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.

See also MPT4 for more details.

The interesting point about the grids is their shape.

The the suppressor grid is close to the anode. The inner grids are held on supports away from the cathode but are shaped to form a small cage around the cathode and close in.

The balloon envelope is 56 mm in diameter and, excluding the B5 base pins, it is 117 mm tall.

References: Datasheet & ¹⁰⁴³. Type MPT4 was first introduced^¶ in 1931.

 
Absolute Maximum Operating Conditions^¶

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs
	4.0	1.0	250	200	-10.5	32.0	5.5

Thanks to Frank Philipse for supplying the above PDF datasheet

More