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The M-OV MKT4/5 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!

However, this exhibit is a genuine MKT4 five pin and it has three wire grids. It is not the only example of a MKT4 pentode.

As the MKT4/5 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.

See also MKT4 for a version with helical grids and a cylindrical anode that looks to be an actual M-OV valve.

In single ended audio configuration would provide an adequate 2.5 Watts.

Turned slightly the screen grid side contact can be clearly seen.

This valve took a lot of identifying. This image shows the scraps of lettering remaining and has been intensified. The valve is very hard to interpret with the naked eye. The oval surround is typical of M-OV and the K and the T reduced the possibilities. Under magnification the digit can only be a 4. Finally a visual check with another MKT4 confirmed the identification.

The box structure connected to the copper support rods of the control grid is there only to dissipate heat. Copper as a good conductor will feed heat to the fins better than nickel etc. Keeping the control grid cool reduces the chances of secondary emission of electrons.

The base of the main electrode assembly. The insulated heated threads the cathode tube and the grids, wound on notched rods, can be seen surrounding one another. Note that all grids are wire.

An enhanced picture of the grids. All three are wire wound. Strange for a beam tetrode. This valve is a pentode. As the structure matched the B7 version, it is possible that this is also a late production valve from Cossor that has been labeled and sold under the M-OV banner. M-OV had connections to Cossor in that they part owned it.

The classic envelope is 57 mm in diameter, and excluding the B5 base pins, is 124 mm tall.

References: Data-sheet & ¹⁰⁴⁰. Type MKT4/5 was first introduced in 1937. See also 1937 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.