Not only did these tubes yield the now-famous Marshall crunch tone, the sound that helped define the genre, they could be driven at higher voltages to provide that louder amplifier that players were demanding, even in its “smaller” 50-watt version… an amp that could actually put out around 60 watts when pushed. The most obvious evolutionary trait, and the one most amp fans will point to first, is the move from the 5881/6L6/KT66 output tube types of the JTM45 (and Fender Bassman) to European E元4 tubes, bottles that became synonymous with British rock tone from 1967 onward. But the changes from JTM45 to JMP50 are certainly more impacting on this amp’s performance than those from 5F6A Bassman to JTM45, and while indeed relatively few in number, are indeed “significant.” Poke your nose inside the chassis of the two – figuratively, please – and the component layouts are still virtually identical, aside from the mounting positions for the tubes. The plexi still had many significant elements that helped form the way the tweed Bassman did business, including the three-knob cathode-follower tone stack, Presence control, long-tailed-pair phase inverter, and a class AB output stage in fixed bias with a negative feedback loop to tighten up the performance. This is mainly because, despite being a very different-looking amp, the JMP50’s circuit is still a direct descendent of the circuit found in the 5F6A tweed Fender Bassman of 1958-’60, upon which Jim Marshall based his original amp, the JTM45. That’s “despite some significant changes” even if analyzed against the big picture these changes might appear relatively slight or at least far less significant, say, than the changes wrought by Fender between the tweed Bassman of 1960 and the blond Bassman of 1960/’61, or by Gibson between the Les Paul of 1960 and the Les Paul/SG of ’61.
Let’s take a look at a gorgeous plexi-panel JMP50 from around 1968, by which time the archetypal plexi formula had been firmly established.
As hallowed as the 1962-’66 Marshalls might be, and as desirable as those later metal-panel heads might still remain, the late-’60s plexi is undoubtedly the pinnacle of Marshall amps for the rocker and typifies what we think of as “the Marshall sound” more than any other product the company has ever produced. Marshall amps of the company’s first 10 years are interesting for having thrown up classics of different eras – from the original JTM45 to the plexiglass-panel JTM50/JMP50 of 1966-’69 (from whence the “plexi” nickname is derived) or the metal-panel JMP50 of 1969-’72 – that all bore the Model 1987 Lead Amp designation, despite some significant changes. Kids wanna rock? Plug into your plexi and bow to the mighty crunch. Output: 50 to 60 watts RMS into a 16-ohm load. Speakers: four 20-or 25-watt Celestion G12M “Greenback” speakers, wired in series-parallel, in closed-back 4×12 cabs. Output tubes: initially two E元4 Class AB, fixed bias.Ĭontrols: Vol Normal, Vol Bright, Treble, Bass, Middle, Presence.
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