'69/70 Marshall JMP 50 Restore Project

shaunf · 2011-11-11T17:19:50+00:00

So, following from my NAD thread earlier in the week, http://www.guitarforum.co.za/amps-speakers-and-valves/noad/ I thought it might be interesting to start...

'69/70 Marshall JMP 50 Restore Project

Norman86

Thanks Shaun!
She's looking good hey!!!

shaunf

Thanks for the comments guys. Can't wait for the amp chassis to come back.

Nicholas-L

mmmm sexy ?

shaunf

Chassis is home and operational but will be going back once a few more parts arrive...

Danny-B

I hate marshalls, and even I want to come and steal that amp. Beautiful and flawless restoration, well done!

Nicholas-L

That pic brings tears to my eyes :'( its so pretty.

Norman86

absolutely perfect! well done!!!
the cab looks good under your AFD!!
It actually looks as if it was made for it!
Same goes for the 1960 cab under your restored head! ?

ezietsman

I love the look of those old Marshalls, esp when they're still new.

Can you crank that plexi or is it too loud for the house?

shaunf

Thanks for the comments guys.

I had the JMP dimed on Monday for about 10mins, testing it out and it's pretty loud, but nowhere near as loud as the AFD at full taps, which I've only done once, just to see what would happen. That's actually a funny story. One afternoon I was home alone and decided to see just how loud 100w through a 4x12 really is, so plugged in my 5m cable, cranked the amp to max, stepped out the room, closed the door and let rip. When the dust had settled, I went back into the room and noticed that my pc, which sits on a desk opposite my amps, at the other end of the room, had actually woken up, so the amp had actually moved the mouse enough to wake the pc up. ?

I can't say that I specifically sought out this amp and that I had any notion of it being a practical or even suitable amp for my purposes. It was something that presented itself as a trade for the TSL combo I used to have, and I thought it would be cool to get my hands dirty and restore it, and then play it occasionally when the opportunity to crank it up presents itself. I'm planning on moving house later this year or next year and I'll be certain to ensure that the new house is better equipped to handle cranked amps without pissing the rest of the family off.

What I am really, really stoked with is the cab. My goodness but it really does sound so sweet, and will now see daily use with the AFD. The irony is that the cab was merely a deal sweetener, so to speak. I also feel it's a little safer playing the AFD through it, rather than the JMP, since I seldom go over 20-30w on the power scaling, and the cab is only rated for 100w and the speakers are really old, so would hate to blow one.

Anyway, just picked up 2 full sets of tubes, a new indicator light, and 2 new speaker jacks at the post office. More to come...

shaunf

I need some help please guys.

It's in my nature to use projects like these as an opportunity to learn new things. Electronics is something I currently have only a basic understanding of.

So, I've spent the last couple of evenings trying to get to know a bit more about this amp, and to try to understand how it works, which has meant learning to read schematics, learn how resistor coding works, comparing the schematics to the actual circuits, determining if there were any differences etc. I now believe I have a basic understanding of this amp. This is what I have learnt thus far:

The amp actually has the 1986 Bass circuit
It has a shared Cathode. The cathode capacitor appears to be of different spec to the schematics I've seen (220uf vs 330uf on the schematics)
It has a 56K resistor and 250pf capacitor installed in the tone stack
It has no bright cap installed
It has screen resistors installed, but I don't know if they are the correct sizes as they look to be 470ohms compared to 1K5W ones I've seen installed on other amps
There's a bit of weirdness with the filter caps. Of the 2 on top of the chassis, one is 50uf (The blue one), one is 32uf, and the preamp filter cap inside the chassis is 32uf. The schematics I have say all should be 50uf, however I've found a lot of examples of amps on the web that have all 32uf, or at least a 32 for the preamp.

So, I thought I'd be clever and make a list of all the differences in the components between the 1986 Bass and 1987 lead circuits, and then order all the bits and pieces so that I could switch between the 2 and see which one I liked better, and it's at this point that my head exploded.... ?

