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Punchy and full with extra crunch added to the mix when the preamp gain was turned up. The tube helps contribute to a much “fatter” tone with better bottom end response.My favorite JMP series master volume amplifiers were made in 19.
The ’79-’81 models are easily identifiable since they marked the beginning of Marshall adding its serial numbers on the amplifier front panels beneath the power and standby switches.
Even with the minor circuit tweaks that only amount to a personal preference toward the ’77-’78 models, the ’79-81 amps were still punchy, and great all around rock and roll amplifiers.
The JMP “Metal Panel” Era from 1969-1981 In July 1969, Marshall replaced its gold plexiglass front and back panels on their amplifiers and replaced them with gold aluminum-brushed panels with black-screened lettering. The mark to the aluminum metal panel Marshalls also marks a significant reduction in comparative value between them and the earlier plexi-paneled Marshalls, though basically by late 1968 into early 1969, the circuit was essentially the same.
I say “essentially” because as was often the case, Marshall circuits varied.
I believe this is due to the incorporation of a new style of capacitor which was now a square plastic cap.
Again, in stock configuration, the higher-output pickups which inherently have emphasized bass but reduced highs seem to balance out the circuit.
While this is merely speculation on my part, the mid to late ‘70s Marshalls and especially the Master Volume series introduced with the 1976 model tend to sound more balanced with a high output pickup design. General consensus is that the EL34-equipped Marshalls were and are thought of more highly, with the added headroom of the 6550 not being as desired in the non-master volume Marshalls of the day.
This is the opposite case with the first plexi Marshalls which are really tuned to favor PAF-style humbucking pickups that are more tonally balanced. In 1973, Marshall introduced its printed circuit board (PCB) design to replace the laborious hand-wiring process though some models continued to be handwired until circa ’75.
Part II: Vintage metal panel through JCM 2000 series The early JMP Master Volume amps achieved their distortion through the preamp stage when played at lower volumes, raising the master volume controlled overall level, but power tube distortion could also be achieved just like with the non-master Super Leads by cranking it up. export versions of these amps continued to use the 6550 tubes, while all of Europe and Canada retained the use of EL34 tubes.
The sound of these ‘70s JMP master volumes was pure rock. Interestingly enough, and again in my opinion, the 6550 tube is my favorite when used in the master volume circuit.
2) The era of the custom pickup or high-output design had begun and was in full swing by the mid ‘70s.