The first few production units were engineered to be almost exactly the same as the Bassman circuit, with US-origin transformers and military surplus 5881 power valves. The major difference however was the cabinet used, as Marshall decided to build separate amplifiers and speaker cabinets, and as they were originally intended as bass amplifiers, Marshall chose to use four 12-inch Celestion speakers in a closed-back cabinet instead of the Bassman's four 10-inch Jensen speakers in an open-back cabinet. This new amplifier, tentatively called the "Mark II", was eventually named the "JTM45" after Jim and his son Terry Marshall, and "45" as, in theory, it produced 45W of power.

This gave Marshall amplifiers a more aggressive voice which quickly found favour with players, most notably a young Eric Clapton, who would sit in Jim's shop practising his playing. When Clapton was invited to join John Mayall's Bluesbreakers, he asked Jim Marshall to produce a combo amp with tremolo which would fit in the boot of his car, and one of the most famous Marshall amps was born, the so called "Bluesbreaker" amp. This is the amplifier that gave Clapton that famous tone on the Bluesbreakers' Beano album.

The amplifiers from this era, are easily identifiable by their acrylic glass (a.k.a. Plexiglas) front panel, which earned them the nickname, "Plexi"s. These now have significant collectors' value. Amplifiers from the 1970s onwards can be distinguished most easily by their brushed metal front panel, and are known as "ali panel" Marshalls. After 1973, in order to streamline production, the inefficient hand wiring was discontinued and Marshall valve amplifiers were switched to printed-circuit-board paths (pcb). The result is that some vintage purists claimed the sound to be more "sterile" whereas this is incorrect as the pcb itself does not change the sound. There was probably a slight change in components on the pcb which is a hotly debated topic within the musician community to this day. Also, much of the tonal debate between the plexi- and aluminium-panel Marshall amps comes from the fact that in 1974 Marshall's US distributor had them change all of the amps sold in the US and Japan over to the much more rugged 6550 instead of the EL34 output tube. This produced a much different sound than the EL34—a sound perceived as less smooth and more metallic when overdriven. This change was brought about due to reliability problems with the EL34's, and the 6550 generally allowed the amps to make it through warranty without problems. The circuit changes required to switch the amps were very minor, and it was easy to change from the 6550 to the EL34 or vice versa by changing a few resistor values, moving the tap for the feedback loop and rebiasing the amp. The mystique surrounding the Plexi series only increased due to the roar first heard on Van Halen's 1978 eponymous debut. Eddie van Halen modified his amps and cranked them up to get his now famous 'brown' sound.

In the mid-1970s, Marshall introduced the "master volume" ("MV") series, which was initially called the "JMP." This was in response to the demand for yet more distortion, and many techs had been modifying the amps for years by connecting the two input stages in series rather than parallel as in the original Marshalls. A master volume was introduced to make the volume levels more manageable. This gave the new breed of Marshalls a different voice, more cutting and edgy, which later found favour with players such as Randy Rhoads, Zakk Wylde and Slash. Soon after the Rose-Morris deal had ended in late 1980, Marshall began calling this series the "JCM800" series (named after both Marshall himself and the license plate of his car, which he'd had since 1972). Marshall made several amplifiers under the "JCM800" name, but most noticeably were the 50 watt 2204 and the 100 watt 2203. Because the valve industry had begun to fade and Marshall became worried that the standard power valve, the venerable EL34 would soon become unavailable, a number of JCM 800s were factory equipped with the 6550 beam pentode power valve, a valve with a different tonal character. Some people (notably Ozzy Osbourne guitarist Zakk Wylde) loved the resulting sound, but it was generally considered to be a downgrade in sound quality. The 6550 is a high power tube that is more common in valve-driven bass amplifiers because it allows a higher output power over the EL34 for a given distortion level and because it has a very crisp low and high end sound. Because many players desired more power amp distortion (which is considered more "pleasant" than pre-amp distortion, which these Marshalls still had plenty of), the prices for EL34s, especially vintage ones, skyrocketed for use in Marshalls. Marshall would not return to full time use of the EL34 in all of its valve amps until the rise of vacuum tube factories in the mid and late 90s when former Soviet countries made most valve types plentiful again.

Marshall began to see more competition from American amplifier companies such as Mesa Boogie and Soldano. Marshall then updated the JCM800 range with additional models and new features such as "channel switching," which meant that players could switch between clean and distorted tones with the push of a foot-operated switch. This feature debuted in the 2205 series and these amps contained more pre-amp gain than ever thanks to a new innovation; diode clipping. This meant a solid-state diode added additional distortion to the signal path, akin to adding a distortion pedal. While hotly criticised today among valve purists, these amps were more popular than ever, finding mass acceptance within the hard rock community and still in use today by many. Marshall around this time began further experiments with solid-state amplifiers, which were increasingly improving in quality due to technological innovations but were still considered beginner level equipment. Regardless, solid-state product lines with the Marshall name on them were and still are a wild (if critically discounted) success for the company, allowing the entry level guitarist to play the same brand of amp as his or her heroes.

In the 1990s, Marshall updated its product line again with the JCM900 series. Reviewed by Guitarist magazine in the UK and given the line, "Shredders, here is an amp you won't need to have modified," this move by Marshall was again an outgrowth of musicians' desires, featuring more distortion than ever and retaining popular aspects of the late JCM800 models. However despite such marketing claims there were not as hi-gain as advertised and lacked a full gain stage. Marshall rectified this with the SL-X series (as used by Tom Johnson of Darkhorse or the group KISS). This model was one channel and was given an additional pre-amp ECC83/12AX7 instead of diode-based distortion. Still, if not for shredders, the JCM900 was well received by younger players associated with pop, rock, punk and grunge which was widespread by the early 90's.

Although the EL34 had at this time begun to return to prominence, a number of these were shipped with 5881 valves, a now uncommon valve similar in tone and build to a 6L6. Around this time, Marshall released a few "special edition" amps in this range, including a "Slash Signature" model, a first for the company, and the Silver Jubilee.