According to Jim, Ritchie Blackmore, Big Jim Sullivan and Pete Townshend were the three main guitarists who often came into the shop and pushed Marshall to make guitar amplifiers and told him the sound and design they wanted. then expanded, hired designers and started making guitar amplifiers to compete with existing amplifiers, the most notable of which at the time were the Fender amplifiers imported from America.
These were very popular with guitarists and bass players, but were very expensive.
To comply with his contract stipulations, these amplifiers had minor circuit changes compared to the regular Marshalls, and minor changes to the appearance.
For instance, often the Parks had silver or black front panels instead of the Marshall's gold ones, some of the enclosures were taller or shaped differently, and controls were laid out and labeled differently.
Marshall also manufactures less expensive solid-state, hybrid (valve and solid state) and modelling amplifiers.
After a successful career as a drummer and teacher of drum technique, Jim Marshall first went into business in 1962 with a small shop in Hanwell, London, selling drums, cymbals and drum-related accessories; Marshall himself also gave drum lessons.
To reduce costs Marshall started sourcing parts from the UK.
Marshall's contract did not prevent him from building amplifiers outside the company, and so Marshall launched the Park brand name, inspired by the maiden name of Jones's wife.
At the request of Pete Townshend, Marshall produced an 8×12-inch cabinet (soon replaced by a pair of 4×12-inch cabinets) on top of which the 1959 amplifier head was placed, giving rise to the Marshall stack, an iconic image for rock and roll.
The size of the wall of Marshall stacks "soon became an indicator of the band's status", even when rendered obsolete by improved PA systems; indeed, many of the "ridiculously huge arrays of heads and cabs" included dummies.
These circuit changes gave the amp more gain so that it broke into overdrive sooner on the volume control than the Bassman, and boosted the treble frequencies.
This new amplifier, tentatively called the "Mark II", was eventually named the "JTM 45", after Jim and his son Terry Marshall and the maximum wattage of the amplifier.