Looking for information on how JMPs changed through the years.

[Matthews Guitars]

As some of you are aware, I'm deep into a project to make replica metal face and back plates for JMP era Marshalls.

The project is moving along nicely but I encountered an unanticipated snag that caused me to have to revisit some of my designs.

I had to remove the metal front panels from both my 1973 Superlead (model 1959) and my 1977 master volume (model 2203) heads. This was to get exact measurements off the panels and off the unused holes in the chassis pans, as well. (And the same goes for the back, though only one amp still had its back panel.)

I found that the fronts of the two chassis are more different than I thought. While all the pot holes match up exactly, the four input jack holes do not. They have been moved slightly, about an eigth of an inch, and their spacing is slightly different.

I had initially thought that the front panels for Plexis such as a 1959 would have the same hole patterns and spacing as for later year 1959s. But this isn't the case at a certain point in production. I'm trying to nail down when that transitional point came.

If you have any information that would help pin it down, please feel free to post it here.

My '73 Superlead is BELIEVED to have the "original" hole pattern of a Plexi. The '77 master volume has a newer hole layout. I'm THINKING, without proof, that a Superlead made around '77 would use the same chassis and hole pattern as the 2203 master volume model.

I'm also thinking that the chassis hole pattern probably changed with the introduction of the master volume models in 1975 or so.

I'm thinking that there were stages in the transition from JMP to JMC800 familes and the chassis changes were just part of it.

Correct me if I'm wrong but it looks to me like that transition went about like this:

1973: Late in the production year, the turret board was replaced with the first edition ST1 PC board.

1975: The cabinet cosmetics changed with the introduction of the master volume models. At the same time, the updated issue 202 ST1 board was introduced into all models. The chassis hole punch patterns also changed at the same time.

A few minor cosmetic changes happened between 1975 and 1980.

1976 seems to be the first year of the rocker type power and standby switches. Not every model changed over to those at the same time, though. I've seen '76 models with rocker switches and '78s of the SAME model with toggle switches.

1981 marked the introduction of the JCM 800 models.

I have no information on chassis hole patterns in the JCM 800 models. I don't know if the 1959 or 2203 models made in that series follow the plexi or the later JMP hole patterns or are something diffferent.

 

[Geeze]

I have two of the JMP 2203's '77 & '78 and they have only two inputs - I always believed this was the advent of the master volume series.

Russ

 

[Matthews Guitars]

Yes, the 2203/2204 should always be two input master volume models. I've seen some from the JCM 800 era that had their two inputs arranged side by side in addition to versions with the inputs arranged vertically like the previous editions.

But under that gold anodized aluminum front panel, there are four holes for input jacks. Makes sense if you want to build different versions of amp in a single chassis.

 

[ampmadscientist]

Yes, the 2203/2204 should always be two input master volume models. I've seen some from the JCM 800 era that had their two inputs arranged side by side in addition to versions with the inputs arranged vertically like the previous editions.

But under that gold anodized aluminum front panel, there are four holes for input jacks. Makes sense if you want to build different versions of amp in a single chassis.

Not always. There is a version with 4 jacks, not just 2. It was the same as 1959 but it was a JCM 800.
We consider this a "transition model."

 

[ampmadscientist]

As some of you are aware, I'm deep into a project to make replica metal face and back plates for JMP era Marshalls.

The project is moving along nicely but I encountered an unanticipated snag that caused me to have to revisit some of my designs.

I had to remove the metal front panels from both my 1973 Superlead (model 1959) and my 1977 master volume (model 2203) heads. This was to get exact measurements off the panels and off the unused holes in the chassis pans, as well. (And the same goes for the back, though only one amp still had its back panel.)

I found that the fronts of the two chassis are more different than I thought. While all the pot holes match up exactly, the four input jack holes do not. They have been moved slightly, about an eigth of an inch, and their spacing is slightly different.

I had initially thought that the front panels for Plexis such as a 1959 would have the same hole patterns and spacing as for later year 1959s. But this isn't the case at a certain point in production. I'm trying to nail down when that transitional point came.

If you have any information that would help pin it down, please feel free to post it here.

My '73 Superlead is BELIEVED to have the "original" hole pattern of a Plexi. The '77 master volume has a newer hole layout. I'm THINKING, without proof, that a Superlead made around '77 would use the same chassis and hole pattern as the 2203 master volume model.

