Many older amps were designed with 2 channels, each channel was voiced slightly differently (higher frequency [guitar] and lower frequency [voice and harmonica] for example) with each having 2 inputs. The inputs of each channel allowed different amounts of input signal to the first pre-amp stage. With a single coil pickup equipped guitar the amp sweet spot (on the edge of breaking up) is different than with a humbucking pickup equipped guitar (higher signal), so the single coil pickup equipped guitar can sound best in the higher sensitivity channel and the humbucking pickup equipped guitar sound best in the lower sensitivity channel. Really most guitar players determine which is louder and only use that from that point on.
Many players use a short instrument cable to connect both channels and use the Volume controls to mix them as desired.
Some amps don’t work for this (Fender DR, TR, VR, VV) because the two amp channels are not in phase, but the tweeds all do, and Marshall vintage amps like the JTM45 and plexi amplifiers (Model 1959 or Model 1987x) have unique channels (different frequency range and gain) so there are many combinations.
Fat One Amps original designs have two internally bridged uniquely voiced (frequency and gain) channels and allow the guitarist to push as much Lows or Highs as they wish
