The AC5/Pen is an audio output beam tetrode on a B7 base.
With a cathode resistor of 180Ω and a 5,200Ω load the output is 4.85 Watts with 7% distortion.
The coating inside the glass makes seeing the electrodes difficult but in this example there is sufficient clear glass at the base for an oblique look inside.
Looking up from the base. The cathode is a wide flat box and the grids can be seen to pass flat across the face of the cathode. The heater is heavily insulated and passes four times through the cathode as a flat M configuration. The anode has additional heat sinking below the lower mica.
The classic envelope is 54 mm in diameter, and excluding the base pins is 108 mm tall.
References: Data-sheet, 1040 & 1043. Type AC5/Pen was first introduced in 1938. See also1938 adverts.