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 cylindrical anode and the rear of the Marconi brand label.
The suppressor grid can be seen to be connected to the cathode at the top of the valve.
The lettering is etched into the dome. The inner two grids are shaped around the cathode whereas the suppressor grid is helical and placed near to the anode.
A closer view of the inner grids.
The balloon envelope is 56 mm in diameter and, excluding the B5 base pins, it is 112 mm tall.
References: Data-sheet & 1043. Type MPT4 was first introduced in 1931.