The KT41 was M-OV's version of a second-generation output valve, with the 1930s standard 4.0 V heater, it replaced Type N41. The KT type number proclaims that it is a beam tetrode, not a pentode. The KT41 was unexciting (though quite adequate) in terms of power output but gave better quality sound than most of its pentode competitors and was a successful valve.
Its manufacturing life was truncated when M-OV changed over to American-style 6.3 V valves from around 1938 but in 6.3V guise on an IO base it lived on into the 1950s as the KT61.
Through a clear part of the glass the pinch and lower part of the mesh anode can be seen.
The cathode is a flattened oval and within can be seen the two insulated heater strands. The mesh anode is cylindrical with four side flanges for greater area. Two of the flanges are used to connect to the supports. What is strange is that there is a third wire grid and no beam plates. This KT41 is a pentode.
The anode and above the bottom mica is the suppressor grid wire.
Taken from the top: the control grid heat sinks are on the left, the suppressor winding is just within the mesh anode.
Very difficult to get all grids in focus with the microscope, but all three can be seen.
The classic envelope is 44 mm in diameter and, excluding the B7 base pins, is 118 mm tall.
References: Data-sheet, private communication & 1043. Type KT41 was first introduced in 1937.
Absolute Maximum Operating Conditions¶
Updated March 19, 2018.