1974 120W SL/A Restoration Project v2.0

glpg80

Well-known member
I’m documenting my journey to take an already modified (poorly), for parts 1974 Marshall Superlead MKII and restoring it with all of the creature comforts of a modern amplifier.

The amplifier landed in my hands with the original board already swapped out and the original faceplate stolen along with it. This amplifier had its board and faceplate transplanted from what seems like a late 70’s JMP master volume board.

It has the original dagnall iron.

The JMP has received extensive tonal modifications which sound stellar and will remain. It’s already modified well beyond turning back to stock and I also prefer the master volume 2203 platform.

The mod list outlined below is what is described:

Design and fabricate a dual bias control board separate from stock that is more efficient and safer than the original design. The board was designed and simulated in LTSpice.

Install a metroloop with footswitchable bypass.

True dual master volumes that are footswitchable.

Install a global resonance control.

Modify the chassis to accept and install a modern Marshall 15A rated impedance selector for safety of the OT.

Replace the incorrectly installed fuse holders.

Replace the damaged pilot light.

Make the Hi/Lo input footswitchable. The design must allow for override for HI or LO input by the user when the footswitch is unplugged.

Design and fabricate the relay control board.

Modify a replacement Marshall 3 button footswitch to control the functions.

Install a dedicated 12.9V DC supply.

Convert the preamp section to DC heaters

Float the power tubes on 12VDC.

Repair a vented original Daly capacitor by recapping the entire amplifier. Repair and replace the damaged and corroded control switches due to the vented Daly capacitor. Clean the chassis.

New preamp tube sockets to replace the worn and damaged originals.

Replace all damaged or compromised wiring. Whoever did the previous work was a hack and the soldering iron compromised the shielding to a lot of wires.

Full retube.

Laminate the ID tag and reinstall to the chassis to prevent further damage from age.

Replace the headshell with a purple headshell. Use the original hardware.

Replace the faceplate with a period correct 4 hole input faceplate.

After all is said and done, I hope to have the best of both worlds. A Marshall that is true to its tone yet complete with all of the functions to make it gig friendly in our modern world while also being reliable with the decreased load on the PT.

Cheers as I walk you along this journey and I hope you enjoy my build.

The original post:

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This thing is a mess. Just ordered new fuse holders because some dip decided it would be a good idea to use rubber cement on the original ones because they couldn't find the correct nut for the inside.

Cord was incorrectly spliced and the shielding had broken where the strain relief existed which is a hazard.

Caps were leaking and beyond need for replacement

So far I've got the caps on hand and have replaced the indicator light which was broken and superglued by the same dumbass. It too was also incorrectly spliced.

I've pulled so much electrical tape and just junk wiring out of this amp that I had to order more wire on hand.

Can't wait until I can get all of these remedial repairs completed and actually start doing fun mods.

IMG_5616 - Copy.JPG
 
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That’s where it gets super cool. The amp was modded by someone who knew what they were doing - proper grounding techniques and shielded wiring is there. Cap values aren’t stock. The important guts of this thing makes it a screaming Marshall. When I sound tested it at a local guitar store, it literally drew a crowd and had people in awe. I get asked about this thing to this day from the employees.

My gameplan for further mods are a foot switchable metroloop, new impedance switch, resonance control, and another foot switchable master, pre gain, and presence to what’s in it now. It will also be getting an individual tube bias control circuit I designed, and it will be getting dc constant voltage regulated heaters on the preamp tubes. Lots of good stuff!
 
Cool! What impedance selector are you using? My '76 has one like that but I'm not a fan of those types.
 
I just decided to go with the newer style impedance selector since it’s genuine Marshall and easier to replace/install in the current configuration. It’s always good to get rid of this style of selector because if the plug ever makes poor or no contact while the amplifier is on and out of standby, then there’s increased risk of open circuit on the secondaries of the OT which can cause OT failure.
 
They did, good eye.

This is a 74 chassis with original 74 iron with a late 70’s JMP 2203 master volume board. The amplifier has a few other modifications too like preamp out and power amp in. I believe it was an amplifier that was started in the factory but not completed until much later for whatever reason.

I’ve been working to design a new faceplate and also started reverse engineering the circuit to add in the extra features in schematic form as to what I’d like to have :)
 
glpg80":3bdov55g said:
They did, good eye.

This is a 74 chassis with original 74 iron with a late 70’s JMP 2203 master volume board. The amplifier has a few other modifications too like preamp out and power amp in. I believe it was an amplifier that was started in the factory but not completed until much later for whatever reason.

I’ve been working to design a new faceplate and also started reverse engineering the circuit to add in the extra features in schematic form as to what I’d like to have :)
Replacing the Marshall board with a Reproduction Turret Board would be cool.

http://www.kurtsequipment.com/reproboardsgallery/
 
I've heard mixed reviews on Mustards. They are typ held in high regard but there are some builders (I can't remember who) that aren't crazy for them. Not that well versed in all the different ones, I've seen the square red ones and I thought people liked those as well. Not familiar with the little green ones.
 
