6900

About the 6900

The Bendix 6900 is a medium-mu twin triode vacuum tube housed in a 9-pin miniature hard glass envelope, specifically engineered for high-reliability pulse applications such as those in US Air Force intercontinental ballistic missiles and aircraft. Developed circa 1954 by Bendix Corporation, it is often regarded as a super-durable version of the 5687 tube, with some debate whether it is a heavily ruggedized 5687 or a distinct design with similar specifications. The tube features a Nonex glass construction, making it notably heavy—about the weight of two typical 9-pin tubes combined. It employs a tungsten stem seal and three ceramic spacers, each equivalent in thickness to four standard spacers, which eliminate common failure sources like gas evolution from mica spacers. The ceramic spacers and multi-pillar supports locked by 12 welded eyelets contribute to a very sturdy structure. Special alloy snubbers maintain position and spring properties at high temperatures, enabling the tube to withstand shock accelerations up to 500Gs, making it highly resistant to microphonics and mechanical shock. The 6900 was tested extensively under severe overload, vibration, and shock conditions, with a typical life expectancy of 5,000 hours under pulse operation (heater voltage 6.3V, heater current 1.0A, plate voltage 500V, grid pulse +50V, grid voltage -100V, plate current 4.25A, grid current 0.25A, pulse time 10µs, pulse repetition rate 250pps). Audiophiles note that the 6900 offers transistor-like detail and bass control while retaining the tonal richness, 3D imaging, and soundstage depth characteristic of high-quality tubes. It is pin-compatible with the 5687 and sometimes used as a substitute in audio applications, though originally designed for military pulse use.