The Osram PT4 is an output pentode that was introduced in 1931. It gives less output than the MPT4 but is for use where the indirectly heated valve cannot be used.
This exhibit has the original balloon envelope. Later production moved to the shouldered or classic envelope.
The box anode is substantial and held by rods in each corner. The rods extend down to a clamp around the pinch stem.
The production is inbetween the use of the glass rod for insulated supports and mica sheets. Here both are employed.
The filament can be seen to be oxide coated and held in hooks attached to spring arms that themselves are fixed into the glass rod.
The pinch end. The control grid and suppressor grids are visible but the screen grid is difficult to see. The U support at the base of g3 is mirrored by a similar U shaped support for g2 seen beside it.
The balloon envelope is 65 mm in diameter, and excluding the B5 base pins is 142 mm tall.
References: Data-sheet, 1043 & 1040. Type PT4 was first introduced in 1931.