I got this JMP 50 lead (1987) circa 1974.
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) :
OK, here is the schematic of this beast :
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 :
In this case, we get a “normal” bandwidth for the normal channel :
When bright channel and normal channel are both mixed through 470k resistors :
Here is the bandwidth of the normal channel, highs are greatly attenuated :
Simulation of both channels mixed (90% normal, 10% bright) :
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 !
Vintage style impedance and voltage selectors restoration