Vintage Marshall 1968 JMP 50W Plexi (1987 model)

Candyman

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Hi everyone, how's it going?

First post here, hope you're all well. I'm in the market for a late 60s Marshall and have an opportunity to buy this amp. Hoping to get some insight from you knowledgeable folk about this 50W Marshall head from 1968. I have been told that:

  1. This is a super clean and mainly original JMP 50W small box plexi head from the early 1968
  2. It has the original transformers
  3. It has the original Mullard EL34s and NOS 12ax7s
  4. Has a new Marshall logo fitted but also comes with the original
  5. Filter caps are newly replaced with F&Ts but it also comes with the original caps still intact
  6. It has retro fitted rubber feet (the original feet are also included)
  7. From 6 to 8 on the volume dial it starts to grind up
  8. From 8 to 12 it becomes a rock beast with an amazing natural breakup
  9. It has had 6 minor component changes, and the tech has apparently said:
"We changed all the electrolytics, including the bias circuit. We added the .68 cap and 2.7k resistor to the bright channel cathode (as in most lead plexis) and changed slope resistor & treble cap in the tone stack from 250pF/56K to 500pF/33K to match most slightly later lead plexis. It fattens up the tone. The bits we changed were not original anyway."

I've got lots of pics, which should help us determine how original and desirable the amp really is.

I would like to find out what you think about this Marshall, what would be a fair price for it, and if there's anything questionable about it. All feedback is welcome!

PS I'll attach more pics to this thread shortly.

1968 JMP 50W Plexi (1987 model) - Front.JPG 1968 JMP 50W Plexi (1987 model) - Back.JPEG 1968 JMP 50W Plexi (1987 model) - Chassis 2 copy.JPEG 1968 JMP 50W Plexi (1987 model) - Chassis.JPEG 1968 JMP 50W Plexi (1987 model) - Chassis 3.JPEG 1968 JMP 50W Plexi (1987 model) - Choke.JPEG 1968 JMP 50W Plexi (1987 model) - Transformer 2.JPEG 1968 JMP 50W Plexi (1987 model) - Transformer 3.JPEG 1968 JMP 50W Plexi (1987 model) - Impedance Selector.JPEG 1968 JMP 50W Plexi (1987 model) - Impedance Selector 2.JPEG
 
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Seanxk

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Looks a very clean example, gut shots are needed really to give a better evaluation.
Shame they changed all those parts. But they left the Bulgin lol, which is good by the way.
 

Candyman

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I'm told that any pics with black filter caps are current. The ones with grey and blue caps are before it was fixed up.

BEFORE:

1968 JMP 50W Plexi (1987 model) - Guts copy.JPEG 1968 JMP 50W Plexi (1987 model) - Guts 3.JPEG 1968 JMP 50W Plexi (1987 model) - Guts 8 copy.PNG 1968 JMP 50W Plexi (1987 model) - Guts 4JPEG.JPEG 1968 JMP 50W Plexi (1987 model) - Guts 10 - Copy.jpg 1968 JMP 50W Plexi (1987 model) - Guts 7 - Copy.jpg 1968 JMP 50W Plexi (1987 model) - Guts 11 - Copy.jpg 1968 JMP 50W Plexi (1987 model) - Guts 9 - Copy.jpg 1968 JMP 50W Plexi (1987 model) - Guts 5 - Copy.jpg

AFTER:

1968 JMP 50W Plexi (1987 model) - Guts 2.JPEG
 
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kuli

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Hard to tell, these are before and after pictures I guess, but which ones are like the amp are now?
Nice amp by the way it looks!
 

Candyman

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Hard to tell, these are before and after pictures I guess, but which ones are like the amp are now?
Nice amp by the way it looks!
You're right, good point! I was told that any pics with black filter caps are current. The ones with grey and blue caps are before it was serviced. I'll try and split the pics into before and after.
 

neikeel

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Early mid 68 originally bass or PA amp.
Looks nice and clean. Work seems pretty good (only issue I have is the flying screen resistors.
V1 socket an both output tube sockets are replacements.
You can clearly see the new parts to make it a part lead amp (a few other changes needed for full conversion).
It is not a collectors piece but above average players amp. If the price is right and it sounds good go for it.
 

Candyman

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Early mid 68 originally bass or PA amp.
Looks nice and clean. Work seems pretty good (only issue I have is the flying screen resistors.
V1 socket an both output tube sockets are replacements.
You can clearly see the new parts to make it a part lead amp (a few other changes needed for full conversion).
It is not a collectors piece but above average players amp. If the price is right and it sounds good go for it.
Great feedback, and exactly what I was looking for - thank you! Looks like there's nothing on the back panel that would indicate which 50W non-MV model it was originally (1987 Lead, 1986 Bass or 1963 PA).

Is determining if it's a Lead, Bass or PA circuit from this era only possible by inspecting the circuit / guts?
 

neikeel

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Great feedback, and exactly what I was looking for - thank you! Looks like there's nothing on the back panel that would indicate which 50W non-MV model it was originally (1987 Lead, 1986 Bass or 1963 PA).
Is determining if it's a Lead, Bass or PA circuit from this era only possible by inspecting the circuit / guts?
Orignally a little gold dymo label on the back after the serial number (same style as the fuse rating sticker). Then yes just pictures of the guts (turret layout and components type and layout).
 

Candyman

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Orignally a little gold dymo label on the back after the serial number (same style as the fuse rating sticker). Then yes just pictures of the guts (turret layout and components type and layout).
Great info, cheers. There is definitely no label after the serial number, so I guess we'll never know which model it was originally.

If I buy it, one of my questions was what few other changes are needed for full conversion to Lead (1987) spec and would it be considered an affordable mod?
 

Seanxk

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Mods certainly affordable, but it's already not really a true 68 lead with it's new components, it's not cheap I presume, so why not get a 69/71 if you want that kind of Lead amp. What era sound are you looking for?
 

Candyman

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Mods certainly affordable, but it's already not really a true 68 lead with it's new components, it's not cheap I presume, so why not get a 69/71 if you want that kind of Lead amp. What era sound are you looking for?
I haven't had a chance to discuss the price after this thread, but it was ridiculously expensive to start with. I am after a late 60s plexi Lead amp, 50W or 100W.
 

neikeel

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A true one without mods and no issues in U.K. is about £4500
If it is ridiculously expensive and they will not haggle given the basic nature of their mods and modern parts I would walk away.
 

Derrick111

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Nobody mentioned this, but your opening info stated that they said the amp goes to 12?!?! Check if that's true, because an original faceplate will only go to 10.
 

Matthews Guitars

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Set your camera to a higher F-stop number to get less depth of field so more will be in focus in your pictures. Ideally, go for the highest F-stop that allows you to still get a good exposure.
 

Derrick111

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I think he meant o’clock?

I don't believe so... he said:
  1. From 6 to 8 on the volume dial it starts to grind up. (6 o'clock would be before "0" yet he's already breaking up.)
  2. From 8 to 12 it becomes a rock beast with an amazing natural breakup. (Chernobyl must happen then after 12 o'clock. Maybe he's using Sovtek tubes)
 

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