Sensibly equivalent¶ to:
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.
In the end they solved the problem by buying equivalents from other manufacturers and re-labeling them.
Part of the problem was that M-OV had invested heavily in the azide process during the late 1920s and were either unable or unwilling to switch overnight to manufacturing oxide-coated cathodes of the rating and quality demanded by powerful, for that time, audio output valves.
Another factor was that M-OV were strongly anti-pentode at that time and may have been unwilling to invest in proper, full-scale development in what was, for them, a new and uncharted field. Instead, as is well known, they put their money into circumventing the pentode - by inventing the beam tetrode, with some help from RCA in America.
The Marconi brand label.
The screen grid connects to the side contact.
The Three wire grids can be seen through the top mica
The classic envelope is 50 mm in diameter and, excluding the B5 base pins, it is 104 mm tall.
References: Private communication, datasheet & 1040. Type MPT4 was first introduced in 1931.
Absolute Maximum Operating Conditions¶
Updated May 13, 2016. shape:classic construction:pinch type:pentode age:1930.1940 base:b5 heater:4.0v pins:5 pin:1.a pin:2.g1 pin:3.h pin:4.h pin:5.k.g3 pin:sc.g2