2001 THD Uni-Valve

The THD Uni-Valve was a fairly popular boutique amp from the 2000’s that I remember seeing in Guitar Player magazine winning accolades. The amp has some unique features including a true Single Ended Class A Output, a rheostat controlled attenuator, an incandescent lamp noise reduction circuit, and a baxandall tone stack. Also it has the ability to take almost any octal power tube and many types of preamp tubes as well.

The PCB construction makes it fairly tedious to work on but luckily this amp had pretty simple problems. The Input gain stage GE/JAN 12AX7 was oscillating and making weird noises so it was replaced with a brand new EHX 12AX7. This tube also fit with the tonal style of the amp providing lots of gain and clarity. Secondly, the rheostat attenuator had contacts that were corroded and coils that were not making physical contact with the wiper causing signal dropout. The rheostat was cleaned with De-Oxit and a wire brush as well as having individual coils mechanically set back into place for seamless contact. The amp now functions as intended.