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 it lived on into the 1950s as the KT61.
The envelope is coated to obscure the electrodes but with back lighting some detail can be seen. The anode is shaped with an extension in the centre of the working face that increases the separation between the grids and the anode.
The osram label.
The top mica is a tight fit toe the shoulder of the envelope. The control grid is wound on copper supports and each support has an L shaped heat sink fin. The screen grid is fitted with and S shaped tape heat sink.
The classic envelope is 45 mm in diameter and, excluding the B7 base pins, is 98 mm tall.
References: Datasheet, private communication & 1043. Type KT41 was first introduced in 1937.