The Dallas Rangemaster Treble Booster was an effects unit made for guitarists in the 1960s. Its function was two-fold: to increase the signal strength of the guitar going into the amplifier, and to increase tones at the high end of the spectrum (a treble booster).
The need for a treble booster arose in the mid-1960s, as British tube amplifiers such as the Vox AC30 or Marshall JTM45 tended to produce a slightly dark, muddy sound when overdriven, particularly when used with humbucking pickups. A pre-amplifier that also boosted treble proved a solution. Additionally, the vintage components in the Rangemaster circuitry could add characteristic distortion and overtones to color the guitar sound, much in the way of the more modern overdrive pedals.