Is the 78,79,80 JMP same exact gain as JCM800 81-83?

keano · Post by **keano** » Fri Apr 27, 2007 2:52 am

I have heard these years are the same that the circuits aren't as bright yet. More punch. Is this true?

I just got a JMP 78 and it is freaing amazing. Has 6550 tubes. International tubes? Does anyone know which tubes would be comparale to these old 6550 not made any more?

Another reason for quesiton is am I losing gain by going for an older JMP then a JCM800 81-86?

johniss0001 · Post by **johniss0001** » Fri Apr 27, 2007 5:28 am

congrats on the amp those late 70's amps are pretty decent. Your amp has the 6550s as it was an export amp from the uk. The american distribuitor put 6550s into their amps because the el34s were too fragile and didn't travel well.

6550's sound superb in the mv circuit. 6550s sound cleaner an stay focused because if i remember correctly correct me if i am wrong here buds those jmps are high on pre amp gain so the 6550s keep it tight. Most of these amps are usually converted to el34s just a simple resistor change el34s might give you more bite well more of a traditional marshall sound.

I don't know much about the circuit changes from the late 70's into the early 80's but it might come down to transformer changes and the quality of parts marshall decided to use

wdelaney72 · Post by **wdelaney72** » Fri Apr 27, 2007 8:53 am

Consider me jealous... nice pickup.

The JCMs are thought of as a little thinner. The caps changed and possible some of the filtering. This article explains it a little bit.
http://www.legendarytones.com/marshallshoppers2.html

In the MV circuit only, some prefer the 6550. To me that or a KT88 makes a little more sense given the preamp design than the EL34, but again, that's the MV amps only.

You won't find a "comparable" current production 6550 tube out there. If I had to choose one, I'd go with Winged-C based on their quality of EL34.
http://www.thetubestore.com/6550types.html

If you have money to spend, your BEST option would be to get a set of NOS from TubeTramp.

Bunnest · Post by **Bunnest** » Fri Apr 27, 2007 11:02 am

I would convert it to EL34s, I used MVs and JCMs for twenty years (until I learned about this forum and the way of the world) BUT , I still do like my friends '81 2203 JCM, its a ripper. When he put in NOS siemens EL34s, it sounded ALOT better, more sustain, more compression and less fizz, which is what you want out of those MV amps. the JCM went to all plastic coated caps, red leggos....still better than most of the new caps today, according to MightyMike, the source of info.

keano · Post by **keano** » Fri Apr 27, 2007 12:16 pm

Thanks guys. do you think there is a noticable gain difference between 800 81-83 era and JMP 78?

I read somewhere that JMP tend to be slightly smoother. I imagine with a EL34 that would be a good things. With 6550 it sounds massive at the moment.

Devon8822 · Post by **Devon8822** » Fri Apr 27, 2007 12:59 pm

I have done a bit of research ont hsi so I will tell you what i know. Really the only difference between the 77 -78 jmps and the 79 - early 80s is that the 79 - 80s had a hair more gain and are a hair brighter. I believe the hair more gain is because of it being slightly brighter. But, really other than that nothing. It is very un-noticable aswell... I'd take a 77- 78 over a later one anyday. most people get rid of the bright cap in the jcm800s anyways, which makes them a 77 - 78. A 76 is pretty much the same as the 77 - 78 except still has the earlier cosmetics. Dude, you got yourself a dandy amp. I have a 72, if I get another at some point it will be a 77 - 78. ENJOY!

keano · Post by **keano** » Fri Apr 27, 2007 1:54 pm

most people get rid of the bright cap in the jcm800s anyways, which makes them a 77 - 78

Just curious so a JMP doesn't have a bright cap per se.

I am loving it. Love the way it looks. got to offload a 2203 reissue now. Ha!

sweetwilliam · Post by **sweetwilliam** » Fri Apr 27, 2007 4:57 pm

A few years ago I was in a band with another guitar player. We both went and purchased 2204's. His was an '82 JCM 800 vertical input, mine was a '78 JMP. I thought the JMP smoked the JCM. We used every variable possible and each time the JMP won. It was warmer and smoother than the JCM.

BTW, he sold his and bought a JMP later. I still have mine. You got a good amp.

johniss0001 · Post by **johniss0001** » Fri Apr 27, 2007 6:22 pm

Devon nothing wrong with 78 marshalls

devon there is more to it but the late 70's amps still had mustards like your mate's 77 but tbh i think a lot of it lies in the transformers as well eh guys? also " marshalls run better on 240V" lmao

there are so many factors but i have a feellin marshall changed the transformers and 1 or 2 components

GTRroddin · Post by **GTRroddin** » Fri Apr 27, 2007 7:47 pm

I picked up this '77 a few weeks ago

With regard to the JMP's vs. the 800's I don't really think one has more "gain" than the other. The different caps make them sound a little different and some may perceive the brighter hairy sound in the 800's to be slightly more gain. But the circuits in the two are identical overall, although the filtering changed on the 800's in about 84 or 85. The caps are the big difference. Notice all the mustard's in mine. I have seen some '78 models that did not have the mustard's (they started using the lego caps or the little white chicklet looking ones). I am not 100% on this but I think around '77 was when Marshall stopped using the mustard's in these amps......I could be completely wrong though, this is just based on the ones I have seen. At any rate I would take a JMP over an 800 any day of the week. The 6550's sound good but I think I am going to change out mine to EL34's to be more traditional. These amps will blow you through the freakin wall (ala' Marty McFly style at the begging of back to the future!!!)

PlexiMan · Post by **PlexiMan** » Fri Apr 27, 2007 8:53 pm

Funny..i have 2 79 JMP's and also an 82 JCM..Circuits are identical. One of the 79's has a mix of mustards and lego caps. The other no mustards. All 3 sound a little different form each other but all really good. Gainwise i think they are pretty close. I have also played tons of JCM 800's. And even they sounded different from each other and i mean earlier ones not he the later ones with the filter changes. I will say the 79 used different transformers than the JCM 800. The 79's use a T4074 power tranny while the 800 uses a T4145. The 79 also has slighlty higher B+ which could also contribute to a little different sound. All in all i love these amps so great score.

johniss0001 · Post by **johniss0001** » Sat Apr 28, 2007 2:23 am

pleximan interesting i thought that but wasn't 100% sure

neikeel · Post by **neikeel** » Sat Apr 28, 2007 4:17 am

Interesting to see the white gloop and the pink link wire on thsi one too, just like Devon8822 s old amp.

http://forum.metroamp.com/viewtopic.php?t=11258

GTRroddin · Post by **GTRroddin** » Sat Apr 28, 2007 11:58 am

Yea that is interesting hu? And it is funny to see that his is a '77 and has mostly green chicklets where my '77 is mostly mustards with only one green chicklet. It is strange that they mixed the two up like that. Just another one of those Marshall things that don't make any since (WTF?)

I love the sound of this amp though!

johniss0001 · Post by **johniss0001** » Sat Apr 28, 2007 5:55 pm

I stand corrected about devon's amp now thanks very much after confirming that white goop