were used for high quality audio.
One source of distortion from a valve amplifier comes about from the variation in inter-electrode distances in conventional valve design. Planar construction has a flat cathode with the grid passing flat across it and the anode as a flat plate parallel with the grid and cathode. For disc seal valves this construction places the grid across the width of the valve and the grid connection is at that plane. SeeLighthouse Valves for details. As this valve is not designed for UHF frequencies, or above, the construction is a double sided planar construction.
The plates at the sides are connected to the grid supports at the bottom and probably both aid alignment of the grid in assembly and help cool the grid in use. Each anode has been folded to form a flat bottomed U shape, enough metal for cooling but most of it away from the electron flow.
The double sided grid is close to the cathode with a larger separation to the anodes on either side.
The grid wire is gold plated and fixed to copper supports.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 58 mm tall.
Reference: Datasheet. Type 12B4A was first introduced in 1955.