The 30P12 beam tetrode was designed as a single ended audio amplifier output valve for television receiver use or as a TV frame scan output valve. The Mullard Type designation of PL801 is also on the envelope. The prominence of the 30P12 suggests that Mullard were secondary makers of the design.
To operate successfully on the low anode voltage available in a TV receiver the valve has to operate at higher current to achieve the target output power of around 2 Watts.
The copper supports of the control grid are connected to the carbonised heat sink seen above the top mica.
The hole in the end wall of the anode reveals the beam plates.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 69 mm tall.
References: Data-sheet & 1040 Type 30P12 was first introduced in 1958.