Some of the coupling caps are different values between the 2 circuits, so I began to explore what to get, and the next thing I new I was looking at Mallory 150's, Vishay MKT1813's, Jupiter this and Mojo Dijon that, Sozo Vintage and Sozo Standard. Hell, I even found myself on Ebay a couple of times and nearly clicked Add to Cart on some NOS Philips/Mullard Mustard caps, the price of which made my eyes water!

Please someone talk some sense into me and help me move forward. It feels like I forgot to properly form my brain's capacitor before I flicked the mains switch into this world I know nothing about. I think I need a lie down... ☹

nick

shaunf wrote:
The amp actually has the 1986 Bass circuit

My JMP50 is a bass model too, but it means the first channel is the normal lead and the second is bass. Plug a guitar in the second and its sounds nothing like the 1st channel, is yours the same?

bottledtone

As with many old production amps, If they could purchase 32uf caps for less than the 50uf's would cost, thats what they would use. Or if they had a surplus of one value, they would rather use them up than buy more components to finish. Schematics were drawn, but this didn't insure that the amp would be built exactly to this spec. Some marshalls even had mains transformers that were wound incorrectly, and instead of 450v on the plates, they had 540v.
I would suggest changing screen resistors for EL34's to 1K5 to 2K2. It would make those valves a bit happier.
And 330uf to 220uf.....you won't hear the dif. ?

Buzz

Can you post a pic of your board so we can see how it is at the moment?

shaunf

here's a link to a hires guts photo:

http://img.photobucket.com/albums/v206/cliosport/IMG_1260.jpg

This was taken before the work just undertaken, which included replacing all the pots and the input jacks. The tech also replaced a lot of the resistors in this photo with ones that are now grey and I think are metal oxide. Are these any good ?

Oh, the bias thingy has also now been replaced with a trim pot.

I can post an updated pic tonight if required?

*Edited to add info about those Philips caps. I had a close look at them last night and they have the following info on them:

One side says Philips HQ
Other side says: 0.041/K/400 and then below that says 341-MKT N7.

Buzz

Most people seem to think that metal oxide are harsh or cold sounding - generally they seem to be only used as power resistors in the B+ circuitry ( the 8k2 and 10k in you amp) and maybe the 100k plate supplies. See if you can get back the resistors from your tech, most of them were the original Iskra's, which are much sought after. If not, buy some 1 watt carbon film resistors - their leads are long enough to stretch between turrets. The 1/2 w ones are too short.

The Philips caps are strange - someone did not have the correct values, so they were put in series to get the correct values! 2 x .041 in series = .020 roughly, which are the correct values for a 1986 -normally .022 same thing as 22n. On a lead amp, the one for the bright channel is usually a 2n2 with 22n for the other. Now that I look more closely at you pic, I can see that they are not original

It is easy to change to a split cathode for V1 - I have hot glued a 68n cap parallel with a 2k7 resistor with on the turret board next to the existing cathode resistor/cap. You can then just jump a wire to pin 8 on V1 and remove the existing bridge between pins 3 an 8. You should also solder a 500pf cap across the corresponding 470k mixer resistor.
The original cathode cap should hav been a 250uf as far as know - it doesn't really make much difference above around 10uf iirc. There is a great calculator here http://www.ampbooks.com/home/amplifier-calculators/cathode-capacitor/ You can plug in different values to see how the response and gain change.

The coupling caps between the PI and power valves are 100n on a 1986 and 22n on a 1987
There is an excellent PDF from Metro amp here http://forum.metroamp.com/viewtopic.php?f=24&t=14850 which shows the board for the different models, as well as explainging the building of a clone.

Make one change at a time and see what it sounds like - if you do a whole bunch at once, you never know what effect each component has on the sound.
I like the 1986 configuration for my strat/tele, but it has too much bottom end/not enough top for my son's SG. I am planning to change one of mine back to a 1987 for him to use. You can use any poly caps to experiment with - the value affects the sound far more then the type. Find the value that you like best and then experiment with different types. Just make sure all coupling caps are at least 400v

shaunf

Thanks Buzz,

I have all the old components the tech removed, including all the resistors. There were one or 2 that were out of spec value wise, but the rest were ok from that perspective. The reason he removed them was because he found a fair bit of corrosion on the leads, particularly on the undersides.

