NAD: 1976 Marshall JMP 2203 with Superlead cosmetics

[JPB]

New Amp Day. I'm stoked I found one of these! I've only seen a couple of these transitional master volumes.
I was even more delighted to find out that the amp was made on my birthday! June 27th.
and had this guy Jim's sig inside.


I'm definitely liking the solid low end of the 6550's in this head.
I noticed the PT was replaced. OT looks stock.


I'm wondering how much of this JMP is stock (especially the preamp).
Can anyone help me there? I would love to know how many of these parts are original. I dont think I've ever played an entirely stock 70's JMP.
Looks like no one clipped out the treble peaking circuit.





Thanks, I appreciate any insight!

[neikeel]

The PT is a replacement.

Someone has tried to fatten up the preamp with slightly larger (0.027uF couplers in first two stages and removed the 470pF cap on the inter stage treble peaker as you suggest.

Converted to run 6550s too.

I really like the early cosmetic 2203 but have not found one cheap enough to grab!

[demonufo]

Bias capacitors have been changed for 15uF's too, rather than 10uF's. I doubt very much these would have been original, but that's not a bad thing, in itself.
There is no real benefit of 15uF though.

Original Daly filter caps by the looks of it.

Personally I'd put the .22's back to stock in the pre-amp, and if a touch more bass was required afterwards, a 400V .047uF cap in series on the NFB line might be just the ticket.

Also very unusual for a master volume amp to have those two .1uF coupling caps.

Would like to see the whole board...

[JPB]

Demonufo, I was hoping you'd drop by! From studying your posts I know you have a lot of knowledge when it comes to these amps! Thanks for looking.
Neikeel, thanks for your insight as well!
Here's a board shot. It was tough to get it all so I went as far as the rectifier diodes and the Bias caps.

[demonufo]

Well, apart from the two powertube biasing resistors, those two obvious coupling caps and the possibility of the bias circuit electrolytics (and of course the piggybacked bias feed resistor) it looks pretty well stock. Including the two .1uF coupling caps in the PI. Normally the only .1uF on the board would be the green cap to the left. Curious.

That really must be a bit on the bass heavy side with all those coupling cap changes.

[neikeel]

Personally I'd put the .22's back to stock in the pre-amp, and if a touch more bass was required afterwards, a 400V .047uF cap in series on the NFB line might be just the ticket.

Also very unusual for a master volume amp to have those two .1uF coupling caps.

All correct of course, although before you go getting 0.22uF caps his finger slipped and he means he needs 0.022uF!

That tropical fish is unlkely to be standard in one half of the PI, it would normally be a green cap like the other 0.022uFs and the other of the pair too.

How does it sound?

[julkke]

Nice! I've never seen a 2203 with .1uf pi couplers stock. Pretty cool, goes to show that there is not one definitive "stock" spec on these amps either. I bet it sounds killer!

[JPB]

The JMP has a huge low end! It also has a tighter response than my 81' JCM 800 (EL-34 loaded) . I attributed this difference to the 6550's alone not knowing any better. I'm really happy about the response of lows on this JMP. It does have the original Daly electrolytics.
These two heads have opposite problems if you want to call them that.. The 800 is smooth and warm but the lows are sluggish. Replaced the Daly's with F&T a few weeks back and the problem is only slightly better.
The only area I'd like to work on in this JMP is the overly glassy highs. There's a bit too much of it compared to the smooth highs of my 800 (which had some mods in the front end). I would love to get closer to the high end I'm getting from my 800.

Here's a pic of the 800. The yellow block is clearly not stock and I've suspected may be letting too much lows pass causing the sluggish lowend. The highs are perfect however. Really smooth and still mix right with the guitar's vol. control set lower for cleaner sounds.

[JPB]

Personally I'd put the .22's back to stock in the pre-amp, and if a touch more bass was required afterwards, a 400V .047uF cap in series on the NFB line might be just the ticket.

Also very unusual for a master volume amp to have those two .1uF coupling caps.

All correct of course, although before you go getting 0.22uF caps his finger slipped and he means he needs 0.022uF!

That tropical fish is unlkely to be standard in one half of the PI, it would normally be a green cap like the other 0.022uFs and the other of the pair too.

How does it sound?

Neil, I did notice the tropical fish, thanks! Not sure what effect it's having in the PI however.
This amp sounds great but isnt quite there yet in the high end.
Rolled some preamp tubes and it made a difference. I figure it doesnt hurt to investigate a few options in taking away a little more of the "glass" in the highs and replacing it with some warmth but I dont think the amp needs anything too drastic either.

[demonufo]

Actually, I take it back, ignore my previous comments. Hadn't eaten.

Okay, so the left hand .1 is normal, the right hand one in the PI is not. I find it hard to believe that 1 mustard coupled with a tropical fish is stock. I suspect these would have been the same .022uF ( ) green caps as the others.
They are rather a fragile, easy to damage cap. I'll bet underneath that left hand PI .1uF cap are some narrower holes similar to that which the tropical fish is mounted in. (Personally I think tropical fish sound a bit nasty in amps too, so if it were me I'd be pulling that out pretty quick. The hard bit is finding something stockish to fit.)


