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 specimen shown in the photo was probably made in the late 1930s.

The classic envelope is 42 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.

 
Pin Connections

B7	1	2	3	4	5	6	7
	-	g1	g2	h	h	k,g3	a

 
Absolute Maximum Operating Conditions^¶

Beam Tetrode	Vh	Ah	Va	Vs	Vg	mAa	mAs	gm	Pout	D
	4.0	2.0	250	250	-4.4	40	8.5	10.5	4.2W	8%

Thanks to Frank Philipse for supplying the above PDF datasheet