The PT16 is a directly heated audio output pentode from 1933. It was believed to be obsolete by 1934 when indirectly heated output valves were being successfully made. The first of the successful indirectly heated audio output pentodes was the Mazda AC/Pen.
The top of the envelope. The filament tension springs are held in the glass bar at the top of the picture. The three grids can be clearly seen.
The box anode has four rod supports at the corners. The oxide coated filament is broken and one of the strands can be seen. The release of tension on the right hand spring can be seen to have caused it to twist.
The base of the anode. The four supports are projected down into a clamp that fastens around the pinch. The clamp is tightened by a nut and bolt - clearly a hand operation adding to the cost of production.
The balloon envelope is 65 mm in diameter, and excluding the B5 base pins is 142 mm tall.
Reference: 1043. Type PT16 was first introduced in 1933.