The 85 is a 6-pin UX-base duplex-diode medium-mu triode vacuum tube introduced in the early 1930s for superheterodyne radio receivers. It combines two signal diodes for detection and automatic volume control (AVC) with a medium-mu triode section for audio preamplification or driver service. Featuring an ST-12 shoulder glass envelope and a top grid cap for high control grid isolation, it operates on a 6.3V heater. Designed as a lower-amplification alternative to the high-mu 75 tube, the 85 provides enhanced linearity and higher audio output drive capability. It serves as the 6-pin predecessor to octal types like the 6B7 and 6G6G. Available NOS and tested used.