Yes exactly this board has square red wima capacitors which are also equally desirable.

I like the way the amplifier sounds and have no desire to change it to a reproduction turret board at the moment as I play more aggressive ozzy style songs - this amp does that in spades.

Thanks for sharing the link for a repo board though! Might come in handy for a future project :)
 
New custom faceplate arrived!

Working on the bias schematic and have decided to change the mod list a bit:

Dedicated line out
Second master (footswitchable)
Hi/lo input (footswitchable)
Global resonance
Metroloop 4v1 (footswitchable)
Dual bias control

I plan to use one of the unused front jacks for the dedicated preamp-out. Resonance and presence will go in the back of the amp.
 

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So the old pre out/power in were just passive, and you'll be replacing that with a Metro loop and also have a Line Out right off of the OT? Just curious but if you have the Metro loop, why the line out?

Is that the new panel in the pic? Stupid question and I should know this but some 4-holers were later produced as MV models with two inputs plugged. I assume they said Master where Volume II would be on a plexi. Is that true and can you get those?
 
SpiderWars":33bx5tsu said:
So the old pre out/power in were just passive, and you'll be replacing that with a Metro loop and also have a Line Out right off of the OT? Just curious but if you have the Metro loop, why the line out?

Is that the new panel in the pic? Stupid question and I should know this but some 4-holers were later produced as MV models with two inputs plugged. I assume they said Master where Volume II would be on a plexi. Is that true and can you get those?


The old preamp out and power amp in were a poor mans loop after the master volume. I plan to keep the preamp out after the master volumes but before the effects loop for a dry signal to use for future WDW if desired.

There was a line out off of the OT on this amp as well with level control, but that will be removed in favor of a load box as the line out only worked with a cabinet inserted.

Yes new panel! The knobs are labeled volume 1 and volume 2 making it correct to have 4 holes in the front and not 2 holes. The faceplate that came with the amp fell right off and was not original to the amp. I saw marring on the chassis where it was once a 4 hole converted to a 2 hole, so I made a new custom faceplate that’s period correct to turn it back into a 4 hole.

Historically there were JCM 800’s that were 1959 circuit based non-master volume to which this JMP is the original father of. If you cascade mod that JCM800 platform, you basically have my 74 amp.
 
Totally odd how the indicator lamp is on the far left, never seen that before.

Yeah this is a 74 chassis first and foremost. If you look at other 74 Marshall’s, they still say volume 1 and volume 2
 
Another thing I forgot to mention was my intent to convert the preamp to 12V DC heaters and float 12V DC on the center tap of the power tube heaters off of the stock PT to help combat noise a bit.

Because the 12V switching supply can have a very high in rush current, a dedicated high current relay has to be used to turn on/off the 12V DC switching supply which can be actuated by the original power switch. This allows you to keep the stock switch to isolate the mains safely when in the off position, and doesn’t draw any additional current that could damage the oem power switch when in the on position. It also allows you to keep the stock switch, as the relay will draw mAs of current which is insignificant.
 
Update time. I decided I didn't want to drill a hole for a dedicated footswitch and decided to nix the line out in favor of using the two un-used inputs on the front for dedicated footswitch controls.

I do have to drill holes for the bias potentiometers and the boards to be mounted to the chassis.

So the front will become HI input, LO input, Footswitch 1, Footswitch 2

The rear will have the global resonance and the 2nd master control in the holes that were previously drilled by previous owners, along with the metroloop I/O.

I've mainly been in limbo waiting on parts to arrive (Proto boards, standoffs, connectors, resistors, caps, diodes, wire, etc). I finished designing the dual bias circuit and have parts on the way for that as well.

I also finished the power control circuit and placed an order tonight. This allows the stock power switch to turn on a relay, which will turn on the preamp and phase splitter DC heaters as well as the necessary DC circuit for all controls running off of 12V DC.I initially ordered a smaller relay for controlling the module but had to order another larger relay tonight. I decided if there was an error in the relay of any kind, or the mains fuse were to blow for any reason, that I wanted the AC/DC converter to turn off safely by shutting down the master relay.

The power tubes will get their heater filaments floated but still be on AC since I am keeping the 6550s which pull 1.5 amps each. They aren't made to be run on 12V at a lower current like the 12AX7/ECC83 varieties. I don't plan to fan cool my AC/DC module and need the headroom to control everything reliably within the enclosed chassis.

Once I have parts in and inventory completed, I'll hopefully have more pictures to upload.
 
The few amps/schematics I've seen that use DC for the heaters typ only use it for V1 and sometimes V2 (I've never done it, I've only lifted the CT). Is there a specific reason for DC heaters for the PI or is it just a matter of; "12vdc is now available, might as well use it"? Or maybe current-handling issues if running more than 1 or 2 tubes? I noticed Ceriatone uses 7812 regulators for V1/V2 but there appears to a 14v tap on the PT.
 
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