NAD - 1969 Marshall JMP 50 in need of some TLC

Phatfrank

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Hi guys! Been lurking here for a while, but finally took the plunge the other day and brought home a 1969 JMP 50. Sounds amazing, but needs some cosmetic TLC:).

Firstly, the previous owner had the front tolex and head cabinet partially painted in gold (!). It actually looks kinda cool, but I would like to have it restored to former glory if possible. I do prefer the old beat-up look so not wanting to get it "as new", and as such re-covering it is not an option for me. As far as I can tell (and according to the guy I bought it from) the tolex is original. For the faint hearted, please avert your eyes:).

jmp50-002.jpg jmp-004.jpg

I was wondering if you guys could help me with the following;

What is the best way to remove old paint (from the early 80ies)? I've read about duct tape, paint remover, naphtha, etc. I tried with a soft toothbrush and some mildly soapy water but that did not work.

What are the correct dimensions for the piping labeled1, 2 & 3 in the following picture (not my amp in the picture) and where can I find decent repros? Also, I have been told (as mentioned) that the tolex is original but shouldn't there be visible grooves where the horisontal piping (#3) and top piping (#1) are?

Piping.jpg

I've tried finding a decent "step by step" guide to replacing piping but can't find it. It seems fairly straightforward but all tips are welcome:).

I also need a couple of new knobs, any links to decent sources (or where I could find some originals) is much appreciated.

Otherwise the amp seems to work well. AFAIK one cap has been changed, and at some point an effects loop was added (!) but has since been completely removed. I am completely ignorant to the workings of amps, but thankfully there are a few knowledgeable amp doctors here in Norway that can assist when/if something goes haywire. A couple of images from the innards are following.

I have found an all original 69' cab locally that hopefully will be joining the top shortly, will keep you posted. In the meantime, thanks in advance for all tips and thanks for a great forum!
317_56614166.jpg 317_80272603.jpg 317_938715368.jpg
 

neikeel

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Welcome
In general the piping available now is not the same so you should try to save the original stuff.
I would use cellulose paint thinners very sparingly (test in unseen area first)
Replacement metal panels are available from @Matthews Guitars .
The amp is more likely 1970 but could be wrong
Board looks like a Bass (1986) or Organ (1989)
Should be a laydown Drake 1202-118 Power transforme
Needs a clean, correct PT and a couple of Iskra 100k plate resistors for V1
New bias caps and tidy the wiring around the PT and Marshall spec pilot lamp
We'll ignore the added preamp tube !
 

Matthews Guitars

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The added preamp tube socket gives you the option for some mods without having to commit acts of violence up on the metalwork, that having alkready been done. If you're looking for a moddable candidate, you've got it.

It's an interesting mix of replaced components, and original components, some of which I'd have expected to be replaced by now. Such as the bias capacitors.

This is an amp that should be appreciated best for its tone, and not for its condition. It's no museum showpiece. But it'll be a fine honest rock and roll machine, with a little work.

I'm pretty sure I've got a replacement front panel for that.
 

neikeel

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No it has not.
See the short white wire to the bias feed comes off the diode block. This is only activated by standby being live. So sure the snubbers are off after PT blow but bias still wrong.
 

Phatfrank

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Thanks for all the replies, I am quite the novice when it comes to the workings of an amp so the knowledge here is very much appreciated.

I'll give the paint job a try next week, in the meantime; @neikeel - I actually did some research (read; 5 minutes with google:)) and I found info on this forum that many JMPs headed for the US were fitted with 1202-164s originally? The seller bought this amp in 1980 and he was 100% sure that it was fitted to the amp when new... I'll forward your insight on recommendations to my amp tech, was planning to have it seen over shortly. And forgive me for asking, but what do you mean with "We'll ignore the added preamp tube !"? And again, please do the dumbed down version for me:).

And thanks for tips on replacement panels, but I intend to keep this beater "original" and use it as it was intented:).
 

shakti

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Congrats, that’s the one on Finn.no right? It would have had a lay down PT originally. What is the plate voltage with that 1202-164 PT? IIRC the later 164s had a lower plate voltage around 370VDC.

Intrigued about the 69 cab you allude to, so tell/show us more about that!
 

Garth Rocket

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Hi guys! Been lurking here for a while, but finally took the plunge the other day and brought home a 1969 JMP 50. Sounds amazing, but needs some cosmetic TLC:).

Firstly, the previous owner had the front tolex and head cabinet partially painted in gold (!). It actually looks kinda cool, but I would like to have it restored to former glory if possible. I do prefer the old beat-up look so not wanting to get it "as new", and as such re-covering it is not an option for me. As far as I can tell (and according to the guy I bought it from) the tolex is original. For the faint hearted, please avert your eyes:).

View attachment 118024 View attachment 118023

I was wondering if you guys could help me with the following;

What is the best way to remove old paint (from the early 80ies)? I've read about duct tape, paint remover, naphtha, etc. I tried with a soft toothbrush and some mildly soapy water but that did not work.

What are the correct dimensions for the piping labeled1, 2 & 3 in the following picture (not my amp in the picture) and where can I find decent repros? Also, I have been told (as mentioned) that the tolex is original but shouldn't there be visible grooves where the horisontal piping (#3) and top piping (#1) are?

View attachment 118025

I've tried finding a decent "step by step" guide to replacing piping but can't find it. It seems fairly straightforward but all tips are welcome:).

I also need a couple of new knobs, any links to decent sources (or where I could find some originals) is much appreciated.

Otherwise the amp seems to work well. AFAIK one cap has been changed, and at some point an effects loop was added (!) but has since been completely removed. I am completely ignorant to the workings of amps, but thankfully there are a few knowledgeable amp doctors here in Norway that can assist when/if something goes haywire. A couple of images from the innards are following.