That lower cathode bypass cap on the 800 could well be the culprit of the flub. That's pretty hard work for a master volume circuit amp to add more bottom end that early in the gain stages.

[JPB]

Actually, I take it back, ignore my previous comments. Hadn't eaten.

Okay, so the left hand .1 is normal, the right hand one in the PI is not. I find it hard to believe that 1 mustard coupled with a tropical fish is stock. I suspect these would have been the same .022uF ( ) green caps as the others.
They are rather a fragile, easy to damage cap. I'll bet underneath that left hand PI .1uF cap are some narrower holes similar to that which the tropical fish is mounted in. (Personally I think tropical fish sound a bit nasty in amps too, so if it were me I'd be pulling that out pretty quick. The hard bit is finding something stockish to fit.)


That lower cathode bypass cap on the 800 could well be the culprit of the flub. That's pretty hard work for a master volume circuit amp to add more bottom end that early in the gain stages.

I think you kind of hit it there with "nasty". The highs dont break up quite as smoothly as i'd like but I don't know what to attribute it to.

I need your opinion on this for the JMP.
I usually run the pre-gain full up and control the gain ranges with the guitar.
In this case do you think clipping out the 1st treble peaking circuit cap would have any effect in the darkening the highs? Am I right to assume that with the preamp pot full up the treble cap would not be active in the circuit as the signal would follow the least resistance through the pot?
Also, What would be a more appropriate replacement cathode cap to replace the yellow .47K block if my goal was to tighten up this bass response some?

[JPB]

Here's the PT replacement. Not sure what brand this is by the identifying numbers on the side.
Previous owner didnt know either as he was the second owner.

[demonufo]

I usually run the pre-gain full up and control the gain ranges with the guitar.
In this case do you think clipping out the 1st treble peaking circuit cap would have any effect in the darkening the highs? Am I right to assume that with the preamp pot full up the treble cap would not be active in the circuit as the signal would follow the least resistance through the pot?
Also, What would be a more appropriate replacement cathode cap to replace the yellow .47K block if my goal was to tighten up this bass response some?

Clipping the treble peaking cap WILL have a definite effect on darkening the highs. Personally I think it just muddy's the circuit up though. Likewise clipping the brite cap of the pre-amp pot is a real bad idea for those that do use the guitars volume control. It makes it real dull and lifeless when rolling back, and neck pickups in particular lose all clarity. And personally I notice a difference in the way the amp responds with guitar flat out, and preamp vol flat out at lower volumes on the master. But these amps do sound better loud for a variety of other reasons, too.
I've played with these areas many times and mostly prefer stock MV circuits, albeit with the V1a 10K cathode resistor lowered to 8.2K for just a little extra dirt. Personally I'm a fan of this cold biased cascade arrangement, flaw an' all.

As for the V1b cathode cap, I'd just go back to stock .68 and work from there. There is every possibility the filter caps aren't up to scratch too. Short of removing them from the circuit and testing them properly for insulation breakdown resistance, and leakage currents, there is nothing you can really do to test these though.

[demonufo]

Oh, and the PT I think is a Schumacher.

[JPB]

Thanks a lot for the input here men.
So, just an update, I did remove the treble peaking circuit with the 470k over the 470pf and I didn't like it. The tone became dull and didnt quite resemble the highs that really make this thing sound like a Marshall. So back in it went and instantly the tone was much better.
Curious as to why you see a lot of people (who should probably know better) suggesting the removal of this circuit to attenuate highs. It does remove highs but not in any good way.
That being said I did remove the treble peaker tied to the preamp pot. My gain control is usually up all the way and I didnt hear any adverse effects with the guitar's vol. control down as I run a tapped humbucker for single coil-ish cleans. Still plenty of clarity.

Next, realizing that the amp had been converted to 6550's I looked at the NFB wire. It was still connected to the 4 Ohm tap. I quickly moved it over to the 8 Ohm tap for a little less NFB and WOW, what a difference! I certainly wasn't expecting such a significant change as I had experimented with this before on EL-34 loaded amps but always found 4 Ohm to sound best for that particular tube..
This was my first experience with a 6550 loaded 70's Marshall. Seems the 6550's really do benefit from less NFB as the highs were now much more pleasant and smoother sounding. I would say it just about solved the harshness issue I was having.

During this experimenting process I got a little crazy and damaged one of the green .022 PI coupling caps.. Ooooops. Guess they are quite fragile indeed.
Its the cap that's coming off of the grid of V3a to the wiper of the Master volume pot.
Can any of you suggest a suitable cap for replacement? I was looking at the Sozo mustards as people claim to hear a noticable difference in warmer highs in Marshall's with these caps.
While I'm ordering, what would be an improvement for these .027uf caps or in general?
Might change out V1a's 10K cathode resistor with the 8.2k as Demonufo suggested. Carbon comp or Carbon film?
Once again, thanks for sharing your knowledge and opinions with me, I really do appreciate the input!