Tube mic preamp
In a studio or home studio, this is an essential part of recording. Whether old or new, these can be very expensive but there are indeed priceworthy preamps. A tube mic preamp work a low power so the effect on the audio is not the same as for tube power amps. Instead, input transformers may have a certain effect on the sound in mic preamps.
Tube preamps are not always best for mic recording. Even though tube mic preamps are preferred for its sound in music recording, there are situations when an unaffected sound is wanted and then a clean solid state mic preamp would be a better choice. That is in situations when recordings are to be made for an authentic reproduction.
The most common tube in mic preamps is probably the 12ax7.
Some mic preamps, either solid state or with tubes, have an impedance transformer at the input that colors the tone at least a little. The
"warmth" that is commonly associated with vintage or expensive preamplifiers, may (also) come from the tone characteristics in the impedance matching input transformers.
If the impedance matching can be adjusted (like 150 - 600 Ohms), the interaction between the transformer and microphone interaction can be adjusted. Different settings may change the tone a little.
http://www.tubepreamp.net/