'72 Marshall Jmp 50 Organ

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Rasputin1176
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'72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Thu Mar 10, 2016 12:23 pm

Been hoping for a vintage Marshall long while & found an almost all original JMP and jumped on it!
I don't have any knowledge really on Organ models. A tech worked on it before I bought it and just replaced a bias resistor, lamp, output Bulgin jack & 2 electrolytics. Soon I will have tech I know give it once over, may need filter caps replaced, was recommended by previous tech but wanted to keep as original as possible.
I didn't want to mod this amp, but I'm not sure how diff from Lead spec it is? I heard its closest to Bass spec with added bright cap on vol pot, but then I seem to see a schematic indicating change in tone stack?
So far it seems bright even for 72 jmp....unfortunately don't have my preferred speakers yet so not best bench test for me on sound.

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ezs
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Re: '72 Marshall Jmp 50 Organ

Post by ezs » Fri Mar 11, 2016 7:25 pm

Neat find. Almost identical to the 1964 Lead & Bass spec. 100p vs 500p on the Vol pot.

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Re: '72 Marshall Jmp 50 Organ

Post by neikeel » Sat Mar 12, 2016 1:06 pm

The organ is the nearest the metal panel era mps get to the 60s plexi vibe.

In original spec they are quite sweet toned with adequate gain when cranked (like all good plexis)

I had a 70 a few years back and converted it to full lead spec (for about a week) it was bright and harsh in that set up so it went back to stock. Really weird as I have had lead amps of identical specs that were totally different.

I strongly suggest you post a good set of pics here before it goes to your tame tech.
Neil

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Re: '72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Wed Mar 16, 2016 9:48 am

Thanks for all the feedback!
I was using it the first week & all the sudden few pops & then no sound. One of the JJ el34 looked little burned, didn't catch any red playing but red paint all turned grey on tube logo... The HT fuse was shot as well.
I took it to my studio tech who I trust. I had fear for the OT, but all is ok. Other than one dead power tube & fuse, he found only 1 bad resistor in preamp section. I don't have more details, it's up and running but he's going to make sure everything is fine.
The 3 filter caps are getting replaced too. I figure rather be safe (I know seems to be debated issue on whether to swap filter caps preemptively). F&T are going in.
So I know may not sound identical after however least no real issues & still basically all original. I'll get all parts back. It has reissue Mullard el34 in it currently that he had. I also have a set of NOS '69 dual halo getter Miniwatt Mullard made el34 coming soon. Preamp tubes all still original Mullard.
I was pleased overall with sound before crapped out however I don't know if it was accurate because the failed resistor in preamp section he believes was like that a while.
Photos are coming!

Next I'll consider some of the things I've heard about adding screen resistors for safety?

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Re: '72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Wed Mar 16, 2016 9:58 am

Here is bad photo from original seller, but I'll take good quality ones myself and tech takes them first before working

https://www.dropbox.com/s/j6sekmlp651uu ... d.png?dl=0

Image

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Re: '72 Marshall Jmp 50 Organ

Post by neikeel » Wed Mar 16, 2016 10:28 am

It is not an organ, looks like a stock lead amp to me.

Can't see what the NFB resistor is, with that V2a (no 0.68uF) should be 100k on 4ohm tap to be stock.
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Re: '72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Wed Mar 16, 2016 3:47 pm

You may be right! The label inside says "Organ" and it's printed clearly not hand written however my tech already had to go thru it to identify circuit before working & he said it was actually Lead spec. I'm happy either way! Even has lay down OT which from my looking around is little rare for '72.
Dying to get this back!

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Re: '72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Fri Mar 18, 2016 6:00 pm

One other question to any others who've used more models than me; is a 72 jmp 50 lead similar sounding to 69 jmp 50 lead?? I've heard a 69 jmp50 lead I loved! (I know sounds can vary even between same year & model, depending on components tolerances , tubes ect). That being said some have that killer sound like the 69 I heard. Dying to get my amp back, any day now

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Re: '72 Marshall Jmp 50 Organ

Post by neikeel » Sat Mar 19, 2016 7:17 am

I have had (and still have some of) the full range from 65 to 77.
The 68/69 lead amps are pretty similar to the 72. The 72 does not have the 0.68uF on V2a but has 100k/4ohm NFB i.e. has less NFB so will be a bit looser and feel a bit gainier without the upper mid boost the V2a cap gives. The 72s have a reputation for being quite fierce vs the earlier, not sure if that is justified (the 5k cap on the volume pot brings the gain in quickly but my 68 had the same cap and I never found it a problem. The 77 JMP50 was the brightest and gainiest probably due to the different presence set up (25k/4k7/0.68uF cap) and lower voltages (385v B+).

