'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 a project thread to keep track of the restoration. As I said in the other thread, the amp is in working condition, but does have some issues, and also has had some work done to it that I would like reversed in favour of returning it as close to original spec as possible/practical. The 4x12 cab I got with the amp has some wonderful 1968 Celestion Greenback speakers in it, but the cab is really ugly, having being painted and recovered rather badly at some stage. So far, My Program of Work for the project is going to look something like this: Amp Head 1.) Recover head in original Marshall Levant tolex, including original spec gold piping. 2.) Replace hard wired power cord with kettle style power recepticle. 3.) Replace all pots. 4.) Replace all input jacks with original mono Marshall vintage style ones. 5.) Replace front face plate. 4x12 Cab 1.) Recover Cab with original Marshall Levant tolex, and install original spec gold piping. 2.) Replace grill cloth with original Marshall Salt and Pepper basketweave grill cloth 3.) Rewire cab to 16ohm.

Buzz

Hi Shaun
The white and blue powercons are different ie the blue and white ones will not mate happily!
Doesn't really matter, it just means that in an emergency another standard powercon mains lead
will not fit your amp.

The 680n caps at 400v are actually huge! That is why Marshall use 160v ones - they will fit onto the board. Anything over 150v should be fine

Silver Mica should be fine MAKE SURE THE ONE IN THE TONE CIRCUIT IS 400V OR MORE- you can use ceramic disc as well
Apparently the sound is different - try and see!

No problem to mail a list to me. jeremybatglobal.co.za at work or jeremyrobertbusbyatgmail.com at home
Metal film are usually a shiny blue colour, metal oxide are usually a matt powdery looking blue (or other colours)

Have you had a 450v DC shock yet?
You don't want one
Make a lead to drain the caps - 2 x INSULATED croc clips with a 100K resistor in series. Clip it on between ground and any of the power caps.
The resistor slows down the discharge, but saves the amp and fuses when you forget to take the drain off and turn on the amp.

I have got into the habit of clipping it on every time I work on an amp. I only needed one shock to learn my lesson.

shaunf

I'm stoked! I think I found a black power socket. It's from Switchcraft:

http://www.alliedelec.com/search/productdetail.aspx?SKU=70214199

Re the 450V DC shock, I've never had one and if I can help it I never will. I have a very healthy respect for electricity and won't take chances, especially with something like a guitar amp. I won't be sticking my fingers inside any part of the amp without first discharging the caps and then verifying that they have no stored voltage with a multimeter. That process has kept me out of trouble so far.

The technique I was taught and have been using is slightly different to what you describe though. I use a crocodile clip lead and clip one end to pin1 on V1, and the other end to a ground point on the chassis. Apparently this has the effect of placing a whole lot of the amp's resistors in series which then assist in bleeding off the caps. From what I've seen, the voltage stored in my amp's filter caps drops to 0V in about 2min's using this method.

Will drop you an email tomorrow sometime or over the weekend. The list of stuff I want keeps growing...

ezietsman

Buzz wrote Hi Shaun
The white and blue powercons are different ie the blue and white ones will not mate happily!
Doesn't really matter, it just means that in an emergency another standard powercon mains lead
will not fit your amp.

The 680n caps at 400v are actually huge! That is why Marshall use 160v ones - they will fit onto the board. Anything over 150v should be fine

Silver Mica should be fine MAKE SURE THE ONE IN THE TONE CIRCUIT IS 400V OR MORE- you can use ceramic disc as well
Apparently the sound is different - try and see!

No problem to mail a list to me. jeremybatglobal.co.za at work or jeremyrobertbusbyatgmail.com at home
Metal film are usually a shiny blue colour, metal oxide are usually a matt powdery looking blue (or other colours)

Have you had a 450v DC shock yet?
You don't want one
Make a lead to drain the caps - 2 x INSULATED croc clips with a 100K resistor in series. Clip it on between ground and any of the power caps.
The resistor slows down the discharge, but saves the amp and fuses when you forget to take the drain off and turn on the amp.

I have got into the habit of clipping it on every time I work on an amp. I only needed one shock to learn my lesson.

Doesn't this amp have bleeder resistors over the caps? If not, why not?

Buzz

Connecting it to Pin 1 is much the same thing - it discharges through the 100k plate resistor and the 10k's upstream.

There have never been bleeders on the Marshall 50's (older ones anyway)- some of the 100's do. Some of them have 2 series 100uf caps to make up the voltage handling, which is usually more than 500v. You should always use bleeder resistors with series caps so that the voltage is shared equally between them.

ezietsman

Buzz wrote Connecting it to Pin 1 is much the same thing - it discharges through the 100k plate resistor and the 10k's upstream.

