In the words of The Electronic Audio Experiments:

The Model feT is a transistor-based preamplifier designed around the legendary Sunn Model T amplifier. Those who have played the original know that while it excels at heavy tones when pushed, it is far more than just an amp for stoner rock and doom metal. In fact, on its own it is a fantastic mid-gain amplifier with a tone that is dark, yet clear and presentmaking it an exceptional pedal platform.