1976 JMP restoration

[Riffmonster]

Hi all,

This is my first (I think) post here but I have been reading the forum for a long time now. Thanks for all the nice and free info provided here, really much appreciated.

I decided today to open up my JMP from 1976. It is a rather odd version with the MK1 headshell and knobs but it has only two inputs and master volume. I guess it is from the time when Marshall was switching to the new design but still used some leftover parts. Anyways, I played this amp for the last 7-8 years without problems but I had some crackling noise issues lately when turning up the volume. I found out that the input jack was loose so I opened it up and decided to change it. I also wanted to remove that second master volume that was already added to that amp when I bought it.

So far so good. When I opened up the amp I was a bit puzzled about the filter caps I saw. There are four 200uf filter caps from the company Hunts. From my little research, Hunts existed only until 1969? This seems rather strange to me. Why would anybody put in those old caps with wrong values? I assume this is not stock, is it? Maybe someone has some more insights about this. Also, if i were to change those caps, it would be best to go with the stock values wouldn't it?

Thanks so much and all the best,

Simon

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[Roe]

A few marshalls did come stock with 200uf caps on the mains ans screens. Often 220 uf on the mains is good, but on the screens 100 uf seems sufficient

[Riffmonster]

Thanks for the reply. So, it might be worth sticking to the values that are in the amp right now. The amp always sounded great with lots of bass response but not as loud as other JMPs.

[_Steve]

Wow I just looked at mine which is also a '76 transitional model with the small logo and toggle switches and I have the same cap configuration! I'd never bothered to look closely at them before. The amp is a keeper. I just plugged it in after 6 months in a closet (the amp not me) and i'd forgotten how good it sounds! Its a monster.

Thanks for posting!

[Riffmonster]

Wow I just looked at mine which is also a '76 transitional model with the small logo and toggle switches and I have the same cap configuration! I'd never bothered to look closely at them before. The amp is a keeper. I just plugged it in after 6 months in a closet (the amp not me) and i'd forgotten how good it sounds! Its a monster.

Thanks for posting!

That is very interesting, so we can assume those caps are stock! I will change them but stick to the values. 200uf is kinda hard to get in Europe, so I will probably go with 220uf 350v from Tube Amp Doctor. Has anybody experiences with their caps or knows another supplier in Europe?

Thanks

[Colossal]

That is very interesting, so we can assume those caps are stock! I will change them but stick to the values. 200uf is kinda hard to get in Europe, so I will probably go with 220uf 350v from Tube Amp Doctor. Has anybody experiences with their caps or knows another supplier in Europe?

Most electrolytics have a wide manufacturing tolerance with the actual, measured value often above the nominal value. Just looking at a F&T 32-32uF 500V cap can, the tolerance is stated as -10/+30%.

[Riffmonster]

That is very interesting, so we can assume those caps are stock! I will change them but stick to the values. 200uf is kinda hard to get in Europe, so I will probably go with 220uf 350v from Tube Amp Doctor. Has anybody experiences with their caps or knows another supplier in Europe?

Most electrolytics have a wide manufacturing tolerance with the actual, measured value often above the nominal value. Just looking at a F&T 32-32uF 500V cap can, the tolerance is stated as -10/+30%.

You are completely right, I should not worry about finding a 200uf one! Thanks, I will keep you guys updated when I am done.

[_Steve]

You are completely right, I should not worry about finding a 200uf one! Thanks, I will keep you guys updated when I am done.

Hey would you mind posting a few pics of your trannies and board? Im trying to work out what the OT in mine is. Here are some pics of mine for reference, and the Hunt caps:

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[Riffmonster]

Hi Steve,

Thanks for the pictures, your amp looks great! I am happy to share more photos, but I probably will not have the time to do so before the weekend.

[_Steve]

No problem, its appreciated

I've already noticed a couple of differences from your board shot. Keen to do more digging - especially around the output transformer!

[Colossal]

Gents,

Thanks for documenting your amps. Always nice to see these old Marshalls. How thick is the lamination stack on your output transformers? 1.5" or 2.0"?

Thanks.

[Roe]

For the mains, I would get Ars or F&T 220uf. For the screens 100uf is preferable, unless you prefer a stiff and tight amp for metal.

[Riffmonster]

Gents,

Thanks for documenting your amps. Always nice to see these old Marshalls. How thick is the lamination stack on your output transformers? 1.5" or 2.0"?

Thanks.

I will try to find that out over the weekend too!

For the mains, I would get Ars or F&T 220uf. For the screens 100uf is preferable, unless you prefer a stiff and tight amp for metal.

I do prefer a more end 70s/ early 80s tone but still I prioritize a hard rock sound over a metal sound. Maybe I will try both values and see what I like best. It seems rather difficult to find single can caps for 100uf around here ( I would prefer filling all the cap holes in the chassis and not rely on multisection caps).

[Roe]

Marshall used 50+50uf caps most of the time

[_Steve]

Gents,

Thanks for documenting your amps. Always nice to see these old Marshalls. How thick is the lamination stack on your output transformers? 1.5" or 2.0"?

Thanks.

Mine is 1.5"