The MKT4 was a beam tetrode version of the earlier (1932) MPT4 output pentode and was intended as a plug-in replacement, production of the MPT4 being discontinued.
The beam tetrode version gave slightly more output power than the pentode but with significantly lower distortion. Whether its performance was any better than that of Mazda's landmark (1930) AC/Pen is open to question!
As the MKT4 was a plug-in replacement for the MPT4 pentode, the B5 base and side contact for the screen grid was the required configuration. Most MKT4's made were to this pattern. Later M-OV fitted the valve with a B7 base. Looking at the MKT4/7 one can see that the electrode assembly remained unaltered, just the base changed.
In single ended audio configuration would provide an adequate 2.5 Watts.
The Marconi label on this M-OV valve.
The screw side connection the style of which was used with the early top-cap connections.
The lower mica and electrode connections. The heater can easily be seen to pass through the round cathode tube four times. The control grid is wound on copper supports.
The cathode and grids with beam forming plates and outer cylindrical anode. Our other MKT4 (4) has a flatter anode.
The classic envelope is 44 mm in diameter, and excluding the B5 base pins, is 96 mm tall.
References: Data-sheet & 1043. Type MKT4/5 was first introduced in 1937.