I bet your 72 will sound great with a decent set of valves and correctly biased up.
Last edited by neikeel on Sat Mar 19, 2016 7:38 pm, edited 1 time in total.
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Re: '72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Sat Mar 19, 2016 6:35 pm

Thanks again Neil! I will keep you posted :)

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Re: '72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Mon Mar 28, 2016 10:43 am

Sorry been busy. I got the amp back. It's 100% working as I've had it cranked for least hour or so a day. I do have a Rivera RockCrusher attenuator running because I really like my 2 Scumback 25w m75's and don't want to destroy them.... It seems to have quite a bit of gain.
Now the interesting part, and I looked over a lot of the other forum posts, photos, some schematics ect, is that my tech saw the jmp 50 lead for 68-72 schematic matched all the values in my amp. However it was missing that .68uf cap on V2a so he added a large Sozo back in. The NFB is 100k though and like most say it shouldn't really have both. I think because the schematic he had listed either 100k or 47k. Not big deal. His work is very solid and I do trust him. I was able to find few photos of 70-72 jmp 50 leads that had both 100k NFB & the bypass cap on v2a but seems rare.
So, that's the current setup mine is now. I'm likely going to keep the v2a cap like some good '69s I've seen have, and change the NFB to 47k. (Next to impossible to find original ISKRA 47k though). I'll prob worry about trying to get authentic parts for what he replaced later. Mainly a real mustard .68uf which also hard. The only other replaced parts were like 4 large resistors, ones that see higher voltage and you could visibly see the discoloration compared to the other same resistors in tone stack area.

I'm still learning, and least tech walked me thru everything he did.
I'm hoping to least get to the point of doing minor work on amp myself. I used to work as electrician for few years, and I do solder well too. I've done quite bit research into safety working on amps as well. See if my tech will walk me thru some basics maybe.
I didn't have my new NOS MiniWatt Mullards made xf2 el34 at time so unfortunately it has Mullards reissue. The preamps I have 2 orgingal Mullards in v1 v2 and PI is a nos GE short plate. I have some more nice NOS to try in preamp, Mullards, briamar, GE, RCA and tungsol black plate, long plates ect. The orgingal Mullards in v1 v2 may not be greatest sounding who knows but the power tubes I really gotta get in there. I want to get bias probe do myself.

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Re: '72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Mon Mar 28, 2016 10:52 am

Also with my Strat it's currently kinda too bright I think. It gets gain pretty quick, like 2-3 even with strat, only light if you dig in. I think that's the bright cap though doing it just like a treble booster. I got some more Hendrix tones going into top right Vol 2 input, then jumpers from bottom right vol 2 into bottom left Vol 1. Going just into top left vol 1 I def need the amp on like 8 vol knob to sound good. It's got quite bit gain then but can just roll back for clean on guitar which I do anyway.
Again It's probably from having both the v2a cap and 100k NFB. Plus no NOS power tubes. The F&T filter caps need to prob break in little? All 3 50+50 but I hear some like 30+30 on preamp one. So there's room to mess around little. Get it sounding good with right values then spend time maybe hunting down NOS cap and resistors to use for what's been replaced.

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Re: '72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Sat May 28, 2016 10:17 am


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Re: '72 Marshall Jmp 50 Organ

Post by Rasputin1176 » Sat May 28, 2016 10:21 am

Can anyone help determine if this OT is an original or of this codes means it's '83 replacement? It doesn't look correct for a '72 and I paid for an amp listed as original! My fault to not check clearly when I did but so far 2 techs never noticed it may not be, so maybe it's not?
I really hope it's not replaced

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Re: '72 Marshall Jmp 50 Organ

Post by Tazin » Sat May 28, 2016 10:37 am

It's a replacement unfortunately....Probably from '83 as indicated by the stamping on the end bell.

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