There have never been bleeders on the Marshall 50's (older ones anyway)- some of the 100's do. Some of them have 2 series 100uf caps to make up the voltage handling, which is usually more than 500v. You should always use bleeder resistors with series caps so that the voltage is shared equally between them.

In my amp, if I switch it off but leave the standby switch to on, the caps discharge in about 20 seconds. I always measure them all before I fiddle, I'm just thinking, these old amps should probably get these safeties installed.

Life >> Historical Correctness

shaunf

Mine, in all likihood will have a set of bleeder resistors fitted soon, but more for convienience sake than anything else.

killafo

Hey Shaun

Man i'm so stoked that you have actually taken the time to restore my x-baby, ?.
SHE LOOKS ABSOLUTELY FANSTASTIC !!!

Q: Can i buy her back ?

Lolz, nah i'm kidding, but seriously man it looks awesome. Well done hey! Glad she went to a home where she could get a makeover.
So obviously i need an invite to come and play her in her full glory!!! ?.

The markings on the faceplate where done by the prev owner, and not me. I would never !!

Really...well done :woohoo: :goodtimes:

shaunf

killafo wrote Hey Shaun

Man i'm so stoked that you have actually taken the time to restore my x-baby, ?.
SHE LOOKS ABSOLUTELY FANSTASTIC !!!

Q: Can i buy her back ?

Lolz, nah i'm kidding, but seriously man it looks awesome. Well done hey! Glad she went to a home where she could get a makeover.
So obviously i need an invite to come and play her in her full glory!!! ?.

The markings on the faceplate where done by the prev owner, and not me. I would never !!

Really...well done :woohoo: :goodtimes:

Hey Juan,

Good to have you here! Glad you like the makeover too!

You're welcome to come and give her a rev once she's done. Still trying to sort out a couple of issues, but it won't be long now.

I know it wasn't you who took a marker to the faceplate! Still, it ended up being a non issue as they all wiped off with a bit of rubbing alcohol.

I can see why you wanted to lose the head and cab since you're a giggling muso. I took the cab down to the tech's house a couple of weeks back to show him. Holy crap, it is heavy! Think I popped at least 2 vessels and gave myself a hernia carrying it down the stairs to my bakkie. It's way heavier than my new 1960 cab, that's for sure!

Anyway, hope the TSL is treating you well.

shaunf

An update:

With a new set of tubes in it the amp now sounds fantastic! Running a full set of Tung Sol reissue tubes, in preamp and power amp and they seem to be delivering the goods.

These is still one problem that I need to a.) either solve, or b.) determine whether its a quirk that I'm going to have to live with seasonally, and that is that there is DC voltage present on the inputs. This results in amplified mechanical noise when I adjust the volume pots on the guitar, switch pickups, or switch a pedal on or off. Besides this issue, the amp is performing flawlessly, and is generally very quiet.

Further testing found that this DC voltage was present at various places on the turret board. Cleaning the turret board down with 100% pure alcohol improved things temporarily, but unfortunately the problem has come back. Since this treatment temporarily resolved the problem, I suspect that the conductive turret board is merely a surface issue, and not that the turret board has absorbed moisture and is thus internally conductive like what happens to old Fender amps with their fibre board turret boards. One of the environmental issues that I need to mention is the crazy humidity here in Durban at the moment. I'm using a dessicant dehumidifier in my study to stop my guitars etc going mouldy. The dehumidifier uses a large salt tablet that is supposed to last 5 weeks in a room the size of my study in relatively high humidity conditions. The last tablet I put in lasted less than 2 weeks.

I strongly suspect the DC voltage issue on the turret board is related to the humidity, especially since it didn't seem to have the problem when it was less humid back in November when I got it. Nevertheless, I'd appreciate any advice anyone may have to get rid of the problem.

bottledtone

A quick fix would be to put a cap on the input of the amp. Even one on a lead would be a very easy way out. Value of 220n should work. SRV had a small value cap in a box before amp to be able to run the amp full out and not flab out the bass. Has benefit of blocking DC to the guitar pots. Hope its not a conductive board.

shaunf

bottledtone wrote A quick fix would be to put a cap on the input of the amp. Even one on a lead would be a very easy way out. Value of 220n should work. SRV had a small value cap in a box before amp to be able to run the amp full out and not flab out the bass. Has benefit of blocking DC to the guitar pots. Hope its not a conductive board.

