NAD! '78 Marshall JMP 1987 - no mods, mostly all original, some issues!

andyk

Silver Supporting Member
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Just bought this one on Craigslist (although it was listed on eBay and Reverb too). All original transformers, decent shape, although the caps are shot and the amp makes strange ghosting noises that fizzle out after each note. I ordered new F&T caps that will go in ASAP.

Build date says March of 78. Does the board look stock? How about the bias caps?

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sickboy79

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14,248
Sweet! Nice score! Can't help with the caps as I'm not a vintage Marshall expert but, that looks great. Nothing looks odd though. Enjoy and crank it up after you cap it!
 

AXEnGEAR4J

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5,898
Owned several. Sold one almost a year ago. It looks great clean and original to me!
I bet it's loud as hell! Mine was pure AC/DC sounding, wished I'd kept it but the money was good!
 

andyk

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2,241
Cool! I have my TAC low watt adapters ready to go, bringing the amp down to 4 watts or so. That plus my Hotplate may mean no LarMar after all!
 

michael.e

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20,762
Sweet amp! Is that an OJ in there? My uneducated guess is that the siliconed in parts are not original.
 

andyk

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2,241
Yeah, I have an old mustard .68 I'll be putting back into the presence to make it stock. What would changing the value do??
 
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DrainBamage

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2,414
The smaller the cap .1uf lets higher frequancys through. The .68uf has a lot more mid range. Makes for a gainy power amp. Didn't you do this to you last one? I remember you made the presence more plexi like.
 

andyk

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2,241
Nope, my last one has the stock .68 on the presence.

Wonder what values lie under the silicone for the orange and the yellow caps? I'll have to check it out.
 

Ultra GP

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1,998
Besides the presence circuit, it's mainly just the V1A cap and resistor. It should be 220uf and 820 ohm. The yellow cap is stock (you can see another one in the amp too - my super bass from the same year has several of these on the board as well).

There are a few other resistors changed (bias splitters) but I wouldn't worry about those. It might have been setup for 6550s originally.

Not sure about the bias caps. They look a lot like the ones Marshall used around that time anyway.

Also, obviously I can't really judge by this, but it looks like that EL34 on the left has been running pretty hot... I'd put a new set in.
 

andyk

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2,241
The tubes are shot and will be tossed. Some amp sellers just stick in their oldest spare tubes.

Why was the silicone slopped onto just the two areas on the board? And, what presence cap value(s) should the amp have to be stock - .68, or .1? I can't find any late 70's 1987 simple diagrams showing the presence pot and caps.
 
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Ultra GP

Gold Supporting Member
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1,998
The tubes are shot and will be tossed. Some amp sellers just stick in their oldest spare tubes.

Why was the silicone slopped onto just the two areas on the board? And, what presence cap value(s) should the amp have to be stock - .68, or .1? I can't find any late 70's 1987 simple diagrams showing the presence pot and caps.

Well the orange drop is a replacement part, so whoever did the work probably wanted it to be extra secure. But I doubt it's needed in this case. As for the bias caps, I'm not entirely sure, especially if they are stock.

The correct value would likely be a .68uf, but not sure if your particular amp would have had a mustard there or something else. I'd save the .68uf mustard in case you need it for something more important in the future, and either put another .68uf there (doesn't need to be NOS) or a .1uf (which I prefer the sound of personally)

If it's of any interest, I think I might have a few 160v .1uf mustards left. These are pretty good for the presence pot - much smaller, and you don't have to 'sacrifice' another 400v one when it can be used elsewhere.

Do you still have that pair of RFTs from the '76 1987 I sold you? Or did you sell them with the amp?
 

andyk

Silver Supporting Member
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2,241
Well the orange drop is a replacement part, so whoever did the work probably wanted it to be extra secure. But I doubt it's needed in this case. As for the bias caps, I'm not entirely sure, especially if they are stock.

The correct value would likely be a .68uf, but not sure if your particular amp would have had a mustard there or something else. I'd save the .68uf mustard in case you need it for something more important in the future, and either put another .68uf there (doesn't need to be NOS) or a .1uf (which I prefer the sound of personally)

If it's of any interest, I think I might have a few 160v .1uf mustards left. These are pretty good for the presence pot - much smaller, and you don't have to 'sacrifice' another 400v one when it can be used elsewhere.

Do you still have that pair of RFTs from the '76 1987 I sold you? Or did you sell them with the amp?

Interesting! I sold the RFT's with the '76. That was a cool amp. I forgot why I sold it!
Probably to buy the Bray modded JMP. I have a bag of old mustards, removed by Dave Bray. I'll see if there's a smaller .1uf in there. If not I may take you up on your offer, thanks!
 
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andyk

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2,241
This is all I have - both .68uf. The schematics all seem to call for a .1UF - which I don't have.

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DrainBamage

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Interesting! I sold the RFT's with the '76. That was a cool amp. I forgot why I sold it!
Probably to buy the Bray modded JMP. I have a bag of old mustards, removed by Dave Bray. I'll see if there's a smaller .1uf in there. If not I may take you up on your offer, thanks!
What amp and why did Bray remove the Mustards? Mustards rarely go bad tbh.
 

andyk

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2,241
I think Brays mod changed the values, hence the mustard removal.
 

Ultra GP

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When are the schematics from? I'll see if I still have the .1uf mustard I can send you. I think they're actually 200v, like these.

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I know one (which I've already used for another amp) was pulled from the presence pot on an old Traynor amp. They seem to be a decent alternative if you don't want to use up the 400v ones.
 

andyk

Silver Supporting Member
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2,241
Schematics from 1970, and 1988 show .1uf as the right value. Not sure what's in there now though. It's not marked.
 

Fulldrive-1

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5,926
This is the M1987 amp, i.e. the four-jack, non-master 50 watt head. By the time this amp was made, it was much less common than the master volume version.

My favorite amp of all times is my year - 1973 version of your head. The main difference is that yours came equipped with 6550 tubes whereas the oldies came with EL34.

Jumper the southwest and northeast inputs with a short cord (they sell purpose made ones) and dial it in. Put a good OD or distortion or fuzz in front of it and it's better than non-pharmaceutical stimulants.

The only "mod" worth considering is to have a tech re-bias it (requires minor component changes) and put in EL34s.
 
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