C18Q1 - Marshall rebuild... Now complete

normula1

This is a 1973 Marshall JMP 50 Lead and Bass (Model 1974) that my parents helped me purchase when I was eighteen or so from HW Audio in Bolton IIRC for the princely sum of £120. I toyed with a 100 watt master volume lead model but that was an extra £20 which I didn't have so this is the one that came home.



Over the years it's had several modifications starting with having a Master Volume fitting via the house technician at Dr Rock in Bury. This had been done in the standard manner between the treble pot wiper and the phase inverter input replacing the lower  Normal (Bass) channel input jack.

Fast forward a few years to the late 80s / early 90s when amp modification was all the rage, I put my electronics qualification to some use and thanks to a few schematics freely sent out by Marshall, I started in anger first re-configuring the input stage to cascade as per a standard Master Volume amp which upped the gain nicely and then fully reworking it by adding an IEC mains socket in stead of the trailing mains lead, beefing up the power supply caps, an extra transformer for the output valve bias (no idea why I did that now), adding extra preamp valves for a second channel with the gain / volume controls where the normal channel jack would be, DC heater supply for the preamp valves, an indicator LED and footswitch socket for the channel switching and a line out. It sounded great and as the cottage we lived in had 9 inch thick walls I could crank it in what passed for a spare room. 




When Child #1 came along I built a dummy load and ran it into that splitting the output into Boss CE2 / DM2 and into a reworked hi-fi valve power amp so that I didn't deafen the poor mite

It was used like this for several years until child #2 arrived and the Line6 POD came out. I had traded a load of gear for one and the Marshall got relegated to the cupboard for a good few years.

Eventually I dug it out and decided to get rid of all the channel switching nonsense and rework it in a similar vane to the Vintage Modern series. I really didn't like it in this guise as it went into distortion way to early on the volume control. I did like the post phase inverter master volume though, so I reworked it again putting it back to something like stock (or so I had thought)

So back to the plot.... the C18Q1 challenge is to remove a lot of the crap that's been fitted over the years, put it (almost) back to stock add a few blanking plugs into the various holes and hopefully get rid of an annoying fizz that occurs when the PPIV is on full. (as it happens I've a feeling that I may have found the cause of that already)

So here's the internals as they are now, the sensitive types should look away now...



WHen I was creating a bill of materials of bits I would need I was pleasantly surprised to see that quite a few of the capacitors had been replaced with reasonable quality ones but I was flummoxed with the values of a few components until I realised that the normal channel was still wired with the Vintage Modern values. Goodness knows why.

The power amp wiring will stay as that's fine and I fitted new sockets etc. in the last round. The treble, middle and presence pots will stay as they're original and still in fine fettle. The two volumes and bass controls are being replaced (again) purely because I'm being pedantic as they have plastic shafts. I've a feelingthe two bias electrolytics are original (I certainly don't recall ever changing them myself) so they'll be replaced along with all the preamp wiring which was non-standard and TBH I don't think is of particularly good quality. I've also got a new set of input sockets and knobs as the originals are long gone. I normally a hoarder and keep hold of stuff in old biscuit tins but I've no idea where they've ended up. Even worse, I've no idea why I changed them in the first place.

Now the eagle eyed amongst you will have noticed a broken logo on the first pic. This happened when the amp was in it's cupboard and it lost a fight with a computer case that knocked against it. I've had the two bits on a shelf for an embarrassingly long time so the first job was to glue them back on with a bit of aircraft model adhesive.



As I'm going to be replacing a lot of stuff, I've removed the PCB and the rest of the gubbins to give me a start.



And now for a really annoying thing I hadn't noticed. The left hand "can" capacitor had been replaced with the two RS once fitted to the PCB (please don't ask why as I've had far too many glasses of red wine since then to remember) and in its place was the smoothing capacitor for the DC heater supply. This had been disconnected some time ago but left in place. It turns out that the clamp is a wierd once that must have come with the capacitor (I've a feeling that I robbed it from some datacentre computing kit that was being scrapped many moons ago). I've no idea where the original clamp is so I've had to order one separately as it wasn't in my list of bits.

