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From JTM45 to JCM800: Marshall Amps and Tonal Influence

I spend so much time talking about pedals that sometimes I lose sight of the other integral electronic pieces. Admittedly, it’s probably because I’ve built hundreds, if not thousands of pedals, and not one amp (yet). When I was young, I worked in a casino repairing slot machines, and got hit with a live wire inside a machine. Ever since then, high-voltage design has always left me a little on edge, but part of that nervousness is intrigue. With that said, I’ve consumed my fair share of amplifier text, and circuit design IS circuit design, after all. And who better to start with than Jim Marshall and his modified Fender Bassman that launched an empire. 


There are few things more confusing than Marshall’s numbering system; model numbers are oftentimes four-digit numbers starting with “19,” such that release years and model numbers are often just a few digits apart. There is no “definitive” schematic for any of these models, seemingly changing on a whim throughout the years. If Marshall changed the model number every time there was a schematic change, they would have run out of numbers beginning with “19.” From here onward, anything written references the model number and not the year of release. 


The first amp Marshall ever released was the JTM45, named for Marshall’s two sons and its wattage. Despite Marshall’s later reputation for manufacturing amps used by hard rock musicians, the first model was a combo amp that was designed as a “girthier” alternative to the Fender amps that had been making their way across the Atlantic. This model was closely related to a “tweed” Bassman but swapped out the input tube to a higher-gain ECC83, closed the speaker cabinet and swapped the power tubes for 6L6s. Most importantly, Marshall’s engineer Dudley Craven modified the negative feedback circuit to inject a little more harmonic content into the signal path, giving Marshalls their signature “kerrang.” These amps are renowned for their comparatively gentle breakup and sparkling cleans.


Marshall’s 1959 is unofficially known as the “Plexi” model due to its Plexiglas-like faceplate, and features 100 watts of power. It was released in 1965, and by that time, rock music was increasing in popularity, and the venue size for concerts increased in stride. Suddenly, combo amps just weren’t cutting it anymore, and amplifiers required outboard speaker cabinets. One of Marshall’s early adopters was Pete Townshend, and he commissioned a custom 8x12 cabinet to use the Plexi to its fullest potential. Marshall obliged, but as any bass player knows, “fridge” cabinets are a beast to maneuver. To compensate, Marshall released the 1960A and -B cabinets, each containing four speakers and shaped to accommodate amps up top and more speakers down below. This became known as the “full stack”, a bit of nomenclature that’s still in use today. Consequently, one’s amp sitting atop just one of these became the “half stack.”


Marshall eventually switched from hand-wiring to printed circuit boards and redesigned the panels to denote an MKII design, as well as the text “Jim Marshall Products”, and so these amps became known as JMP models. This is where the first seismic shift of Marshall lore begins; Marshall had signed a US distribution deal with a company called Unicord, and Unicord’s engineers modified every JMP amp sold in the US and Japan through them, changing out the power tubes to a more reliable model. Combining the circuit changes with the new tubes and eventual master volume control in 1975, Marshall’s amps had officially made the leap to hard rock and a nascent genre known as “heavy metal.” Eventually, Unicord’s engineers monkeyed around with the gain controls, adding even more dynamic range after cracking the Master and inspiring metal players the world over. Eventually, this circuit morphed into the JCM800, available in 50- or 100-watt models.


This is by no means a comprehensive list of Marshall amp models, but they are the ones I sought to emulate with my latest design, the Dirty Little Secret Deluxe. With a Boost channel, amp channel and the ability to switch these two circuits in any order, you can cop any of these tones with ease. The Boost circuit is a mu-amp type, built out of a fragment of the main amp emulation circuit. This means that with just the Boost section engaged, you can get those slightly gritty, chewy cleans, then combine them with the amp channel to get some liquid sustain with plenty of harmonics, reminiscent of a cranked 2204. Switching the order lets you experience the thrill of playing a non-master volume amp and all the saturation it provides. The amp channel also features a real MOSFET-based phase inverter and a real transformer that gently saturates depending on the rest of your settings, just like a real amp. Perhaps best of all is the ability to switch between Super Lead and Super Bass modes in any configuration to channel our most refined versions of the greatest rock amps in existence.