A: Some Marshall amps, such as the JTM45, 1959HW, and 1987X are two-channel amps. However, these are not traditional channel-switching amps. Each channel has its own input jack(s), and to access that channel you need to plug into the appropriate jack.
On these Marshalls, though, the channels are also voiced quite differently. The “Normal” channel has a bassier, darker tone, while the High Treble channel has a much brighter tone.
Many players actually prefer hearing a mix or blend of these two channels, and use the volume controls to craft the tone they want to hear — turn up the Normal channel for more thump and bass, turn up the High Treble channel for more cutting, bright tones.
To access both channels simultaneously a short jumper cable is run between the two channel inputs. For example: plug your guitar into input I of the Normal channel. Then connect a jumper from input II of Normal channel into input I of the High Treble channel. The signal is now being sent to both channels simultaneously. Turn up the Normal and High Treble volume controls to achieve the tonal balance and gain level you want, then use the amp’s tone controls for final shaping of your sound.