Marshall experts needed (paging John Phillips!)

pluto

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Has anyone seen a late 70's JMP Master Volume with an extra preamp tube hole on top the chassis? I've seen extra preamp tube holes on early JMPs when the supertrem was still being made, but I've never seen them on later JMPs. The amp is a 1980 2203. Thanks in advance, Mark
 

pluto

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I would but I figure he's always on the board and I know there are other Marshall experts here.
 

Topcat

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I'd shoot this question over to the Amp Technical Info section of this forum. John seems to pick up more questions over there.

Terry
 

John Phillips

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Well, since I'm always here... ;). (Note to self: must try - again - to get a life :).)

I've seen one, on a very early JCM800 2203, but I didn't believe it was original when I first saw it. However, it was extremely cleanly done - it was punched, certainly not a DIY mod with a drill and files like most 'extra' holes are - and more strangely, although the amp had been modded a little, there was no evidence of large-scale preamp mods having been fitted and removed, which would be the only justification for fitting an extra tube socket... odd. I have to put it down as possibly original, but I don't know why it would need to be. There's no other model I can think of that shares the same chassis and uses four preamp tubes.
 

cameron

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Originally posted by John Phillips
There's no other model I can think of that shares the same chassis and uses four preamp tubes.
No production model. It could have been that there was a model designed and prototypes built (or at least chassis for the prototypes were built) but which never went into production. The chassis punched for that model that never was could then have been used for amps that didn't need the extra hole.

The above is just wild speculation, but it's in the realm of the possible.
 

pluto

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Thanks to everyone (and John, if you had a life, we wouldn't have as much help here would we?). Hmmmm, I have 2 pics (top and bottom of chassis). Can I email them to you guys so that you can see it? The hole is plugged but you can at least see the circuit board to figure out if there were mods done to it. I know that would be difficult to figure out but it's worth a shot.
 

Swarty

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I'd welcome the opportunity to take a look. There were a few oddball amps in the late '70s like the Club & Country, there was also an amp with some sort of built in boost circuit, so maybe they ran out of "regular" chassis one day? Does the hole line up and space consistently with the others? Does the edge/finish on the hole look like it was done at the factory? Another thought is that someone started a mod, got the hole done and went no further. I've actually seen one like this.
 

Lavely

Silver Supporting Member
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1,550
Here is the amp pluto is asking about:





The tricky question is that there seems to be very little fiddling with much on the circuit board that would indicate a major mod. There is an obviously different wire with shielding and a replaced big orange cap (and one small blue resistor), but no major stuff.

Also note - you can see the "plugs" on the back of the chassis.

Thoughts?

Lavely
 

Lavely

Silver Supporting Member
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1,550
In this shot, you can see the plastic cap covering the last hole.



Lavely
 

Swarty

Member
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I'd like to see the hole without the plug to see how it compares to the others. That yellow cathode bypass cap on the far left is also not legit as is the input lead to the first gain stage.
 




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