So once that arrives part two can commence....

webrthomson

Nice - you going to do your own board or are you going to spruce up the ST1 PCB?

normula1

I was thinking of getting one of @Modulus_Amps jmp50 turret boards but as this amp model was always PCB, I'm going to spruce the existing one up with some new components where needed and some new wire to the chassis mounted components apart from where the original connections are still in place.
What I've yet to 100% decide is whether to put it truly back to spec with shared V1 cathode components, no bright cap on the lead channel volume and 0.1uF PI capacitors or to do some sort of hybrid. I'm half thinking of putting a switch into the existing hole in the front panel to allow for switching between two variants for a while until I decide which I prefer.

Modulus_Amps

nice, has it still got the original output transformer, if so I would try make it as stock as possible, and get blanking plates for those extra holes on the top - to stop the beer getting inside

normula1

The output transformer hasn't been changed by me and I've had it for since 1984ish and it has the correct part number.

The holes have now been bunged up with these: https://www.vital-parts.co.uk/flexible-blanking-plugs-5729-p.asp thanks to @thermionic ;dropping that link into a recent thread and the can capacitor clamp and caps now fitted.







Now onto the board re-population.

normula1

Incidentally, does anyone know where replacement metal faceplates can be procured? I could only find plexi (plastic) ones when I searched the other night. Having said that I was using my phone to search whilst the better half was watching TV..

normula1

It's been a while, thanks to a long-winded Microsoft training course, but I've had some time this week to get back on it in anger.

Board populated and quite of the few tracks that had been cut or had failed now fixed.



Wires added and part way through being connected up. To make life easy for myself I used one of @Modulus_Amps wire kits.



I've given myself the option of a split or shared cathode on V1 and I've fixed up a temporary switch to see which I prefer. Rather than change or make any more holes in the chassis, I nicked a bit of my son's old stash of Meccano and mounted it to that..

normula1

Wiring now complete. After an A/B test I went with the split cathode as the bass was quite mushy on the shared cathode even with the Tele I was testing it with.



A new set of EL34s that had been sat on a shelf for a couple of years. All of the fizziness I had before has now gone.



And now back in its box...



Time to wake the neighbours up from Bargain Hunt

paulnb57

In total admiration of your ability to understand electronics! Nice job!

Corvus

Excellent stuff! How nice to have that going again. I like the battle scarred faceplate fwiw.

webrthomson

Nice work - lets get some clips then?

normula1

If you really must torture your ears...

https://soundcloud.com/jeffbooneuk/noodling

So this is a 2007 Les Paul Goldtop with the stock burstbucker pros into the Marshall running just shy of 8 on the "lead" channel into a Tube-Town ToneHound reactive load into a Focusrite Scarlett 2i2 and then into Ableton with a Celestion G12-65 IR and some of the built in Ableton reverb.

The quiet bits are simply the LP volume pots rolled back and in places I added some extra dirt (mostly boost rather than true dirt) with a TC Nova drive.

webrthomson

Very nice - that's pretty much the definition of how a Marshall should sound - good effort!

AustrianJohn

Great job - sounds wicked. Now you've reminded me that I've got a home built amp that doesn't quite work as planned - perhaps a Q2 challenge for me.

martinw

Good work.
Some of the most satisfying jobs I've done have been restorations of amps that have been, err,  ravaged by inexpert repairers  (Not saying your work was in that category!)
Some of the things I've come across would be hilarious if they weren't also lethal.
You end up with a good solid original amp that might be a bit cosmetically challenged, but is tonally gorgeous and good for another 40 or 50 years.
My favourite was an early JTM45 that looked fit for the scrapheap, but afterwards was a real gem.

normula1

Thanks for the kind words everyone.

I would happily admit to ravaging it if I were to make the mods I did make today.
I would caveat that though as at the time the original mods were done, amps like these were ten a penny, (I think I had a choice of 6 or 7 when I bought it a few years earlier) and therefore not of any great value. Secondly, I couldn't afford / justify anything different and having a keen interest, a few schematics when Marshall would just send them in in the post, and a hot soldering iron I just dived in and made it do what I wanted at the time.

Even with hindsight, the only thing I sort of regret was drilling the holes in the front panel, and I've learned over the years to try to make any modifications reversable with as little evidence as possible of a mod being made.