Advice: 1st Amp Build - Mojotone 1978 Brit Style 50w JMP

[jchrisf]

I've been wanting to build a Marshall JMP/JCM for a few years now and have been doing a lot of research. A couple of weeks ago I purchased Headfirst's ST1 board but with the sale last week on the Mojotone amps I got the British Style 50w JMP kit too. I'm going to start with this Mojotone kit.

I plan on building this exactly like the wiring diagram and then later, after I get some experience playing it, I plan on modding it. It seems from my research, I may need to add a 1ohm resistor between the power tubes lug 8 and the ground lugs next to them to help with biasing it? I've been reading through the directions of an old StewMac JCM build guide and Metropolous' 50 watt plexi instructions. StewMac doesn't mention the 1 ohm resistor but Metropolous does.

Here's the wiring diagram and schematic for this Mojotone kit:
https://www.mojotone.com/Amp_Kits/British/British_50W_1978_2204_WD.pdf
https://www.mojotone.com/Amp_Kits/British/British_50W_1978_2204_SCH.pdf

 

[Techdeth]

Why is 1978 the year they use . What makes it that year ? I’m interested because I have a 1978 JMP

 

[Chubbs]

Yup that 1ohm resistor(s) allows you to bias the amp without a special tool, you can substitute them for just a straight piece of wire to ground if you don’t want to use them.

You do you, but I find if your gonna tweak something point to point boards make it so much easier. It really sucks when you load up and install a pretty PCB board only to find out you have to hack parts out to experiment. Also, point to point boards are easier I find to learn on as everything is laid out linearly, whereas pcbs are laid out to be quiet or fit a given space with no real order.

 

[jchrisf]

Yup that 1ohm resistor(s) allows you to bias the amp without a special tool, you can substitute them for just a straight piece of wire to ground if you don’t want to use them.

You do you, but I find if your gonna tweak something point to point boards make it so much easier. It really sucks when you load up and install a pretty PCB board only to find out you have to hack parts out to experiment. Also, point to point boards are easier I find to learn on as everything is laid out linearly, whereas pcbs are laid out to be quiet or fit a given space with no real order.

yeah, that's why I am starting with the point to point and am going to use it to mod.. though I could easily do the JEL mods Jason demonstrated without messing up the PCB board.

So if I don't install the resistor, the diagram has a ground from this lug (see below). Will I still be able to bias it without a special tool?

 

[jchrisf]

Why is 1978 the year they use . What makes it that year ? I’m interested because I have a 1978 JMP

I hope it is because it is the best sounding year.. but you can also build this kit like the 1976 amp.

 

[Techdeth]

I hope it is because it is the best sounding year.. but you can also build this kit like the 1976 amp.

Very interesting. I know ac/dc used 1978 JMPS on POWERAGE and that’s best rock tone iv heard

 

[Chubbs]

yeah, that's why I am starting with the point to point and am going to use it to mod.. though I could easily do the JEL mods Jason demonstrated without messing up the PCB board.

So if I don't install the resistor, the diagram has a ground from this lug (see below). Will I still be able to bias it without a special tool?
View attachment 270123

So with the above picture you will have to buy a tool to bias the powertubes.
The one ohm resistor allows you to measure voltage across it much like a biasing tool allows you to do, except you don’t have to break the circuit with the one ohm resistors.

 

[jchrisf]

So with the above picture you will have to buy a tool to bias the powertubes.
The one ohm resistor allows you to measure voltage across it much like a biasing tool allows you to do, except you don’t have to break the circuit with the one ohm resistors.

Thanks.. looks like I'm going to have to order a couple of these resistors.

 

[TotallyRadGuitars]

Why is 1978 the year they use . What makes it that year ? I’m interested because I have a 1978 JMP

I would love to know myself. I know there are a few variations on the circuit that aren't shown on any official schematic, but I just did a video about the 2203 circuit and I used these mojotone schematics to talk about how their 1976 schematic only has two gain stages, unless I'm reading it wrong. The "official" JMP 2203 1976 schematic has the standard 3-stage thing just like a JCM800 2203.

So I suppose only someone at Mojotone could answer exactly. If I had to guess it's probably based on a real amp (or amps) they had for reference while making the kit but I'm totally speculating and they would have to clarify.

I was going to buy one of these kits myself actually. I swear I checked to see if they were on sale multiple times over thanksgiving week and weekend and never saw a sale. Oops.

