This MKT4 from Marconi could also be described as an MKT4/7 as it is on a British seven pin base. The MKT4 was also made with a B5 base with the screen grid connected to a side terminal.
The MKT4 replaced the MPT4 that was also made in a catkin version.
In single ended audio configuration would provide an adequate 2.5 Watts.
The side elevation of the anode box. The getter holder can be seen on the bottom left. The Marconi brand of the M-OV company can be seen above the typical identification lozenge. Three wire grids can be seen - a pentode. However, the MKT4 was listed as a beam tetrode. All the older clear bulb versions we have seen are pentodes.
The face view of the box anode. The control grid has a box radiator above the top mica.
The lower end of the anode. All three grids are wire and thus this is a pentode and not a beam tetrode as the data-sheet suggests. Could this indicate a valve supplied to M-OV and not made by them? The control grid is wound on copper support rods for better heat conduction.
The top mica with heat radiator connected to the control grid supports.
The classic envelope is 57 mm in diameter, and excluding the B7 base pins, is 114 mm tall.
References: Data-sheet & 1043. Type MKT4/7 was first introduced in 1937.