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The KT66 is a very well known audio output beam tetrode valve. This exhibit is of Chinese origin and has the Golden Dragon branding. The later GEC production of the KT88 was branded Gold Lion.

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. It is not uncommon to find these valves used in parallel push pull. Here each single valve of the push pull pair is replaced with two valves connected in parallel. A 100 W amplifier results.

The anode. The plates are stitched together and carbonised on the outer surface to make the anode a more efficient radiator. Thanks to Simon Balderson for sending these images and details.

The shape of the envelope is a mixture of the classic shape and the wide tube.

The classic envelope is 50 mm in diameter and, excluding the IO base pins, is 116 mm tall.

References: Data-sheet & 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.