KT61, 6P25, EL33, CV1438, CV2938, N147, OM9, OUTPUT BEAM TETRODE, OCTAL

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 from 1948 is Mazda's 6P25, a beam tetrode just like the KT61, but having slightly higher mutual conductance gm and with its outer envelope screening whilst it may not be as pretty as a KT61 some say it is a better performer.

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 which finds use in many classic hifi amplifiers such as the Leak TL10.

The benefits of a beam tetrode tranlates 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 can be prone to heater-cathode shorts so make sure you test regularly and buy carefully.

KT61

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KT61, OSRAM, GREY GLASS, BLACK BASE, ETCHED CARTOUCHE, BLUE & WHITE SQUARE DECAL, OCTOBER 1946 PRODUCTION

KT61, MARCONI, BROWN BASE, LARGE RED & GREEN DECAL, HAMMERSMITH DECEMBER 1954 PRODUCTION

EL33, MULLARD, OLD SHIELD, WHITE PRINT, PENTODE, 6AG6G, 6M6G, BVA266, CV2938, KT61, N147, PP6BG

KT61, GEC, METAL OVERCAPPED BLACK BASE, LARGE BLUE & WHITE SQUARE DECAL, EX BBC VALVE STORES, NO KEYWAY, HAMMERSMITH DECEMBER 1952 PRODUCTION

EL33, MULLARD, OLD SHIELD, WHITE PRINT, PENTODE, 6AG6G, 6M6G, BVA266, CV2938, KT61, N147, PP6BG (Copy)

KT61, OSRAM, BROWN BASE, LARGE BLUE & WHITE ELLIPTICAL DECAL, HAMMERSMITHOCTOBER 1954 PRODUCTION

KT61, OSRAM, MATCHED PAIR, BROWN BASE, FROM 1954-1955