The Mullard PA1 is a bright emitter audio output valve that dates to 1924. Construction follows the pattern of the R Type with a vertical anode tube and a helical grid and single strand filament. The size is larger and similar to a small transmitting valve of the time such as the O/20.
The PA series from 1924 consisted of three valves: PA1 for highest output, PA2 for medium power and PA3 as an upgrade to the ORA or RA.
The pinch has extra glass insulation/support for the anode and grid wires.
The glass has the earliest of the BBC stamps etched into the surface. This stamp was used from 1/11/1922 to September 1924. The PA1 dates from 1924 and therefore this is one of the first produced.
The BBC stamp.
The grid is a helix that is spot welded to a stiff wire support on one side only. The filament wire is held in a closed fold in the lower support. Tungsten has too high a melting point to be spot welded.
The pinch, electrode supports and red platinum wires enclosed in the pinch. The glass additions around the anode and grid supports can be seen to be sections of glass tube that have been melted into the top of the pinch. This is a hand made valve, and probably expensive.
The top of the filament has broken just below the upper support. The support is made of tape to form a spring to tension the filament. This tape is then folded over at the lower end and welded to the support wire.
Another image of the envelope.
The etched markings are clear in this final image.
The balloon envelope is 55 mm in diameter, and excluding the B4 base pins is 120 mm tall.
References: 1003 & 1004 Type PA.1 was first introduced in 1924.