Marshall JMP 50W '67 and '71 - How original? (Pics + sound clips)

[Jucciz]

Hi everyone and especially you vintage Marshall experts!

I'm lucky to have two old 50-watt Marshall heads: a '67 JMP plexi 1986 bass model and a '71 JMP aluminium panel (1985 to 1986 conversion). I'm interested in knowing how original these babies are, so I really appreciate it if you can help me out even the slightest bit.

The '67 JMP doesn't have the original logo, unfortunately. (Please sell me one if you have!) Other than that it looks pretty good to me, but I hope you experts can tell me more about it.

Here are some detailed pics of the '67: Dropbox - Photos - Simplify your life

The '71 JMP head started its life as a 1985 model PA head. Other than having been converted to a 1986 bass specs and at least partially recapped, how original is it in your opinion? For what I know, the voltage and impedance selectors have been hard-wired by the previous owner to 230V, 8ohm jack and 16ohm jack, but I do have the original "window style" selectors so those can be restored easily if needed. Also the mains socket has been replaced with an IEC type for easier connectivity - I do have a Bulgin socket at hand though.

Here are some detailed pics of the '71: Dropbox - Photos - Simplify your life

I have a question regarding a couple of capacitors inside the '71: the original Hunts 8uF (or were they already 10uF in 1971?) electrolytic caps have been replaced with newer 10uF ones. Is it worth the trouble to replace them with period-correct NOS Hunts/Erie caps now that they still are available on eBay for a few bucks? Also the channel mixer has a brown disc capacitor now. What brand are those red caps in the circuitry and is it possible to find those anywhere? The mixer bypass cap is something I'd probably like to replace with a "red one". Or then again... is it worth the trouble either?

Then of course one thing... I'm definitely not planning on selling either one, but still... how much would one get for these nowadays?

Here's a couple of sound clips of the '67 and a Marshall 1960TV cab from around 2003, pre-China Celestion G12M Greenbacks (just random noodling):
http://mp3.jucciz.com/aikuisten_vahvistimet/jmp_lp.mp3
http://mp3.jucciz.com/aikuisten_vahvistimet/jmp_strat.mp3
http://mp3.jucciz.com/aikuisten_vahvistimet/jmp_tele.mp3

Here's a quick sample of the '71 played with a Gibson Les Paul '59 Reissue, boosted with a Legendary Tones Time Machine Boost - the cabinet was actually a Kingsley D32C combo enclosure with 2x Celestion G12H's from '76: http://mp3.jucciz.com/other/jucciz223.mp3

Btw, It's funny how much smoother breakup the '67 JMP has. The '71 is much more aggressive, but I do like both of them. I try to answer questions if you need more info. Thanks to everyone!

- Jucciz -
Finland

 

[Jucciz]

Found it out already: the red caps are Lemco.

Any comments regarding to the original posting?

 

[boola]

Given that both amps are conversions, they both look very good, all period correct components.

Personally I wouldn't go with NOS electrolytics from 1970, it wont affect the value if they are replaced with new equivalents.

I have a very early 1970 jmp 50 lead on the way next week so it will be interesting to compare them, these amps went through so many changes in this era!

 

[SmokeyDopey]

Besides the amp, I love the photos!

 

[Jucciz]

Given that both amps are conversions, they both look very good, all period correct components.

Personally I wouldn't go with NOS electrolytics from 1970, it wont affect the value if they are replaced with new equivalents.

I have a very early 1970 jmp 50 lead on the way next week so it will be interesting to compare them, these amps went through so many changes in this era!

Thanks for your input. Please post photos of your JMP if possible.

By the way, how did you come to a conclusion that the '67 is a conversion? At least I didn't state that.

Besides the amp, I love the photos!

Thanks, nice to know that. I like photographing these babies almost as much as playing them.

 

[boola]

Sorry, I misread your post! It's in extremely good nick even though it's not a conversion I would think it is very valuable

 

[Jucciz]

Sorry, I misread your post! It's in extremely good nick even though it's not a conversion I would think it is very valuable

Okay, no problem. Thanks for your comment.

 

[Jucciz]

Hi!

