A tube microphone is a type of condenser microphone (often, but not exclusively, a large-diaphragm mic) that is commonly found in recording studios. A tube microphone has a vacuum tube inside of it. The tube acts as the amplifier for the microphone’s signal.
When a condenser microphone captures a signal, the diaphragm outputs a very low-level signal. That’s where the internal amplifier (sometimes called a head amp) comes in. It’s located inside the microphone and amplifies the signal so that it can drive the cable going to the microphone preamp or console. In the early days of electronics when tubes were the only option, these internal preamplifiers were built using tubes to amplify the signal. However, with the advent of solid-state devices, FET preamplifiers could be built into microphones more cheaply, and solid-state mics largely took over the market. Nevertheless, many people still prefer the tonal characteristics of tube microphones, due to the harmonic distortion and tonal response of the tube in the circuit. For this reason, many manufacturers still make tube-based microphones. Some popular tube microphone models include the Warm Audio WA-47, the Lauten Audio Eden LT-386, and the Rode K2.