Mojotone JCM800 Build

[Rotorcraft230]

My son is gigging this weekend with the 800, all finished up. We haven’t done ordered a cab for it so for now I’m putting it in the white Jubilee cab. Made a new faceplate for it. Still have the grill cloth to put in but waiting for the paint to dry more.

The amp is definitely an 800 notes are punchy and full and and plenty of gain it’s very bright, still working in that brightness, want to calm it some, remove the bright cap on the preamp volume but nothing else yet.

 

[paul-e-mann]

My son is gigging this weekend with the 800, all finished up. We haven’t done ordered a cab for it so for now I’m putting it in the white Jubilee cab. Made a new faceplate for it. Still have the grill cloth to put in but waiting for the paint to dry more.

The amp is definitely an 800 notes are punchy and full and and plenty of gain it’s very bright, still working in that brightness, want to calm it some, remove the bright cap on the preamp volume but nothing else yet.View attachment 118427

The higher you dial the pre-amp the darker the tone gets. Max it out and see what that does and dial back if its too much, toss a clean boost out front to push the gain further. I typically keep mine on 5 all knobs and volume to taste.

 

[Jimi Biscuits]

I think that’s a downgrade. Alpha Taiwan have a better taper (CTS don’t leave the CCW track end pad until the wiper reaches nearly 3 on a 0-10 scale) and a smoother action.
CTS track wafer

Regarding the heater wiring, I acknowledge that the normal Marshall arrangement uses sweeping loops to reach the far (4&5) terminal, but it seems ripe for improvement to me. Wiring across the socket eliminates the loop, and if the socket is rotated 180°, the gap between pins 1&9 can be used.

Agree with both of these points. I find Alpha pots superior in their taper and wiring across the sockets per Merlin Blencoe the Valve Wizard a cleaner solution.

 

[Justin Whitstine]

Yeah I would definitely keep the low gain input, as is. That way you can use pedals like a big muff, etc into that.

 

[sri]

I was a teenager in the 80s and the most iconic amp to me was the JCM800 2203/2204. Well here it is 3 decades later I’m building one for my son. Follow along and check out the build. I’m will post lots of pics. I’ve already upgraded to CTS from Alpha, Cliff jacks, and laying out my own circuit board.View attachment 114761View attachment 114762View attachment 114763View attachment 114764View attachment 114765

You are in for a treat during and after the build. One thing to offer as advice: Marshall and almost everyone else do horribly compromised grounding schemes that invite hum and radio interference in '800 builds. Most playets think it inevitable a meduim gain amp will buzz but not so. If you dont know about perfect grounding and cant be bothered paying for Kevin O'Connors genius reference books or studying Valvewizards free texts online, I would point you at Michael Mortimer's website (Modulus Amplification) as his layout and grounding scheme layout for a JCM800 follows most of the best practice and theory there is rather than slavishly repeating Marshall's mistakes. Oh, and also look at heater ground referencing off Valvewizard's site to get things even quieter.

 

[Rotorcraft230]

You are in for a treat during and after the build. One thing to offer as advice: Marshall and almost everyone else do horribly compromised grounding schemes that invite hum and radio interference in '800 builds. Most playets think it inevitable a meduim gain amp will buzz but not so. If you dont know about perfect grounding and cant be bothered paying for Kevin O'Connors genius reference books or studying Valvewizards free texts online, I would point you at Michael Mortimer's website (Modulus Amplification) as his layout and grounding scheme layout for a JCM800 follows most of the best practice and theory there is rather than slavishly repeating Marshall's mistakes. Oh, and also look at heater ground referencing off Valvewizard's site to get things even quieter.

I will definitely go visit those sites and see what they have done on the grounding. I have a modified ground already. I never put the ground buss to the back of the pots, run the heater CT to the input end. I also run the wire in pin 9 on the pream tubes up and over the tube to get it as far away from the grid wires.(there is a pic of that on this thread). The amp has no hum at full volume amp and guitar, it does hiss some.

Thanks for the info, looking forward to hearing this weekend.

 

[AndyD]

I think it has already been stated but reduce the bright cap on the gain pot rather than remove completely. The guitar won’t clean up on the guitars volume if it is omitted completely. Hope the gig goes well. In a band mix, the amp won’t seem as bright as you think it is!

 

[Rotorcraft230]

I think it has already been stated but reduce the bright cap on the gain pot rather than remove completely. The guitar won’t clean up on the guitars volume if it is omitted completely. Hope the gig goes well. In a band mix, the amp won’t seem as bright as you think it is!

Didn’t know about the clean up with the guitars volume. I have a good selection of silver mica caps from 100pf -1000pf. I’ll try a 250pf and see how that does.

 

[Jackco]

Ok, read all this thread this morning. I'm going to buy a kit !!!!
Thank you all for posting
I hope this has inspired others too.

 

[Rotorcraft230]

It’s a pretty easy kit, no instructions just a layout. Also check on lead time, I had to wait for a chassis for 6 weeks, had them ship what they had, still was missing the OT that was another 6 weeks. The kit is very nice, good quality components and it sounds awesome..

Ok, read all this thread this morning. I'm going to buy a kit !!!!
Thank you all for posting
I hope this has inspired others too.

