M-OV had trouble designing the MPT4. Not only did it take them a clear two years to produce technically successful copies of Mazda's landmark AC/Pen but thereafter they had to redesign it several times (including the beam tetrode version MKT4) over the next 5 or 6 years.
This exhibit has the original screw-on side cap. See alsoMPT4 for more details.
The bright cylindrical anode with ridges to give structural strength. The side terminal looks to be original but has lost the outer Bakelite insulation.
The dome carries the identification etched into the glass. The electrode assembly is held in place by a shaped mica that has fingers that are an interference fit to the glass.
The cathode tube is a rectangle with rounded ends. The seam can be seen. The second mica holds the control and suppressor grids.
The classic envelope is 50 mm in diameter and, excluding the B5 base pins, it is 113 mm tall.
References: Data-sheet & 1043. Type MPT4 was first introduced in 1931.