The 12HL7 was designed as a video amplifier in colour televisions. Video signals are high bandwidth and the active device needs an intrinsic high gain before feedback broadens the response. This pentode is built with a frame grid and as a consequence has a high mutual conductance of 21.0.
This image is looking along the grids. Inside the anode is a bright sheet that forms the beam plates. The copper rod is one end of the screen grid. So although described as a pentode the construction is actually beam tetrode.
Above the makers logo can be seen the beam plate ends and above them is the dark rectangles of the control grid cooling fins.
The anode is on the right. On the left is the gettering and the screen.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 59 mm tall.