Marshall JMP 50 Lead (1987 model)

I got this JMP 50 lead (1987) circa 1974.

Marshall JMP50 JMP 50 1987

Marshall JMP50 JMP 50 1987 label

Marshall JMP50 JMP 50 1987 PCBMarshall JMP50 JMP 50 1987 PCB2

Reforming caps test, it seems to work (a few mA through the resistor 100k/2W, for several hours, until the voltage across the 100k resistor is less than 10% of the maximum voltage) :

Marshall JMP50 JMP 50 1987 reforming caps

OK, here is the schematic of this beast :

Marshall JMP 50 1987

Why is the sound of the normal channel so boomy, without highs ? It is because bright caps of the bright channel interact with the normal channel and they suck the high frequencies … Let’s see a SPICE simulation :

If we disconnect bright channel from normal channel :

Marshall JMP 50 1987 Normal channel disconnected from bright(Output of RMix1 is grounded on this simulation : bright channel disconnected)

In this case, we get a “normal” bandwidth for the normal channel ;-)  :

Marshall JMP 50 1987 Normal channel disconnected from bright bandwidth

When bright channel and normal channel are both mixed through 470k resistors :

Marshall JMP 50 1987 Normal channel bright bandwidth mix schematic simulation(Bright volume is at zero)

Here is the bandwidth of the normal channel, highs are greatly attenuated :

Marshall JMP 50 1987 Normal channel bright bandwidth mix bandwidth simulation

Simulation of both channels mixed (90% normal, 10% bright) :

Marshall JMP 50 1987 both channels mixed bandwidth

The bright channel is definitely too crazy bright ! So I replaced the 2.2nF coupling capacitor with a mustard 22nF, and the 5nF “volume pot” bright cap with a 300pF. Still very bright (due to 500pF // 470k as usual in the Marshall world), but mixed with normal channel, it sounds like hell !

Marshall JMP 50 1987 Coupling Capacitor 2.2nFMarshall JMP 50 1987 Coupling Capacitor 22nF Marshall JMP 50 1987 Bright Cap 300pF

Marshall JMP 50 1987 DALY Cap

Marshall JMP50 chokeChoke

Marshall JMP50 output transformerMarshall JMP50 output transformer 2Output transformer

Marshall JMP50 power transformerPower transformer

Vintage style impedance and voltage selectors restoration

Impedance :

Marshall JMP50 vintage impedance selector 1Marshall JMP50 vintage impedance selector 2Marshall Vintage impedance selector wiring color codeMarshall Vintage impedance selector 3

Voltage :

Marshall Vintage voltage selector wiring color codeMarshall Vintage voltage selectorMarshall Vintage voltage selector - 2

Done !

Marshall JMP 50 1987 MetalFace

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One Response to Marshall JMP 50 Lead (1987 model)

  1. Pingback: 1975 4-hole 50w Lead USA - schematic? - MarshallForum.com

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