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The KT66 is a very well known audio output beam tetrode valve. This exhibit has a black base cap and the classic shouldered envelope shape. This suggests a pre-war build. The adverts from the 1940's and 1950's show the later straight sided envelope.

The KT66 was essentially a 6L6 redesigned with a larger cathode and a shorter, fatter anode. The KT66 had less rapid acceleration, longer electron paths, and almost no kink. It was ace for audio hi-fi but the electrons took longer to reach the anode and were more open to influence from magnetic, etc, fields on the way. Moreover, under HF conditions the electron stream was not perfectly in phase with the grid signal and this resulted in a loss of output power if used for RF applications.

The figures given are for single ended use, but a pair run in ultra linear AB2 push pull will deliver 50 Watts with 2% distortion.

The Osram paper label. The lozenge containing the Type identification was M-OV and the branding consisted of which paper label and box came next on the production line.

The smoked glass obscures the electrodes but the copper control grid support can be seen as can the cooling fin attached to it.

The classic envelope is 56 mm in diameter and, excluding the IO base pins, is 123 mm tall.

References: Data-sheet, ¹⁰⁰⁴, ¹⁰⁰⁵ & ¹⁰⁴⁰. Type KT66 was first introduced in 1937. See also 1937 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.