I had a look at the resistors this morning. The leads are still solid. I was able to straighten them nicely with a pair of long nose pliers. Do you know of a good way to remove the oxidation and if so, is it possible to somehow protect the leads to prevent/slow down further oxidation? If so, it may be possible to reuse them.

I also gave you the wrong value off those Phillips caps. They are .047 not .041. Still not correct thorough. Do you have any suggestions for replacement coupling caps? I sent the guy from Sozo an email yesterday but have not yet had a response.

Thanks again,

Shaun

TomCat

Just one comment from my side.....

Those caps in series to get the right values is not a good thing from a phase shift perspective.

Capacitors are reactive components that cause a phase shift to an audio signal as it passes through the cap. By placing two caps in series the phase shift is compounded and could seriously compromise tone if that cap is in the main signal path.

I recommend replacing the two caps with the correct value as a single cap.

shaunf

Thanks Tom, appreciate the input. My plan is to replace them as soon as I can decide on what to replace them with. Hoping to get some suggestions from you knowledgable folk.

Buzz

I don't worry too much about brands of caps - Sure, they may sound different, but it seems everyone has different ideas which are better or worse. Buy some correct values locally and experiment to find the values that sound best to you.

Maybe then you could experiment with different types to see what difference they make. Sozo are expensive to experiment with. I have bunch of Mallory 150 on their way to me - They are inexpensive and look more or less the part. The 2 amps my son and I gig with have all local caps in them, and I can't hear any major differences between them and the olders ones which all have the original mustards.

Just use some steel wool on the resistors - 40 year old amps will always have a bit of corrosion on them! It's only on the surface of the leads, so won't cause any issues. Don't steel wool them too much, otherwise you will go through the tin plating and expose the copper underneath.

Tomcat is spot on with his advice - the nice part of having on old Marshall that has been molested is that you can carry on molesting to your hert's content to find the best sound for yourself.

Did you manage to get a look at the PT/OT to see if they are originals?

shaunf

Morning Gents,

Been struggling all weekend with a problematic internet connection, so wasn't able to reply. Thanks again to all those who have contributed with suggestions and advice, I really do appreciate it.

Buzz, regarding the transformers, here is a photo of the PT:
http://img.photobucket.com/albums/v206/cliosport/IMG_1239.jpg
It has 2 codes stamped into the cover plate; P-6302 and 606910. I haven't been able to find much information from these codes, although I did get a few hits on the 606910 code, which indicated it could be a date code, and if so, puts it's production date somewhere in 1969, which is plausible, given the the amp's date is similiar. However, the hits I did get referred to Fender transformers, not Marshall, so I don't know, and in fact doubt it is original to the amp.

A picture of the underside of the transformer also makes me doubt it is original. The tech who examined it seemed to think it may have been rewound:
http://img.photobucket.com/albums/v206/cliosport/IMG_1259.jpg

The OT - I have no idea. Unfortunately at some stage the chassis of the amp and the transformer covers were painted, so I can't see any codes on them. Do you know of any other way to be able to see if the OT is original?

Here's a pic of the choke, with the OT in the background:
http://img.photobucket.com/albums/v206/cliosport/IMG_1243.jpg

Regarding capacitors, I found these ones on RS Components that I am considering. They are made by Vishay Roederstein. I've seen guys talking about ERO MKT1813's. These are the same, just under a different name since Vishay took over. I would appreciate any opinions or comments you may have on these:
http://za.rs-online.com/web/p/products/1854296/

Finally, my amp was missing the original Bulgin power socket. I was hoping to replace it with a standard IEC socket, but it's not going to fit the current hole which is around 26mm diameter. The IEC socket will leave a gap on top and at the bottom due to it being rectangular going into a circular hole. Alternatively, I could use one of these Neutrik, which should pretty much fit in the existing hole in the chassis?

The only thing is it's not going to look particularly stock. Any other better suggestions?