The amount of distortion produced by a Marshall amp depends on several factors, including the type of amplifier, the type of tubes or transistors used, and the settings of the amplifier's controls. Some Marshall amps, such as the JCM800, are designed to produce more distortion than others, while amps with higher wattage tend to produce more headroom, meaning that they can be turned up louder before they start to distort.
The gain control on a Marshall amp is used to adjust the amount of distortion produced. When the gain is turned up, the amplifier's transistors or tubes are driven harder, which causes them to clip the signal more. This results in a more distorted sound.
The tone control on a Marshall amp can also be used to shape the sound of the distortion. The tone control adjusts the frequency response of the amplifier, allowing you to boost or cut certain frequencies. This can be used to create a variety of different distortion sounds, from a bright and edgy distortion to a warm and smooth distortion.
The master volume control on a Marshall amp is used to adjust the overall volume of the amplifier. This can be used to balance the volume of the distortion with the volume of the clean signal.
By adjusting the gain, tone, and master volume controls, you can create a variety of different distortion sounds with a Marshall guitar amplifier. This versatility is one of the reasons why Marshall amps are so popular with guitarists of all genres.