The 300B is a directly heated power triode using a four pin base, introduced in 1937 to amplify telephone signals. It measures 6.4 inches high and 2.4 inches wide, and has a 40 watt anode dissipation. In the 1980s use of 300Bs by audiophiles had grown. The 300Bs used by these audiophiles were used home audio equipment. The 300B has high linearity, low noise and good reliability. It is often used in single-ended triode (SET) audio amplifiers. These SET amplifiers output about eight watts, while a push-pull pair of 300Bs can output 20 watts.
Current manufacturers of new 300B tubes, and tubes that have similar characteristics include EH Electro Harmonix, Emission Labs - EML, JJ Electronics, KR Audio, Shuguang, Sovtek, Svetlana, TJ FullMusic and Westrex Corporation. Prices for new 300B tubes range from $175 to $900 per matched pair.