One of the curious ironies of the MPT4 saga is that the catkin version of 1933 although a 'blind alley', was technically very superior to the glass version MPT4. Comparison of the two photos makes it quite clear that the large anode used in the glass version could not possibly be used in the catkin version and shows the benefit of the external anode design.
The two versions are, in fact, entirely different designs. The compact catkin version was possible because the external anode was directly air cooled and did not reflect excessive heat back to the cathode-grid region of the valve.
It took M-OV about five years to learn how to make large glass power pentodes with grid structures which did not emit electrons, or buckle, due to overheating.
The paint is in excellent condition and the identification including the Marconi logo is printed. The clip round the copper anode and the connecting wire leads from it to the base pin. The side contact is the screen grid.
The side contact with original screw cap.
The data sheet enclosed in the box with the valve.
Reverse.
The thin metal tube envelope is 18 mm in diameter and, excluding the B5 base pins, is 103 mm tall.
References: Data-sheet 1040 & 1005. Type MPT4 was first introduced in 1931. See also1931 adverts.
