In electronics the 300B is a directly-heated power triode vacuum tube with a four-pin base, introduced in 1938 to amplify telephone signals. It measures 6.4 inches high and 2.4 inches wide, and the anode can dissipate 40 watts. In the 1980s it started to be used increasingly by audiophiles in home audio equipment. The 300B has good linearity, low noise and good reliability; it is often used in single-ended triode (SET) audio amplifiers of about eight watts output; a push-pull pair can output 20 watts.
As of 2012 manufacturers of 300B and other tubes of similar characteristics included EH Electro Harmonix, Emission Labs - EML, JJ Electronic, KR Audio, Shuguang, Sovtek, Svetlana, TJ FullMusic, Westrex Corporation, and Takatsuki. Prices for new 300B tubes ranged from US$175 to $2,000 per matched pair.