The 12D4 is a dual triode preamp and microphone tube that was manufactured in the 1960s by RCA and is still available today. It is a high-gain, low-noise preamp with excellent sound quality. It is often used in high-end audio equipment as well as in guitar amplifiers. The 12D4 has two distinctive sections, one that serves as an input transformer, and another that is used as an output transformer. It has a very wide frequency response and is often used for equalization applications.