Triple-twin
The Triple-twin was a special vacuum-tube design used in audio power amplifiers. It combined two different triodes in one glass envelope: an input triode and an output triode. The output tube needed a positive grid bias and current, which was supplied by the input triode acting as a cathode follower. The input triode’s cathode was connected inside the envelope directly to the output triode’s control grid.
The first model, type 295, appeared in March 1932 from Cable Radio Tube Corporation under the Speed label. It was advertised as twice as powerful as the popular 47 pentode and three times as powerful as the 45 directly heated triode. It could deliver about 4.5 watts at 5% distortion into a 4 kΩ load; at 2 kΩ and 10 kΩ loads, distortion rose to about 8%.
The 295 required roughly +250 V on the plate and about +6 V positive grid bias. The original 295 had a directly heated output section and an indirectly heated input section.
Later versions followed: the 2B6 in 1933 had both cathodes indirectly heated; the Sylvania 6N6G in 1936 had both cathodes indirectly heated and a cathode-follower resistor built inside the tube. A single-ended 6N6G amplifier needed only the output transformer, with currents and bias set by the internal resistor. A push-pull 6N6G amplifier required two tubes and two transformers (input and output).
Despite heavy advertising, the triple-twin was a market failure. The industry preferred general-purpose tubes, and the triple-twin was obsolete by the end of the 1930s.
This page was last edited on 2 February 2026, at 15:11 (CET).