Thanks Bottledtone, I don't think the board itself is conductive. I'm sure it's just the surface. If the problem was internal to the board, I'm sure I'd have worse problems than what I do? The problem was completely resolved for a couple of days while it was still with the tech, just by wiping down the board with alcohol, but then returned after I bought the amp home.

shaunf

Just wanted to say big up to Marshall customer service. When I got this amp and was considering trying to create new front and rear panels, I sent Marshall an email from their website and asked them if they were able to assist me. It took them a few weeks to respond but when they did, they mentioned they couldn't supply me a completed panel, but could source me some blank panel material and asked if this would help me. I said it would, paid about £10 for postage and yesterday 2 brand new blank panels arrived in my postbox. I'm really impressed at the trouble they went to for me.

In fact this is the second time I've had great service of this nature. Around 2001 or so, I dropped my Les Paul when it managed to unhook itself from my strap during a gig. Given the fall, which was directly onto the headstock onto a concrete stage, I'm amazed the headstock didn't break, but the G string tuner was badly damaged. I somehow managed to clip the back cover of the tuner back on and amazingly got the guitar back in tune and carried on, but in the long term I needed to replace the tuner. I couldn't find one locally so I sent Gibson email and they sent me a complete set of tuners free of charge.

Moral of the story: don't write off the big guys if you need help with one of their products, regardless of how old it is or how small the item. You might be surprised.

shaunf

I dunno if anyone is still reading this thread, but if so, I did my first mods to the amp today. I replaced 2 of the .022uf coupling caps with some Vishay mkt1813's. Plugged it back and tried it. Couldn't really hear much difference from the old Phillips ones. Drained the caps and then installed a .68uf cap across the 820 ohm V2 cathode resistor like the lead circuit has. Amp seems to have a brighter response and a bit more mids. I like it so that will stay.

vic

shaunf wrote I dunno if anyone is still reading this thread, but if so, I did my first mods to the amp today. I replaced 2 of the .022uf coupling caps with some Vishay mkt1813's. Plugged it back and tried it. Couldn't really hear much difference from the old Phillips ones. Drained the caps and then installed a .68uf cap across the 820 ohm V2 cathode resistor like the lead circuit has. Amp seems to have a brighter response and a bit more mids. I like it so that will stay.

well I'm still reading it... ? great job so far.

nick

Also still following ?

makepeace

Me too. Cool to hear those Vishay caps work nicely.. When I next need caps, think I'll go for those. Beats waiting for overseas shipping. Anyone know of a local source of adequate filter caps?

shaunf

Cool! Glad you guys are still here. Wish more of the amp guys would join in and throw their 2c in, even if it's just general amp chatter. I'm really enjoying learning about how amps work and fiddling with mine.

Makepeace: The only local source I've managed to find for those big filter cap cans is Mr Valve. From their pricelist, it seems they keep the JJ ones, although their pricing isn't the best.

I've just ordered a full set of F&T filter caps from overseas, as well as a couple of different options for coupling caps, including some Mallory 150's and Illinois Capacitor Polypropylene's to play with and see what I like best.

I've also ordered the parts required to install a Lar/Mar PPIMV, which I'm quite keen to try at some stage too.

makepeace

Ah yes I know about Mr Valves offerings, thanks ? For future, IMO unless you're replacing them in an amp, my advice would be to stay away from those big multi-cap cans. They make your grounding scheme quite a hassle and have other shortcomings..

Anyway, have done some more research on the topic and have found some 450V Evox-Rifa electrolytics. Great prices, around R50 a pop, plus, you only have to wait a couple of days for RS's free delivery, and no customs! Have done some reading and they have commendable specifications.. Low ESR, wide range of operating temperature and are meant to last for 30 years! Quite well spoken of in the DIY hi-fi circles. Does anyone have any experience with these in guitar amps?

Let us know how the attenuator works out!

colt

shaunf wrote ...Having chatted to several laser cutting, engraving companies, it sounds like it may be possible to get a new one made, however all of the companies I spoke to require a to-scale design in either some type of CAD or Corel Draw format. With this in mind, I set about removing the existing panel, carefully measuring every aspect of it, looking at other reference pictures to fill in the missing markings and then working in Corel Draw to produce one:
.

Anyway, so next week it's off to go and see if I can get this faceplate made. If I'm successful, I'm considering getting the a new rear plate made too.

Could I get a copy of that Corel Draw file?

Thanks

colt

Would really like to be able to get a copy that Corel Draw file. Thanks!

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