I have found an all original 69' cab locally that hopefully will be joining the top shortly, will keep you posted. In the meantime, thanks in advance for all tips and thanks for a great forum!
View attachment 118021 View attachment 118022 View attachment 118026
I’ve read tolex can withstand paint remover, it that is it would certainly do the trick. What ever you use remember to keep it away from the piping as that may react to it. Good luck! Cool amp!
 

Phatfrank

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Congrats, that’s the one on Finn.no right? It would have had a lay down PT originally. What is the plate voltage with that 1202-164 PT? IIRC the later 164s had a lower plate voltage around 370VDC.

Intrigued about the 69 cab you allude to, so tell/show us more about that!
Yepp, that's the one. I'll try to have the voltage checked, but as far as I could tell from the Great Internet you're correct. The cab is a friend's, will let you know if it ends up here. 100% original and clean.
 

Phatfrank

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As for paint removal I’ve had great results with Duxola paint stripper:

It’s like a gel you brush on and let work for a while, then use a soft brush to rub off the paint. It doesn’t harm the tolex but you will need to rinse thoroughly.
Thanks! I actually looked at that one - good thing there are more norwegians on here:)
Their was a fourth preamp tube socket bolted in but not wired up, the original ones are riveted in. @neikeel is suggesting that we’ll not mention that mod, maybe suggesting that it goes without saying that it’s extra.
Thanks! That makes sense.
 

neikeel

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Thanks for all the replies, I am quite the novice when it comes to the workings of an amp so the knowledge here is very much appreciated.

I'll give the paint job a try next week, in the meantime; @neikeel - I actually did some research (read; 5 minutes with google:)) and I found info on this forum that many JMPs headed for the US were fitted with 1202-164s originally? The seller bought this amp in 1980 and he was 100% sure that it was fitted to the amp when new... I'll forward your insight on recommendations to my amp tech, was planning to have it seen over shortly. And forgive me for asking, but what do you mean with "We'll ignore the added preamp tube !"? And again, please do the dumbed down version for me:).

And thanks for tips on replacement panels, but I intend to keep this beater "original" and use it as it was intented:).
The amps with the 1202-164 PT did not have a dirty great hole in the chassis for a drop through transformer and have a new one fitted with new holes and shiny nuts and bolts. The wiring loom would be much neater.
I am in no doubt that the amp sounds great (it is to typical late 60s plexi spec). The era of -164 would be interesting as the first ones were same B+ as the -118s and last ones very low. It is relevant as the higher voltages run a bit cleaner for a bit longer, low B+ brown out at lower volumes.
 

Phatfrank

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The amps with the 1202-164 PT did not have a dirty great hole in the chassis for a drop through transformer and have a new one fitted with new holes and shiny nuts and bolts. The wiring loom would be much neater.
I am in no doubt that the amp sounds great (it is to typical late 60s plexi spec). The era of -164 would be interesting as the first ones were same B+ as the -118s and last ones very low. It is relevant as the higher voltages run a bit cleaner for a bit longer, low B+ brown out at lower volumes.
Aha, thanks again. I’d normally prefer to learn more before purchasing old gear, but didn’t have the time so the knowledge here is very helpful indeed👍🙏. Will try and take some better pictures asap.
 

LoudStroud

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There's sumthin kinda cool about that paint job. A rock n roll time piece w some sorta cool story, I'd imagine. Very prog. Have you considered keeping the original painted box as is and buying a repro replacement? Along with what I'm sure is a killer tone, it's an attention grabber.
 

66tele

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I thought I would pass along my successful experience removing spray paint from a Sunn 1200S. This subject has been discussed several times on many forums. I suspect that many posts are from well-meaning guys posting their best-guess, but not necessarily based upon actual experience. So here is some real-world results....

The amp cabinet had been spray-painted gold by a previous owner. I did a bit of internet research on this subject and found recommendation for Motsenbocker's Liftoff Spray Paint Graffiti Remover (also referred to as Motsenbocker's #4). Available from Home Depot in 22 oz. spray bottles or 1 gallon jugs. It's water-based, so not too stinky. And it is recommended for painted vinyl.

This stuff works very well! It takes about 6 applications to get all of the paint off. Here is how I did it.....

Remove the chassis and all of the hardware. Spray the remover on the tolex.....just enough to wet the surface and let it work for a 2-3 minutes. Take a stiff, plastic-bristle scrub brush wrapped with a cotton paint rag (t-shirt material) and begin scrubbing the tolex. The bristles force the cotton rag down into tolex crevices to soak up the dissolved paint. Remember that the goal is to remove the paint, so keep a fresh section of rag on the brush. As soon as it gets soiled or wet, move to a fresh section of rag on the brush. Don't get cheap on the rags or you'll just be moving the dissolved paint around! If you wait too long, the paint will begin to solidify. If this happens, just spray another light coat of the remover.

I found that it's best to do a first application on the entire cabinet to get the first layer off, then do the second application, etc. If you try to completely clean one side of the cabinet, you will inevitably get some spill-over from the next side, spoiling the clean side. You need to be patient, thorough and detailed, but it's worth it. This cleaner will also take the paint off of the hardware.

The cleaned vinyl will be a bit tacky, but it does return to normal in about 30 minutes. You can also wash the vinyl with water to neutralize the cleaner. Re-install the hardware and you are good to go.

I did try this product on painted grill cloth, but it would take a lot more work to remove the paint (possibly soaking the cloth in a bucket of the remover and continually scrubbing it), but I gave up on this after several cycles.

Still, this product works very well on tolex!

Good luck with your restoration!
 

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