Marshall JMP Model 1992 Super Bass 100 Plexi Tube Amplifier (1969)

Marshall  JMP Model 1992 Super Bass 100 Plexi Tube Amplifier (1969)
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Item # 11716
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Marshall JMP Model 1992 Super Bass 100 Plexi Model Tube Amplifier (1969), made in London, England, serial # SB/A 10533, black tolex covering finish.

Marshall amps are certainly a rock legend, with a sound that has defined both an entire style and era of music. This fairly early Super Bass 100 watt head is a real monster, both in volume and tone. Marshall amps owe their humble beginnings to the Fender Bassman 5F6-A, the original subject re-interpreted using English components and originally sold exclusively through Jim Marshall's London shop. Both the circuit design and sound soon evolved from the Fender paradigm into something entirely their own.

Early Marshalls powered the sound of the Who, Cream, Hendrix and beyond, de rigeur among top UK groups by 1969. Although the dedicated bass models were popular with many A-list bassists (arguably more so than the original Bassman) many notable guitarists have also chosen the Super Bass model over the Super Lead; Jimi Hendrix, Jimmy Page, and Eric Johnson, to name a few. Consequently, the Super Bass has developed something of a cult following, but the differences between Super Lead and Super Bass amount to little more than a handful of components; one circuit can fairly easily be modified into the other.

Similar to the Bassman, Marshall's Super Bass features four inputs and two channels, often jumpered with a short lead for more preamp gain, along with controls for each channel: volume, treble, middle, bass and presence controls. The head sports a quad of EL34 power tubes, putting out a whopping 100-watts, typically into the company's iconic closed-back cabinet loaded with four 12" Celestion speakers, another Marshall innovation that owed much to The Who's John Entwistle, ever on the lookout for ways to make his bass louder and an early and eager advocate of this full-stack design.

These amps came of age in the late 1960s, an era when bands routinely played large stadiums and festivals, often without PA sound reinforcement for the guitars. In 1969 amplifiers with this sort of power were a necessity rather than a luxury, and the tone became the new rock standard. These days, we have the luxury of attenuators, to tame these beasts somewhat and protect our ears!

This Super Bass head is one of the later specimens of the revered 'Plexi' era, characterized by the plexiglass panels used in amps between 1965 and 1969. Dating 1960s Marshalls accurately can be somewhat tricky as serial numbers were seemingly applied at random until mid-1969. There are no date codes on electric components to go by either. Luckily, with the inspection sticker on the steel chassis, we can narrow down the production/test date of this amp to March 1969.
 
Height is 11 in. (27.9 cm.), 29 in. (73.7 cm.) width, and 8 in. (20.3 cm.) deep.

Cosmetically, this head has clearly seen some use but remains in a really well preserved condition for its age. The dark Tolex covering shows minor amounts of scuffing, mostly just some corner spots. Both the original handle and the gold piping on the head remain in great condition for their age, as well as the gold mesh on the back panel. The original logo is in nice shape with some of the gold plating still intact. All the original knobs are present and nicely intact, save for the presence knob, which is sadly missing its tip. The plexiglass front panel is in beautiful condition with no visible cracks; it's only showing VERY subtle (commonly seen) heat-wear around the power and standby switches.

Electrically, this amp is nicely original. The transformers and pots all appear original; the power and output transformers are sadly missing their stick-on labels, but they appear to be original to the amp. The choke bears the marking C1999. All pots retain undisturbed red-marked solder joints throughout, save for joints that were touched up and minor modifications that were undone. The amp has otherwise seen our typical servicing and maintenance, including replacement of all electrolytic capacitors as well as any other caps that are out of spec, a grounded 3-prong power cord, cleaning of all sockets, pots and jacks and power tubes biased to spec. This amp sports a nearly full set of Marshall-branded tubes; all test great and run well. The previously missing/broken impedance and voltage selector caps have been replaced with 3D-printed reproductions. The amp is set up for US voltage and as they used to say, "Ready To Rock" Excellent - Condition.