Mojotone British 800 Build

[Chris-in-LA]

I’ve got this Mojotone kit on order and have been comparing different schematics and wiring diagrams ever since. I did a search of the forum and couldn’t find any specific mentions for this amp kit. I have noticed a few differences between kits but nothing major. One larger exception i noticed is that Mojotone only uses two diodes as opposed to four in the rectifier. I guess what I’m really asking is if any forum members have built this kit and is there anything to look out for. I’m going all stock initially but ordered the kit without the cabinet or tubes to keep the price down. I plan to put it in one of my other JCM cabinets for testing and want to use it as a backup for my 2204 for rehearsals and gigs.

 

[charveldan]

I was interested in a kit by Tube Depot til i saw the price.

For what they wanted i could have a real old Marshall.

The Metro kits were the bomb but they dont have those anymore.

 

[paul-e-mann]

I’ve got this Mojotone kit on order and have been comparing different schematics and wiring diagrams ever since. I did a search of the forum and couldn’t find any specific mentions for this amp kit. I have noticed a few differences between kits but nothing major. One larger exception i noticed is that Mojotone only uses two diodes as opposed to four in the rectifier. I guess what I’m really asking is if any forum members have built this kit and is there anything to look out for. I’m going all stock initially but ordered the kit without the cabinet or tubes to keep the price down. I plan to put it in one of my other JCM cabinets for testing and want to use it as a backup for my 2204 for rehearsals and gigs.

For $200 more you can get the Ceriatone ready built for you, unless youre into building them yourself.

 

[ampmadscientist]

I’ve got this Mojotone kit on order and have been comparing different schematics and wiring diagrams ever since. I did a search of the forum and couldn’t find any specific mentions for this amp kit. I have noticed a few differences between kits but nothing major. One larger exception i noticed is that Mojotone only uses two diodes as opposed to four in the rectifier. I guess what I’m really asking is if any forum members have built this kit and is there anything to look out for. I’m going all stock initially but ordered the kit without the cabinet or tubes to keep the price down. I plan to put it in one of my other JCM cabinets for testing and want to use it as a backup for my 2204 for rehearsals and gigs.

The 2 diode and 4 diode do the same thing basically, depends on which power transformer you start with.
Both of those change AC into DC to run the amplifier B+.

 

[anitoli]

I was interested in a kit by Tube Depot til i saw the price.

For what they wanted i could have a real old Marshall.

The Metro kits were the bomb but they dont have those anymore.

I don't know, i am not seeing any vintage JTM45's or SLP's going for $700.

 

[charveldan]

I don't know, i am not seeing any vintage JTM45's or SLP's going for $700.

https://www.mojotone.com/kits/BritishAmpKits_x/Mojotone-British-45-Offset-Style-Amplifier-Kit

It was this one .. $1200 bucks for that i could buy a JMP or Blues Breaker easy.

 

[anitoli]

Ok. I was thinking about maybe doing this one:

https://www.tubedepot.com/products/classic-british-jtm45-plus-tube-guitar-amp-kit-head

 

[Gunner64]

Pick a layout, Buy a chassis and source your own parts.

 

[Chris-in-LA]

For $200 more you can get the Ceriatone ready built for you, unless youre into building them yourself.

Yep, I like building, makes me happy. BTW, the Ceriatone would cost $1010 plus shipping. The Mojotone kit without tubes or cabinet is $500 shipped and I get to build it.

 

[ampmadscientist]

I would use a steel chassis because I like to solder / bolt my grounds to the chassis, besides which steel being stronger.
It's going to cost more, and weigh more, machine slower, but I still like galvanized steel because of how solid it is.
Even stainless steel...a total incomprehensible pain in the ass to drill but I still like it better.

 

[Chris-in-LA]

I would use a steel chassis because I like to solder / bolt my grounds to the chassis, besides which steel being stronger.
It's going to cost more, and weigh more, machine slower, but I still like galvanized steel because of how solid it is.
Even stainless steel...a total incomprehensible pain in the ass to drill but I still like it better.

Hmmm, the Mojotone chassis is brushed aluminum. What did Marshall use back in the 80’s?

 

[ampmadscientist]

Hmmm, the Mojotone chassis is brushed aluminum. What did Marshall use back in the 80’s?

Steel of course.

Two main reasons manufacturers stopped using aluminum
1. Strength
2. Electrical continuity breakdown due to oxidization of the metal.