 

[Rotorcraft230]

Ok, read all this thread this morning. I'm going to buy a kit !!!!
Thank you all for posting
I hope this has inspired others too.

I just checked all the big parts for the kit are in stock, doesn’t look like there will be a wait like I had to.

 

[jchrisf]

Ok, read all this thread this morning. I'm going to buy a kit !!!!
Thank you all for posting
I hope this has inspired others too.

I think if I were to do one of these kits I would probably pick the modulus kit. Supposedly his transformers are identical to the old Marshalls and they are in stock

MOD 800 2204 Small Box kit - 50w Output - Master Volume British Amp Kit

MOD 800 2204 Small Box kit - Master Volume Lead British Amp Kit Power Output 50w 120v/240v - 50/60hz Modulus Kit Suggested Layout DOWNLOAD HERE One

modulusamplification.com

 

[jagrenet]

I was a teenager in the 80s and the most iconic amp to me was the JCM800 2203/2204. Well here it is 3 decades later I’m building one for my son. Follow along and check out the build. I’m will post lots of pics. I’ve already upgraded to CTS from Alpha, Cliff jacks, and laying out my own circuit board.View attachment 114761View attachment 114762View attachment 114763View attachment 114764View attachment 114765

Looks great - LOVE the organization !!! - Noob question - What guage/type of wire is typically used for these builds. Considering building my own. I have done quite a bit of modding up to this point, .. just unsure of the exact specs for wire requirements.

 

[AndyD]

Didn’t know about the clean up with the guitars volume. I have a good selection of silver mica caps from 100pf -1000pf. I’ll try a 250pf and see how that does.

Even a 100pf would work. Btw, I have to say once again how impressed I am with your build, and what a wonderful project to do with/for your son. What a lucky guy he is!

 

[Rotorcraft230]

Even a 100pf would work. Btw, I have to say once again how impressed I am with your build, and what a wonderful project to do with/for your son. What a lucky guy he is!

Thanks for the compliment, I’ve built several for him. He has built a Dumble clone from Ceriatone and did a fine job. I have been working on him how to troubleshoot, which can be a tougher to sort through. I want him to be able to work on ALL his gear. He even brings a tool box and a few spare parts when he gigs.

 

[john l]

When i hear talk about removing the bright cap on an 800 i get grumpy lol. Those are really essential to that tone imo. I prefer these amps fead stock however when i have addressed this for other guys ive had luck with some tasteful plate bypassing along with upping the fizz cap to 100pf? The trick is to just take a little of the top, not enough to change their character

 

[Rotorcraft230]

When i hear talk about removing the bright cap on an 800 i get grumpy lol. Those are really essential to that tone imo. I prefer these amps fead stock however when i have addressed this for other guys ive had luck with some tasteful plate bypassing along with upping the fizz cap to 100pf? The trick is to just take a little of the top, not enough to change their character

Your talking about the cap(fizz) on pin 1 and 3. Marshall shows a 47pf, mine came with a 10pf. Maybe I’ll change that?

 

[john l]

Your talking about the cap(fizz) on pin 1 and 3. Marshall shows a 47pf, mine came with a 10pf. Maybe I’ll change that?

Yep thats the one. Give it a shot. I’ve gone as high as 250pf for people who dont dig the high end with these amps. When it comes to snubbing try to resist urge nip it all in one stage if you want to retain as much of an 800s character as possible. I find it beneficial to use small values on each stage’s plate so that they accumulatively get me where i want to go rather than drastic things on any one stage in particular.

There are lots of ways people approach this. Some guys like to just slam 1000-4700pf across V1B’s 100k plate. Friedman likes to darken things up by putting a 500pf across the cathode follower.

A good friend of mine likes to make a little triange of 250pfs on the the phase inverter plates and fizz cap.

I personally love 800s bone stock. All that high end works great with a meaty Les Paul.

 

[NewReligion]

Outstanding read along. Your work looked clean & solid. Congratulations on the build.

I use many items from Mojotone to build amps. I use their Heyboer (Dagnall copy) 100 watt transformers & chassis then Early 1970’s style head box. Custom Plexi plate Black with Gold lettering. ARS filter caps, JJ tubes when I can. Sometimes use Mojotone turret boards or I have about 30 ST-ONE PCB boards left I had made in Green.

I have two now I am about to start.

David

 

[Rotorcraft230]

Outstanding read along. Your work looked clean & solid. Congratulations on the build.

I use many items from Mojotone to build amps. I use their Heyboer (Dagnall copy) 100 watt transformers & chassis then Early 1970’s style head box. Custom Plexi plate Black with Gold lettering. ARS filter caps, JJ tubes when I can. Sometimes use Mojotone turret boards or I have about 30 ST-ONE PCB boards left I had made in Green.

I have two now I am about to start.

David

I’ve built 2 amps from Mojotone, the 800 and the Studio one. The studio one is his daily practice amp and it’s a 2203/2204 circuit. Even at one watt it sounds like an 800 just not very loud. It’s single ended, sounds great and has 1000s of hours on it, pretty incredible for an amp that cost less than 400 bucks.

Here is the Studio One