OK, let's get this straight right from the start, I do know that the Mullard EL33 is actually a pentode but it’s both pin and electrically equivalent as well as being easier to drive whilst showing lower intermodulation distortion and was introduced at the same time as the KT61 in 1939.
An interesting alternative and the youngest of this type being introduced during 1948 is Mazda's 6P25, a beam tetrode just like the KT61, it having slightly higher mutual conductance gm and with it's outer envelope screening whilst it may not be as pretty as a KT61 some say it is a better performer. But, who knows, why not tube roll with a couple of each and decide for yourself.
The KT61 proper, also introduced during 1939, is an audio output beam tetrode valve, part of a family of similar valves from the Marconi-Osram Group and finds use in many classic hifi amplifiers, the Leak TL10 being one such example, the benefits of beam tetrodes also transfers well to guitar amplifiers where KT61 are seen as a useful modification in replacing 6V6 as the valve of choice to give a distinctive but desirable sound. But watch out, the KT61 is prone to heater-cathode shorts so make sure you test regularly and buy carefully - preferably from us!!!!!!!!