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 :
(Output of RMix1 is grounded on this simulation : bright channel disconnected)
In this case, we get a “normal” bandwidth for the normal channel 
:
When bright channel and normal channel are both mixed through 470k resistors :
(Bright volume is at zero)
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 !
Choke
Output transformer
Power transformer
Vintage style impedance and voltage selectors restoration
Impedance :
Voltage :
Done !
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You wouldn’t happen to have any of the impedance or voltage pieces, would you? The internal switch, not the little plug.
I’m sorry, I don’t understand …
Hi! I have a Marshall JMP 1987 50W MKII 1976 that look very close to your amp. (i know it’s a 2012 publication but i found this tread on the net…) I’m looking to have that “marshall tone” rich, huge and full in the spectral frequencies, but here how i can describe my sound: Fizzy, overtone of high mids in the feedback loop when i hit a chord (a big E let’s say!) and it slowly run out. First hit is ok(when i hit a E), but when i leave it roll, the sound begin to fizz, lack of lows and richness in the mids… sound is much more in the 2khz to 5khz and it’s strange. More like a “zzzzzzzzzzzz” overall. When i’m playing single note(solo with the 1-2-3nd strings E-A-D), it’s even more noticable. So, i’ve red that old NOS preamp and power tubes can help me in this. When i get this amp, it has 3 x Mullards 12ax7 I63 that were completly out of shape (test below normal, 1 microphonic…) and 2 x Sylvania 6CA7 EL-34 (1 out of shape too…). So, my tech replaced them by new JJ E34L and 3 x JJ ECC83s. He also change a 47pf red cap on the PCB (plate cap?), but still have the problem of fizz like describe. So, I decide to replace all the ECC83 and made a LOT of test with preamp tube. I ended up with a Westinghouse 5751 in V1, a Mullard NOS 12ax7 I61 in V2 and a CV4004 M8137 in V3. Power tubes were replace by NOS Mullard XF2 matched pair and are biased. Still have that fizz, but the general tone have change and it began to be a cooler tone. I want to get rid of that final part to have a more rich and full sound in the spectral. I compare my PCB to yours (some difference, yes: i have all green caps .022 400V x 6, Bias caps are are .47uf 63V (ERIE 20103) instead of your 0,1uf-160V
(ERIE 20102) and other things. I also get rid of the bright cap on HI volume pot to reduce all that highs. Can you help me in my project? I can send you pictures of my actual amp. Thanks! Dominic
EL34 oscillations ?
It might be time to replace your electrolytic capacitors. They wear out after 10-15 years. Some will last longer but it’s often good practice to go ahead and get those changed out. Google it and do a little research. It sounds like what you might be experiencing?
Sorry… That should have said ~15+ years on electrolytics. Relative and subjective though… but, look into it if inclined
Hi, I had same problem – together with low voltage by PT (plate 360V) it was horrible sound. I changed 1k 5W screen grid resistor to 180R – big, rich, full dynamic sound.
Hi again…have you installed 1k 5W screen grid resistor or diferent value? There is big. Its value together with the size of the plate voltage affect the sound, the point is that originally these resistors were not installed.
If the amplifier has a lower plate voltage than 400V, the resistance value is 1k too high. It wants to try to reduce up to 150R or no resistance, but then carefully with volume to the maximum to avoid melting power tubes.