I'm also thinking that the chassis hole pattern probably changed with the introduction of the master volume models in 1975 or so.

I'm thinking that there were stages in the transition from JMP to JMC800 familes and the chassis changes were just part of it.

Correct me if I'm wrong but it looks to me like that transition went about like this:

1973: Late in the production year, the turret board was replaced with the first edition ST1 PC board.

1975: The cabinet cosmetics changed with the introduction of the master volume models. At the same time, the updated issue 202 ST1 board was introduced into all models. The chassis hole punch patterns also changed at the same time.

A few minor cosmetic changes happened between 1975 and 1980.

1976 seems to be the first year of the rocker type power and standby switches. Not every model changed over to those at the same time, though. I've seen '76 models with rocker switches and '78s of the SAME model with toggle switches.

1981 marked the introduction of the JCM 800 models.

I have no information on chassis hole patterns in the JCM 800 models. I don't know if the 1959 or 2203 models made in that series follow the plexi or the later JMP hole patterns or are something diffferent.

A. What year did the chassis change from obsolete aluminum to the modern steel chassis?
B. What year was the output transformer beefed up to withstand more heat?
C. What year was the bias winding added to the PT? Previously the bias was tapped from the HT winding.
D. What year were the screen resistors added to compensate for the updated EL34 design?
E. What year were the filter capacitors uF increased? When was the 2 filter capacitors in series adopted?
F. What year were larger output tube sockets mounted in larger diameter holes?

These are all changes which I consider the "MK II" design updates. But the years each change actually started is a little unclear.

 

[Matthews Guitars]

One more data point for the archives! Thanks.

What started out as being simple in principle, just a few models, has snowballed into an as yet uncounted number of major and minor variants for me to keep track of!

 

[ampmadscientist]

One more data point for the archives! Thanks.

What started out as being simple in principle, just a few models, has snowballed into an as yet uncounted number of major and minor variants for me to keep track of!

I would like to know which update applies to which year....it's interesting to explore these changes and why they took place.

 

[Matthews Guitars]

I find it particularly interesting that some changes seem to have gone back and forth over time. I've seen photos of 1975 2203s with rocker switches and 1978 2203s with toggle switches. I haven't been able to verify that the manufacturing dates reported for those amps are correct, though.

It's been my understanding that Marshall had a very economical approach. They'd make changes only when a lack of supplies of parts they use dictated the change. And if they should find a stock of old parts that makes another part useful, like finding a lot of toggle switches so they can use up their inventory of the old front panels that were cut for toggles, then they'd go back and use them. A very thrifty, cost conscious company.

 

[stickyfinger]

Canadian market didn't get rockers.
Must have been a weird Canadian electrical code issue.

 

[Matthews Guitars]

That brings up the possibility that a number of 2203s and 2204s AND 1959s and 1987s, were built on the earlier pattern 1959 and 1987 chassis.

I'd like to find out if that's the case. It's information that's relevant to my making repro panels.

It turns out that the chassis front punch pattern was changed at some point around 1975 or 1976. Only the six holes for the controls stayed the same.

The quad of holes for the input jacks was moved about 1/8" closer to the volume controls when they made this revision.

This raises the possibility that you could have two amps of the same model made in the transitional year and they have different hole layouts. You'd have to know if you have the "early" chassis pattern" or the "late" pattern.

 

[sct13]

The Chassis "supplier" or the part of the shop drilled all four holes. They didn't know what amp was going to be wired in there.

My 77 JMP 2203 has four holes with only two used.

Also, there could have been an overstock of 59 chassis as well. Its easier just to cover the second un needed set of holes rather than split up an order to another department or supplier,

I worked in many manufacturing facilities and rarely do they make an exact # of one component to match an order. So they just make whatever fills a bin or two.

It way more controlled now days.

Its even engineered in, I worked at a Ford assembly plant, and the Lincoln Luxury sedan is really the same exact shell as the Ford Fusion….some holes are covered up with bling....in fact the whole thing is bling.....

 

[sct13]

That brings up the possibility that a number of 2203s and 2204s AND 1959s and 1987s, were built on the earlier pattern 1959 and 1987 chassis.

I'd like to find out if that's the case. It's information that's relevant to my making repro panels.

It turns out that the chassis front punch pattern was changed at some point around 1975 or 1976. Only the six holes for the controls stayed the same.