I got the '71 serviced. I decided to take her to the doctor as well, since my newest acquisition - a '68 JMP 50-watter - needed service anyway. See the other thread about that if you're interested: http://www.marshallforum.com/lets-talk-vintage/31454-marshall-jmp-plexi-50-watter-68-a.html

Anyway, here's how the '71 looks now:
Dropbox - Photos - Simplify your life

Here's what was done to the amp:
- New filter caps: TAD Gold Caps, 50+50uF - all three of them on top of the chassis
- New bias caps (Sprague Atom, 8uF/150V)
- New V1 cathode bypass cap (Philips, 220uF/63V)
- New V1 cathode resistor (820k): the old one had too short leads in order to be connected across the cap.
- A bit of tidying up for the wires in general: a previously audible, albeit not too annoying mains hum disappeared completely. This amp is now very, very quiet, hooray!
- The mixer bypass cap was removed - this thing has enough treble even without it. And bass models shouldn't have it in the first place.
- Some not-so-good-looking wires on one of the filter caps were replaced with more sturdy ones.
- Some solder joints were redone just in case.
- The previous fixed 8ohm and 16ohm speaker outputs were rewired back to stock specs: two jacks in parallel and a correct window style impedance selector.
- The V3 100k plate resistor had drifted to 3Mohm (!) and was first replaced with an Allen-Bradley a couple of months ago and now finally with a NOS period-correct Piher, hooray!
-The amp started to blow fuses at some point earlier this year and the reason was a faulty snubber cap - it was replaced earlier. Also V1 and V2 grid wires were replaced then with shielded wire for even more quiet operation.

The mains selector switch the amp came with falls apart when a jumper is connected to it, so at the moment the amp is hardwired to 240V, but I'll surely install a new switch if I'll find one, or if someone finds one for me. Please let me know if you have one!

Other than that, this amp is now one of the best rock'n'roll machines I've ever come across. Soooo much bluesy, crunchy and roaring tone available. I'll definitely record a clip or two in the near future.

Any comments are welcome!

P.S. I did some rearranging in my photo gallery. The "before" pictures can be found here:
https://www.dropbox.com/gallery/430... & Cabs/Marshall JMP 50W 1971/Before?h=4c13d5

P.P.S. I also purchased new electrolytic caps for the '67 JMP mentioned in the first post of this thread. When I asked my tech about recapping the '67 (which is absolutely dead quiet: no hum, cracks, pops, hiss or anything), he said there's no need to do it. He inspected the amp inside out and found no reason for recapping it yet, since everything sounds and looks healthy. Well, I'll keep the caps at hand in case things change.

 

[Jucciz]

Finally!

Here are some video clips with high quality audio. First the '71 JMP:
[ame=http://www.youtube.com/watch?v=292bC9_Me0g]Tonefest: wine, beer and a bunch of Vintage Marshalls part 1 - YouTube[/ame]

And here's the '67 JMP:
[ame=http://www.youtube.com/watch?v=iPb2_eYFKCE]Tonefest: wine, beer and a bunch of Vintage Marshalls part 3 - YouTube[/ame]

The guy playing in the video is my friend Sam Vilo. Hope you like them. I'm glad to hear comments on the tone of these two babies.

 

[ptrickamp222]

Nice tones, love both. Like the 67 breakup also. I own a 71 and 72 50 watt metal panels. The 71 is stock except for the snubber caps, blew one. 72 has F&T's for filters and new snubbers, came without. These are both 1987 amps and came without the .68 on V2. Bought some original Mustards and installed. I like the tone of the .68 on V2 some don't. Great amps have fun!

 

[Hillcountry]

Love the "quick repair" on the '67. Nothing like a fist to straighten it out!
Great sounds and great playing!
-Hillcountry

 

[Blokkadeleider]

That little "reset" procedure looks somewhat familiar...
Actually I like the 71's sound better.


Gr,

Gerrit.

 

[ptrickamp222]

The older the marshall the cleaner it sounds until it's wide open. The output tranny's had different impedence. If you can get Ken Fisher's Trainwreck pages he explain's it in great detail. That's why the old Plexi's you had to crank to get breakup. When I grew up in my brothers band the guitar player had a full plexi stack and a 62 LP/SG. The marshall had to be around 67' the guy sold it and had to go to Vietnam,that's how Mike intended up with it That thang on 8 or 9 would crack your teeth. Great condition it would be worth a ton today and the guitar / stop tail. The 71 and 72 metal panel plexi's really got the good's on the Marshall Growl..... NOS Mullard Xf2's and preamps and you get the sound you dream about. and with a great LP.

