Reußenzehn is a German manufacturer of hand-wired tube amplifiers based in Offenbach-Bieber, near Frankfurt. The one-man workshop was founded by electrical engineer Thomas Reußenzehn, who began series production of tube devices in 1983 after previously running a shop. The company remains deliberately small-scale, building to order rather than mass-producing, which allows the workshop to accommodate custom requests from individual musicians and audiophiles.

The product catalog spans three areas: guitar and bass amplification, HiFi and high-end tube amplifiers, and studio gear such as microphone preamps and tube mixing consoles. Notable musician products include the Guitar Slave 19-inch tube power amp (introduced in 1985 with 2x 50 watts), the EL-34 series guitar heads, the Motown tube amp, The Bass Studio Amplifier, and the Daniel-D Tubebooster — a minimalist ECC83-based overdrive originally developed in 1983 to push Vox guitar amps into heavier distortion. The Tube Bass Preamp MK1, debuted in 1979, is often cited as an early milestone in the brand's history.

Reußenzehn is recognized in the German boutique-amp scene for all-tube, point-to-point construction and for its custom shop work for professional players, including early master-volume modifications built for Ali Neander of the Rodgau Monotones.

Reußenzehn has 1 product cataloged on Equipboard, including Guitar Amplifier Heads.