Seventeen-year-old Dudley Craven in his tiny ham radio shack, circa 1961. Many of the Marshall JTM45 prototypes would be refined and tested here. Photo courtesy of Barbara Craven.
But the amp’s unique harmonic characteristics caught the guitarist’s ear. “I got very angry, very frustrated,” he remembers. “I kept pushing them. I said, ‘You’d better [expletive] do this—there’s something happening here which is really interesting. You get up to a certain pitch, and something happens between the pickup and the amp. The guitar kind of starts to sound like a symphony orchestra.”
It was almost as if Townshend could peer into the future and see that overdrive would shape the new sound of rock ’n’ roll. “I knew that in distortion there was a music of a much higher harmonic order than anything that I could play,” he said in the aforementioned interview. “So I started that whole trip off.”
Bigger and Louder: Model 1959
In mid 1965 Marshall asked Craven and Bran to begin prototyping what would become model 1959, also known as the JTM45/100—Marshall’s first attempt at a louder amp. The design team increased the power by building up the JTM45 circuit, while taking pains to prevent the components from overheating. All amplifier manufacturers knew heat was the enemy of a reliable amplifier.
The first model 1959 prototype was totally experimental. It used one 5AR4/GZ34 rectifier tube, four 6V6 output tubes, and three ECC83/12AX7 preamp tubes for about 60 watts of power. They used a Radiospares “De Luxe” output transformer, but it couldn’t handle the power. The windings melted.
The second prototype was quite different. This amplifier used two 5AR4/GZ34 rectifier tubes, four 5881/6L6 output tubes, and three ECC83/12AX7 preamp tubes. The amp used two Radiospares 30-watt output transformers, which together could handle the amplifier’s power. This prototype reassured the design team they were going in the right direction.
Third Time’s the Charm
Craven perfected model 1959 with a third prototype in the fall of 1965. The design team had replaced the 5881/6L6 output tubes with KT66 tubes, which were easier to source in England. After testing with the dual 30-watt output stage, it became clear that heavier-duty output transformers were required. Craven selected a pair of Drake 50-watt output transformers (784-74), because at the time no 100-watt output transformers were available.
This third prototype of the Marshall model 1959, serial #6406, may be the first 100-watt Super Tremolo amplifier. These early 100-watt amplifiers incorporated a unique dual-output transformer design. There is no impedance selector switch.
Texas Instruments TS107 silicon diodes replaced the inefficient tube rectifiers, increasing power and changing the amplifier’s sonic character. The mushy “sag” characteristic of tube rectifiers was gone. The bottom end was tighter. Highs were clearer. The response was faster. And there was an added benefit: The amplifier would never fail due to a bad rectifier tube.
The power transformer was a large military/industrial-grade model manufactured by Radiospares. Because this amplifier was used only for prototyping, the power transformer did not have a USA voltage tap.
The chart (right) shows the various voltages that can be obtained from the three taps on the power transformer in amplifier #6406—which is owned by the author and is one of the 12 original, dual-output 100-watt Marshalls.
Dual-Output 100-Watt Marshall Amplifiers
Dual-output Marshall amplifiers were manufactured for a few months in late 1965 and are extremely rare. Decades later, when Marshall conducted research for the 40th anniversary of the 100-watt stack, it was determined that a total of 12 dual-output amplifiers were manufactured, including the third prototype of model 1959. Ten of these 12 amplifiers were built with Radiospares power transformers with the USA voltage tap.
The dual-output model 1959 was available as a PA, bass, or lead model. These early amplifiers were built on aluminum chassis that are prone to warping and cracking under the weight of the transformers. The front panel is gold plexi, a look borrowed from the JTM45. (That’s why some refer to the early model 1959s as JTM45/100s.) The PA amps received JTM100 gold plexi panels. The back panels were white with the “Super Amplifier” logo silkscreened in gold. The first few amplifiers used square power boards, which would have trouble clearing several of the output tube sockets and the internal fuse. Eventually the power board was cut slightly to provide clearance for these components.