 

[Jucciz]

One question puzzles me: has anyone ever seen an orange indicator light in a metal panel Marshall? See, this '71 of mine has a round orange light. It seems to fit nicely in the front panel, but I haven't been able to find another instance of such a lamp anywhere so I have a little doubt about its originality though can't be sure.

Any thoughts are welcome.

 

[AtomicRob]

I've never seen an orange light on anything as late as a '71. My '66 has what I believe to be the original orange light but I think they started using red soon after that.

 

[Jucciz]

I've never seen an orange light on anything as late as a '71. My '66 has what I believe to be the original orange light but I think they started using red soon after that.

I think so too. Hard to believe that they had made one-offs with orange lights in the 70's. Well, either way, I've kind of grown accustomed to the lamp the way it is and it doesn't bother me much since the amp sounds great and has been recently serviced. It will definitely rock hard for many decades to come.

 

[codyjarrett]

Great amps
They both sound amazing.

 

[Jucciz]

I finally took a new set of pictures of my '67 JMP (S/ 10688). Here they are:
https://www.dropbox.com/sh/d913ianxq6dvvyp/0aB6vdCbaM#/

A couple of observations:
- The most recent date code on the Mustard caps is D7N (Channel 2 first coupling cap). I guess this points to the last quarter of 1967 and thus the amp can be a late '67, right? If not, please let me know why.
- The bright channel mixer resistor looks like there might have been a bypass cap at some point: correct me if I'm wrong, but looks like something has been snapped off there. Maybe someone had installed a cap there and (someone else) then took it away.
- Two of the rectifier diodes have been replaced.

The logo is a current-production Marshall logo that obviously won't fit in those vintage holes too well. The logo bent and one day just snapped in two pieces. Well, with a little imagination and a pair of scissors it became what it is now... until I find an original '67-'69 gold logo. Yeah, where?

The amp was recapped a couple of years ago, as you'll notice from the earlier messages in this thread. Also a 10k bias trimmer was installed in series with a mint green 62k resistor.

I have been toying with the idea of trying a mixer bypass cap (the amp is a bit on the dark side at lower volumes), but would prefer one that looks like the old ones: Lemco, RS or whatever they were using. Could someone point me to one of these big, flat, brown capacitors? Or should I try a (5000pF?) bright cap on the volume pot instead? Or both?

Also, if someone knows where to find period-correct rectifier diodes, it would be kind of fun to replace the current ones - not that it would make any difference tone-wise.

This amp has the lowest filtering of all my Marshalls: 2x 16+32uF on top of the chassis and 32+32uF on the board. This must contribute to the nice bluesy and "spongy" feel.

One more thing. If I were to install screen grid resistors to this amp, would my best choice be a Welwyn W22 1k/5W? At least Marshall used them at some point. Any other suggestions? It would be nice for them to look like being the right age for this amp.

Thanks again, all comments are welcome!

 

[66SuperTremolo]

Kickass !!! Both great amps.

I have a '68 JMP and Rare '66 JTM-45 MKIV Bluesbreaker (Head) KT-66 , Mullard GZ-34 Rectifier.

Look after them, that '67 is in great shape. Enjoy

 

[TAZIN]

As for the date of manufacture I'd say very late '67 or early '68. I'm leaning towards very early '68 since the PT has a dedicated 120vac voltage tap, and the Dagnall C1999 choke. You could also check the potentiometers for a date code which might also help narrow down the date of mfg.
It does look like someone might have removed a bypass capacitor from the mixer resistor assembly. Personally, I'd go with a 500pF or 560pF bypass cap for the mixer resistors rather than a 5000pF volume pot cap. I think if your patient enough and constantly hunt of a Lemco or RS cap you'll find one, but in the mean time you should just pickup a silver mica 500pF or 560pF cap to use.
I think the Welwyn W22 1K/5W resistors would be a good choice. For the 100w amps of this era Marshall used Welwyn, the gray RS branded resistors, and the cement DCC resistors.