Jim Marshall agreed, and several of Hendrix's road crew were trained in the repair and maintenance of the Marshall amplifiers through the years. Mid-1970s and 1980s models The JMPs After 1973, to streamline production, labour-intensive handwiring was discontinued and Marshall valve amplifiers were switched to printed-circuit-board (PCBs). Much of the debate about the difference in tone between the plexi- and aluminium-panel Marshall amplifiers originates from 1974 when a number of circuit changes were made to the 1959 and 1987 amplifiers; with the addition of 'mkII' added to the 'Super Lead' name on the back panel and 'JMP' ("Jim Marshall Products") added to the left of the power switch on the front panel. Marshall's US distributor Unicord also had them change all the amplifiers sold in the US and Japan to the much more rugged General Electric 6550 instead of the EL34 output tube. The combined effect of different valves and a modified circuit gave these mid-1970s Marshalls a very bright and aggressive sound that was punchier than the EL34 sound, but not as rich, compressed, and had less poweramp distortion. In late 1975, Marshall introduced the "Master Volume" ("MV") series with the 100W 2203, followed in 1976 by the 50W 2204. This was an attempt to control the volume level of the amplifiers whilst maintaining the overdriven distortion tones that had become synonymous with the Marshall brand. To do this, Marshall designers connected the two input stages in series rather than parallel on the 2203, but not initially on the 2204, and modified the gain stage circuitry to preserve the tonal characteristics of the 'cranked Plexi' sound and converted the now obsolete second channel volume control to a Master Volume by wiring it between the preamplifier and EQ circuit. The 2204 followed suit in early 1977 and changed its preamplifier circuit to match the (then) more popular 2203. Per Rick Reinckens, who was a short-term Unicord employee electronic technician who tested the first units when they arrived from England, Tony Frank, Unicord's chief design engineer, came up with this idea for a dual-volume-control (a preamplifier gain and a master volume). The circuitry modifications were optimised to replicate the sound of the earlier non-MV Marshalls with the Master Volume control set 'low'; however, players quickly realised that 'cranking' the MV of these new Marshall amplifiers would yield even more overdrive distortion, the tone of which was more cutting and edgy, and