The dark heater improves tube reliability

The life span of a heater in an electron tube is in large measure governed by the effects of high-temperature operation on the physical properties of the heater wire. Darkening the normally white heater coating improves its thermal radiation properties and makes it possible to reduce the operating temperature of the heater wire by up to 350 degrees Kelvin for normal cathode temperature. Data obtained from more than a million tube hours of accelerated life-testing on a wide range of tube types demonstrates a tremendous improvement in heater reliability as a result of the lower operating temperature of the dark heaters. Other benefits accruing from the use of this heater include reduced heater-cathode hum and leakage, increased heater surge-voltage capability, and constant heater input power during life. The unique features of this heater also open up new avenues for tube design, such as a reliable, heavy-duty, line-operated (117-volt-heater) rectifier tube for use in TV and hi-fi applications.