The quad of holes for the input jacks was moved about 1/8" closer to the volume controls when they made this revision.

This raises the possibility that you could have two amps of the same model made in the transitional year and they have different hole layouts. You'd have to know if you have the "early" chassis pattern" or the "late" pattern.

this sort of thing happens all the time too...You would have to have access to the drawings and then when that engineering change happened. Then when it was implemented and made its way into the line.

Marshall was notorious for Not keeping track of these things.

 

[Matthews Guitars]

As a matter of fact, one of my acquaintances has an early 2204. I was allowed to photograph and measure it. It has toggle power switches, and a round indicator lamp with bezel, and the input jack locations match that of a 1987 chassis from earlier years. (The two that are on the front panel, anyway.) There are no radial hash marks on the number circles around each knob, just even numbers only as per earlier units. And the sensitivity legend is high up just below the high level jack.

All this research is making me something of a subject matter expert on JMP era Marshalls. In some respects, anyway.

 

[Trapland]

Not always. There is a version with 4 jacks, not just 2. It was the same as 1959 but it was a JCM 800.
We consider this a "transition model."

This ^. I’ve seen several model “1959” jcm800s.

 

[Trapland]

I’ve also seen a 50 watt 2204 built on a 4 hole chassis.

I’d say 1976 was a year of MAJOR cosmetic changes.it seems like the 50 watt models had radical inconsistencies, not Linear change. I know of at least 3 versions of the 50 watt master alone from that year.


looks like it was all sorted by 77.

edit: JUST the 1976 year alone makes me humbly believe I’ll only ever be an enthusiastic student of JMP’s, never an expert.

 

[Matthews Guitars]

I'm starting to think that nearly every change to Marshalls over the years was mandated by running out of the whole inventory of specific parts they acquired from Radio Spares. "Hey, Jim...we're running out of toggle power switches!" "So use rocker switches!."

 

[MickeyJ]

Just look at where they started, in a drum shop copying a fender amp. Jim had the sense to not change a good thing and he basically made the same thing for decades, they weren't trying to change the world over there at Marshall , which is a good thing. It does appear that Jim was more concerned with cosmetics than amp design, and those rocker switches add some red bling to that gold and black, I can see that as being the main reason for the change in switches. As mentioned Canada kept rocker switches when other countries got rocker switches, you can tell a Canada amp as they had ' CSA' on the back of them.

 

[Matthews Guitars]

Plus the Canadian amps are covered in red print warnings on the back panel, and inside, fuses seem to be the most common type of component in the whole amp. Canadians must be paranoid about safety....at least if they're working in the CSA department.

 

[ampmadscientist]

Plus the Canadian amps are covered in red print warnings on the back panel, and inside, fuses seem to be the most common type of component in the whole amp. Canadians must be paranoid about safety....at least if they're working in the CSA department.

OK check it out, another one for the history book:
http://www.marshallforum.com/threads/valuation-out-of-interest.110522/#post-1898190
A 2203 built on a 1959 JMP chassis w/ metal toggle switches.
(100MV model)

 

[Matthews Guitars]

Well, as it happens, a friend of mine owns the 50 watt twin of that amp. Two inputs, sensitivity legend located just below the high sensitivity legend, JMP marked, and the spacing from the centerline of the master volume to the centerline of the two inputs is 1.5 inches, exactly right for the 1959 chassis. Internally, it's a four input chassis but only two are punched through the front panel. The only cosmetic difference is that it has the older JTM45 style indicator lamp, not the red ice cube.

The evidence is that Marshall built the first 2203/2204 models on 1959 and 1987 chassis starting some time in, I am presuming, 1975. Some time in 1975 the 2203 and 2204 got their own chassis which are defined by the changeover to rocker switches (not applicable to Canadian examples, of course) and a change in the spacing of the input jacks relative to the master volume. The jacks got a little closer to the MV by about .1 inches. You can see the difference if you're paying attention to it. The sensitivity legend was also centered up down the panel centerline at some point, certainly by the time the rocker switch changeover happened.

These transitional master volume models also feature 1959/1987 style numbering around the controls, without the radial hash marks at positions 1, 3, 5, 7, and 9.

I would of course be aware of these changes as they're important to me if I am to offer high accuracy replica panels for various JMPs. I have designed and made prototypes of this specific front panel type already. I can provide replacement panels for transitional master volume amps.