Marshall JMP vrs. JCM 800?

[rdrgtr]

I just purchased a 1976 Marshall JMP 50 watt head.  The schematic says 2203 for this head.  What's is the difference between this head and a 50 watt jcm 800?  I sounds like a typical stock Marshall...thin and trebley (is it ok to use that word as an audio discription?).  I know some of you guys will know the subtle differences between the two.

Thanks

[Matty_V]

Weird I thought the 2203 was the 100 watt version. I have a '76 jmp 2204 50w. I rewired it with a turret board using a jcm800 schematic. Almost identical

[rdrgtr]

My bad.  It is 2204.

[shortfuse]

I have a 76 JMP 2203 lead and a 2204 JCM800.  I would say the JCM can get a bit more distorted with the JMP having more head room.  Never really looked at the circuits to see the actual differences.  I liked the 800 more until the JMP OT died on me.  I replaced it with a Mercury Tone Clone and it changed the whole amp to the better I tend to play the JMP more now.  Not trying to promote mercury as I normally dont use them because of the price but was talked into it by a friend.  If I knew it would have made that big a difference I would have done it a long time ago.  Just my .02.

[Matty_V]

What kind of difference did it make? Clearer? Tighter?

[plexi50]

Either circuit you have a great amp /

[shortfuse]

What kind of difference did it make? Clearer? Tighter?

I guess the first thing I noticed was the pre amp and MV controls were much more usable.  It now has volume control from 0 to 10 on both sides.  With the original OT it would explode at 1 3/4 and was pretty much unusable in a small room at home.  Now I use it all the time.  Even my wife commented on how I was able to play the JMP now at lower volumes and that is very unusual for her to say anything about any of my amps.  Also the distortion is much smoother (dont know if that word makes sense or not) and not as harsh.  I used to run the treble at 2-3 now I use it at 5-6.
I am going to order another for the Hoffman Plexi 50 I built I love that amp just the way it is but if it makes that big a difference with that amp I will be sold.  If not I will chalk it up to the the original OT in the JMP having an issue all along.  I have had that amp since 1979.

[rdrgtr]

Well, I opened the JMP to get started on making her sound like the amp should.  I swapped out about 5 parts and she was spot on.  It is amazing how predictable these are and how they all seem to need the same simple mods.  She's fat and rich and full.  This amp now sounds like my JCM 800s (same simple mods to those too.).  I would really like to purchase a Mercury tranny but hate to pay the price.  If I buy one and like it would want to put them in all my amps.  I guess ignorance is bliss...

[Matty_V]

Haha, she's fat and rich and full. I wouldn't recommend you say that to her face! 
So what 5 parts did you swap out? I may try some of those myself.

I hear ya fuse about having the treble way down and getting blasted with the MV. My setup is in the basement and I usually have the pre around 7-8, master around 2-3. Any more than that and it hurts. I still dig it though!

[Willabe]

My setup is in the basement and I usually have the pre around 7-8, master around 2-3. Any more than that and it hurts. I still dig it though!

What kinda floor you got in that basement, concrete, tile? Hard reflective surfaces are hard on the ears.


             Brad 

[rdrgtr]

Well,  this is just me.  I get rid of the .68 bypass cap and put something in a little larger.  I cut out the .001 bright cap.  I remove the 470k resistor in the 2nd stage as well as the 470p bypass cap and solder in a 68k res. with no bypass cap. You can do this in the 3rd stage as well.  This is something that I do that is outside the box, in the 3rd stage,  I leave in the 470k res. and replace the 470p cap with a .001 cap. This gives a nice mid range push with a touch of gain as well.  I did not do this in this JMP mod.  It sounded pretty good without it so I left it alone.  These changes really get the amp sounding bigger and extra gain comes with it.  I have heard of people playing with the 10k cathode res. on stage 2.  For me,  I think that anything lower than 10k is to much gain for this circuit.  Some guys put .1 caps in the place of the .022 caps, but I have never had to do this yet.  I can see to where it would put more beef to the signal.  Oh yea,  I almost forgot.  Changing the celestion 75s to vintage 30s was HUGE game changer...

[Matty_V]

Coolness. I may give some of that a whirl some day. I'd like to try some v30s as well. More amp/guitar parts I haven't thrown money at yet...