1981 Marshall JMP 2203

The Marshall JMP 2203 is the amplifier that made Hard Rock and the beginnings of Metal possible, and then so famous that all of music was influenced by the genres played on this amp. Debuted in 1975 The JMP 2203 was the evolution of the Plexi circuits (1959/1987) that had dominated Rock music and Marshall’s lineup previously. Marshall made the decision to factory install a Pre-Phase Inverter Master Volume in their amps, which was already a common modification many techs were installing to let players get more distortion from the Plexi circuit. Another difference from the 1959 Plexi is that the first gain stage of the circuit is cascaded into the second which was changed to be very cold biased (AKA the Cold Clipper) - this creates crunchy and pleasing asymmetric distortion in amounts that were not available on any amplifier made before. The EQ section and Power Amp circuits were left much the same as Marshall’s previous designs. The Marshall JMP 2203 is the exact same circuit as the JCM 800 released in the early 1980’s, but with 1970’s aesthetics and construction. The reason for this is that in 1981 Marshall’s distribution deal with Rose-Morris ended and Marshall decided to update the looks of their amps and rename the series JCM 800. For whatever reason (probably just marketing) the JCM 800 is more closely associated with Metal and many types of Hard Rock even though the same amount of distortion and same circuit were available with the earlier JMP 2203 since 1975. Differences in construction between the 2203 and JCM 800 are: the 2203 uses upright metal bobbin transformers vs the JCM 800’s lay-down plastic bobbin units, hand-wired pots vs. PCB mounted pots, and different brand caps/resistors. Many people say this makes the 2203 sound less bright/shrill and have sweeter overdrive/compression. This comparison is especially true when considering the horizontal-input JCM 800’s made from 1986 on. These later JCM 800’s featured a big circuit change engineered by Marshall as a cost cutting measure. They reduced the number of filter caps from 6 down to 3 by the end - this made the amps much more shrill and aggressive, they were also less composed when played at high volume. The 1981-1985 models do not have this circuit change and aesthetically can be differentiated by the vertical input jacks on the front panel. For all these reasons many people believe the 1975-1981 Marshall JMP 2203 is the high water mark in high gain amp production. For me personally it is tied as my favorite amp of all time along with the 1964-1967 Fender Twin Reverb.

This amp was made in the very last year the 2203 was manufactured. It is the Canadian version which contains a whole separate PCB for safety fuses that protect all circuits around the valuable Power Transformer and Output Transformer - this is a smart idea that for some reason never carried over to the English, European, or American versions of this amp. I believe the Canadians at this time had much stricter safety regulations for electric devices. Notable users include: Jimmy Page (Led Zeppelin), Angus Young (ACDC), Jeff Beck, Kerry King (Slayer), Jeff Hanneman (Slayer), Andy Summers (The Police), Joey Santiago (Pixies), Tom Morello (Rage Against The Machine), Randy Rhoads, Zakk Wylde, and James Hetfield (Metallica).

This amp came in having already been repaired before, however the quality of the work was pretty bad. Most of the necessary preventative maintenance was left undone. The biggest offender was that all the tube sockets had been replaced but they were mismatched and coupled with lazy and incorrect wiring to the sockets. The heater lines were not done in twisted pair formations which causes 60-cycle hum. It needed a cap job, new power/screen/grid stopper resistors, a new octal tube socket, power tubes, lots of hardware replaced, new fuses, and cleaning.

The amp was given a full restoration. All the axial lead electrolytic capacitors were replaced with upgraded temperature rated Vishay brand electrolytics. The multi-section cap cans were replaced with new German made F&T 50/50uf 500v type units. The power dropping resistors were upgraded to 3 watt Metal Film type and the Screen Grid resistors were upgraded to 5 Watt Cement type units respectively. This was done for better noise floor and reliability from heat damage.

The heater lines were replaced with the correct twisted pair formation and all other associated wiring was taken out, leads twisted, then pre-tinned, and re-soldered in a neat manner. One of the octal tube sockets was mismatched with the others - this was replaced with the correct high quality ceramic socket. 1 ohm 2 Watt Metal Oxide resistors were installed on pin 8 of the power tubes to provide an easy way to check the bias. New brass hardware matching the original type used by Marshall was installed on all the tube sockets and two cap cans because the original hardware in those places was completely stripped out and destroyed by the previous tech. One knob (Presence) was the wrong type for this amp, the set screw was stripped out, and the unit had been glued on - this was cut of with a rotary cutting blade so the correct knob could be installed. Five 3AG type fuses were replaced as the units present in the amp were the wrong values - many far over-rated for the specs requested by Marshall which made them unsafe.

The amp was completely re-tubed - the preamp tubes all tested good but the new TAD brand tubes provided by the customer had less noise and better tone. This was the final lineup: V1-V3 = TAD ECC83-Cz , V4-V7 = JJ E34L Matched Quad. I picked out this lineup specifically for this amp to give the best balance of tone, reliability, and low noise. The adjustable Fixed Bias was set to %63 Class AB Plate Dissipation with a B+ of 466 VDC. All pots were cleaned with De-Oxit and all hardware was tightened.