When 2 dissimilar metals are connected together, a layer of oxide forms between the metals.
This oxide forms a perfect insulator, which stops the flow of electricity.

For example we connect the mains ground to the aluminum chassis with a steel ring terminal.
That connection can fail, and the chassis ground can be lost.

That is why there is special rules and regulations for safe grounding of aluminum.

Connecting steel to steel does not have nearly as much problem, and I can solder it permanently.
That is why most professional tube amps are built with steel chassis (a safety issue).

(aluminum cannot be soldered) but can be welded.

Long ago, aluminum was used but created a lot of safety issues. The codes were changed to make grounding safer. (dissimilar metals are now subject to special grounding rules)

It's easier, quicker, safer, for me to use steel.
Although it is still possible to use aluminum - it's just more effort and money to get it grounded permanently.

FYI : Kit builders and vendors are ignoring these safety rules. (BUT) I am following the rules.

Because: when you build an amplifier - (when you wire any electricity)
you are responsible for other peoples safety. That is a responsibility that "should be" taken more seriously.

 

[ampmadscientist]

(Example) Steel to Steel Grounding:

The ring terminal is permanently soldered to the chassis.
Then the wire can be crimped and soldered into the ring terminal.
The Ring is also mechanically attached to the chassis with a bolt and a lock nut.
This is a method that Fender used for many decades...soldering prevents oxide from forming between the metals.

However what you can do with aluminum: is weld the rings to the chassis...(aluminum cannot be soldered)

 

[Chris-in-LA]

Steel of course.

Two main reasons manufacturers stopped using aluminum
1. Strength
2. Electrical continuity breakdown due to oxidization of the metal.

When 2 dissimilar metals are connected together, a layer of oxide forms between the metals.
This oxide forms a perfect insulator, which stops the flow of electricity.

For example we connect the mains ground to the aluminum chassis with a steel ring terminal.
That connection can fail, and the chassis ground can be lost.

That is why there is special rules and regulations for safe grounding of aluminum.

Connecting steel to steel does not have nearly as much problem, and I can solder it permanently.
That is why most professional tube amps are built with steel chassis (a safety issue).

(aluminum cannot be soldered) but can be welded.

Long ago, aluminum was used but created a lot of safety issues. The codes were changed to make grounding safer. (dissimilar metals are now subject to special grounding rules)

It's easier, quicker, safer, for me to use steel.
Although it is still possible to use aluminum - it's just more effort and money to get it grounded permanently.

FYI : Kit builders and vendors are ignoring these safety rules. (BUT) I am following the rules.

Because: when you build an amplifier - (when you wire any electricity)
you are responsible for other peoples safety. That is a responsibility that "should be" taken more seriously.

Now that I think about it, I’ve built 3 Mojotone Fender clones and every single one of them came with a steel chassis. The chassis that they are selling for the British 800 sure looks like steel in the picture. I guess I’ll find out when it gets here.

https://www.mojotone.com/amp-parts/ChassisCapPans_1/British-800-Style-50W-Chassis

 

[Gunner64]

I have the 50w mojo 800 chassis and it is in fact aluminum. The Fender style chassis they sell are steel, makes me wonder why their Marshall chassis are aluminum?

 

[ampmadscientist]

I have the 50w mojo 800 chassis and it is in fact aluminum. The Fender style chassis they sell are steel, makes me wonder why their Marshall chassis are aluminum?

Aluminum is cheaper / lighter / much easier to machine. Much cheaper to make an aluminum chassis.
But only one rub: lack of grounding terminals that are welded to the chassis.

Some people who make these will weld the ground when they build it.

 

[Chris-in-LA]

I have the 50w mojo 800 chassis and it is in fact aluminum. The Fender style chassis they sell are steel, makes me wonder why their Marshall chassis are aluminum?

How did you handle the grounding?

 

[Gunner64]

I grounded everything back to the cans. And while it's still attached to the chassis, the chassis isn't solely relied upon for ground.

 

[Chris-in-LA]

I grounded everything back to the cans. And while it's still attached to the chassis, the chassis isn't solely relied upon for ground.

I’m thinking about a ground wire on the back of the pots and running everything that I can to the bolts on the PT.

 

[Gunner64]

What I did was more or less a star grounding thing. I didnt solder to the back of the pots, but I did run a ground buss off the turret board, and that went back to the cans as well. The amp, which I followed the triode electronics 2204 layout is dead quiet at idle. No hum or weirdness...here's the amp. Mojotone chassis and faceplates.