Improve JCM 900 1960a cab

matei72

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Hi all...

Posted something similar on FB however this is probably a more appropriate place. I picked up a JCM 900 1960a cab last night, with the scope of improving it a wee bit and adding 4x Celestion Neo Creambacks - 8ohms.

First thing I identified is the need to ditch that stereo jack plate - I don't intend to ever use this cab in stereo.

I assume that for a quick fix I can just take out the existing jacks and PCB gubbins and wire up a Neutrik or Switchcraft 1/4'' jack to the speakers in series-parallel?

Re. the construction of the cab itself... I read elsewhere that the later Marshall cabs don't have a proper 2x2 inside, rather it is shorter and that there is a bit of MDF or particle board used inside for cloth baffles etc. Is there a guide to anything like the proper dimensions of those bits?

Also - the back panel, has anyone fabricated one out of plywood? Mine is obvs not, and I'm trying to lighten this cab a wee bit and improve the sound.

Lastly I realise I'm going to need to ditch the plastic handles as well and put metal repros on, not so much for looks/accuracy but the ones on there are 30 some-odd years old and on their way out. I'm in the UK I'll have to order them from the US.
 

clutch71

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I made some changes to my 1960 B cab over the years. I replaced the center post with a 2x4 (screwed and glued) and new slightly larger plywood base. The existing one was stapled, loosened over the years, and vibrating.

I sanded the inside of all the black goo that holds the tolex.

I ran every speaker combination known to man through it before settling on 4 G12EVH's in 2014.

The 1960 I have is all ply but I have replaced the MDF back panel (the rest was ply) on a Peavey 5150 cab. I felt it was an improvement but no longer own the cab. I even reused to tolex from the original back.

I have not replaced the handles but want to. Scumback Speakers (member here) has direct replacements.
 

soundboy57

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The biggest difference you can make is adding the verticle center brace that they used up until early 1977, on the slant baffles.
Much tighter/focused sound, IME.
Make sure the inside 2x2 center brace is tight with the back panel, and glued tight to the baffle.
I have some stick on thick foam rubber pads, exactly like they used in the mid 70s
for the smaller 2x2 brace, on some cabs I have owned/seen.
I prefer the MDF back, just me.
The ply back can sound a bit "hard" tone wise, depending on the speakers, amd setup. I have had them all at the same time in a room,
years ago.

The only other MDF (chipboard) is the small strips around the edge of the baffle, and behind the logo and model plates, which they started using during the 800 era sometime.
I wouldn't bother with that. The rest of the cab is birch ply, inckuding the front baffle.

I have used the Scumback metal handles and they are wonderful to look at, solid, fit the screw holes exactly, and reasonable.
From 3 feet away they look
like the old Marshall ones on the outside.
Highly recommended.
My two cents.
 

pulsonicsound

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Hi all...

Posted something similar on FB however this is probably a more appropriate place. I picked up a JCM 900 1960a cab last night, with the scope of improving it a wee bit and adding 4x Celestion Neo Creambacks - 8ohms.

First thing I identified is the need to ditch that stereo jack plate - I don't intend to ever use this cab in stereo.

I assume that for a quick fix I can just take out the existing jacks and PCB gubbins and wire up a Neutrik or Switchcraft 1/4'' jack to the speakers in series-parallel?

Re. the construction of the cab itself... I read elsewhere that the later Marshall cabs don't have a proper 2x2 inside, rather it is shorter and that there is a bit of MDF or particle board used inside for cloth baffles etc. Is there a guide to anything like the proper dimensions of those bits?

Also - the back panel, has anyone fabricated one out of plywood? Mine is obvs not, and I'm trying to lighten this cab a wee bit and improve the sound.

Lastly I realise I'm going to need to ditch the plastic handles as well and put metal repros on, not so much for looks/accuracy but the ones on there are 30 some-odd years old and on their way out. I'm in the UK I'll have to order them from the US.
Making a ply back panel will actually make the cab a little heavier, having said that it means you’ll never get a broken corner on the back panel

You’ll need 15mm ply for the back, you can also use 12mm which will be a little lighter but it won’t be flush with the back

Not many builders merchants stock 15mm so unless you want to buy a whole sheet it might be easier to just get a 12 mm plywood back cut up, that shouldn’t cost more than £22, that’s what my local place charges, if you don’t like the way it sounds stick the mdf one back on!

Like @soundboy57 mentioned I wouldnt overly worry about the mdf on the logo mount and edge of the baffle, I don’t think that will have a bearing on the sound

If you do decide to do it just measure what you already have

Penn Elcom do metal panel handles that are dirt cheap, not the best looking though!

Hope that helps
 

Clayton Custom Cabs

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The biggest difference you can make is adding the verticle center brace that they used up until early 1977, on the slant baffles.
Much tighter/focused sound, IME.
Make sure the inside 2x2 center brace is tight with the back panel, and glued tight to the baffle.
I have some stick on thick foam rubber pads, exactly like they used in the mid 70s
for the smaller 2x2 brace, on some cabs I have owned/seen.
I prefer the MDF back, just me.
The ply back can sound a bit "hard" tone wise, depending on the speakers, amd setup. I have had them all at the same time in a room,
years ago.

The only other MDF (chipboard) is the small strips around the edge of the baffle, and behind the logo and model plates, which they started using during the 800 era sometime.
I wouldn't bother with that. The rest of the cab is birch ply, inckuding the front baffle.

I have used the Scumback metal handles and they are wonderful to look at, solid, fit the screw holes exactly, and reasonable.
From 3 feet away they look
like the old Marshall ones on the outside.
Highly recommended.
My two cents.
As a former audiophile speaker builder and the owner of a beat-up JCM800 1960A I can attest to the fact that the Marshall 412's break all the rules of hifi speaker cabinet construction, but we need to remember that's why a Marshcall half stack sounds like a Marshall half stack. Just like the early flimsy Fender pine boxes sound like they do, they were built cheaply. Those are the sounds we like, right? If you run your guitart amp through a hifi stereo speaker you won't likely like the sound it produces.
 

Clayton Custom Cabs

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Making a ply back panel will actually make the cab a little heavier, having said that it means you’ll never get a broken corner on the back panel

You’ll need 15mm ply for the back, you can also use 12mm which will be a little lighter but it won’t be flush with the back

Not many builders merchants stock 15mm so unless you want to buy a whole sheet it might be easier to just get a 12 mm plywood back cut up, that shouldn’t cost more than £22, that’s what my local place charges, if you don’t like the way it sounds stick the mdf one back on!

Like @soundboy57 mentioned I wouldnt overly worry about the mdf on the logo mount and edge of the baffle, I don’t think that will have a bearing on the sound

If you do decide to do it just measure what you already have

Penn Elcom do metal panel handles that are dirt cheap, not the best looking though!

Hope that helps
Good idea, and if that 12mm ply back starts vibrating with out of phase resonance you can do as I did (not on a Marshall) and add 1X2 hardwood stiffeners in a cross formation....but then your weight savings goes by the wayside.
 


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