The Boost circuit is a mu-amp type, built out of a fragment of the main amp emulation circuit. 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. 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. ![]() Eventually, this circuit morphed into the JCM800, available in 50- or 100-watt models. 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. 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. 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. Consequently, one’s amp sitting atop just one of these became the “half stack.” This became known as the “full stack”, a bit of nomenclature that’s still in use today. 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. Marshall obliged, but as any bass player knows, “fridge” cabinets are a beast to maneuver. One of Marshall’s early adopters was Pete Townshend, and he commissioned a custom 8x12 cabinet to use the Plexi to its fullest potential. Suddenly, combo amps just weren’t cutting it anymore, and amplifiers required outboard speaker cabinets. It was released in 1965, and by that time, rock music was increasing in popularity, and the venue size for concerts increased in stride. Marshall’s 1959 is unofficially known as the “Plexi” model due to its Plexiglas-like faceplate, and features 100 watts of power. 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. 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. 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. The first amp Marshall ever released was the JTM45, named for Marshall’s two sons and its wattage. 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. There is no “definitive” schematic for any of these models, seemingly changing on a whim throughout the years. 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. And who better to start with than Jim Marshall and his modified Fender Bassman that launched an empire. With that said, I’ve consumed my fair share of amplifier text, and circuit design IS circuit design, after all. Ever since then, high-voltage design has always left me a little on edge, but part of that nervousness is intrigue. ![]() When I was young, I worked in a casino repairing slot machines, and got hit with a live wire inside a machine. Admittedly, it’s probably because I’ve built hundreds, if not thousands of pedals, and not one amp (yet). I spend so much time talking about pedals that sometimes I lose sight of the other integral electronic pieces.
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