 

[Techdeth]

I would love to know myself. I know there are a few variations on the circuit that aren't shown on any official schematic, but I just did a video about the 2203 circuit and I used these mojotone schematics to talk about how their 1976 schematic only has two gain stages, unless I'm reading it wrong. The "official" JMP 2203 1976 schematic has the standard 3-stage thing just like a JCM800 2203.

So I suppose only someone at Mojotone could answer exactly. If I had to guess it's probably based on a real amp (or amps) they had for reference while making the kit but I'm totally speculating and they would have to clarify.

I was going to buy one of these kits myself actually. I swear I checked to see if they were on sale multiple times over thanksgiving week and weekend and never saw a sale. Oops.

My 78 rips head of it’s crazy

 

[glpg80]

78 has nothing to do with the circuit and everything to do with the power transformer. Dagnalls are in the early 70’s and there’s also Drakes. Marshall was experimenting with all kinds of plate voltages. 78 and 79 they got it right.

They also got it right in 67/68/69 with dagnalls.

I custom ordered from Mercury magnetics a prototype mix of a 67 and 69 transformer for my Marshall. It will be the basis for what I use for other mods if it works out.

 

[jchrisf]

I would love to know myself. I know there are a few variations on the circuit that aren't shown on any official schematic, but I just did a video about the 2203 circuit and I used these mojotone schematics to talk about how their 1976 schematic only has two gain stages, unless I'm reading it wrong. The "official" JMP 2203 1976 schematic has the standard 3-stage thing just like a JCM800 2203.

So I suppose only someone at Mojotone could answer exactly. If I had to guess it's probably based on a real amp (or amps) they had for reference while making the kit but I'm totally speculating and they would have to clarify.

I was going to buy one of these kits myself actually. I swear I checked to see if they were on sale multiple times over thanksgiving week and weekend and never saw a sale. Oops.

I would like to know what the difference is between the Mojotone 800 and Brit 50 JMP. I can see the differences on the schematics and wiring diagrams but I'm not sure what they are doing. There is the obvious.. the 800 has the mains selector... but not sure of the other differences.

BTW, Mojotone had the sale on Cyber Monday.. they sent me an email saying everything was 15% off. No coupon needed, you just had to put the items in your cart and it would reflect the sale price.

 

[jchrisf]

78 has nothing to do with the circuit and everything to do with the power transformer. Dagnalls are in the early 70’s and there’s also Drakes. Marshall was experimenting with all kinds of plate voltages. 78 and 79 they got it right.

They also got it right in 67/68/69 with dagnalls.

I custom ordered from Mercury magnetics a prototype mix of a 67 and 69 transformer for my Marshall. It will be the basis for what I use for other mods if it works out.

What was the right plate voltages in 78'?

 

[glpg80]

What was the right plate voltages in 78'?

50W or 100W?

 

[jchrisf]

50W or 100W?

50w

 

[glpg80]

Bone stock on the lower side between 400VDC and 460VDC depending on primary tap and house voltage. Some amps are even as low as 350VDC. Contrary to belief people think higher plate voltage more gain but that’s incorrect. Higher plate voltage is more headroom but less preamp gain. That’s why people loved the 50W more. They’re all secret Van Halen lovers taking a page out of Eddie’s book.

 

[jchrisf]

Bone stock on the lower side between 400VDC and 460VDC depending on primary tap and house voltage. Some amps are even as low as 350VDC. Contrary to belief people think higher plate voltage more gain but that’s incorrect. Higher plate voltage is more headroom but less preamp gain. That’s why people loved the 50W more. They’re all secret Van Halen lovers taking a page out of Eddie’s book.

This one is supposed to have 440 so we'll see where it is when I finish it..

 

[gtrwun]

78 has nothing to do with the circuit and everything to do with the power transformer. Dagnalls are in the early 70’s and there’s also Drakes. Marshall was experimenting with all kinds of plate voltages. 78 and 79 they got it right.

They also got it right in 67/68/69 with dagnalls.

I custom ordered from Mercury magnetics a prototype mix of a 67 and 69 transformer for my Marshall. It will be the basis for what I use for other mods if it works out.

Everything I’ve heard and read says the 76 50 watt JMP did not cascade the preamp like the 100 watt. One less gain stage leaving you with a VERY clean sounding amp. I made the mistake of buying a 76 2204 and it was a complete dog. Marshall changed this in 77 onward.

 

[glpg80]

Everything I’ve heard and read says the 76 50 watt JMP did not cascade the preamp like the 100 watt. One less gain stage leaving you with a VERY clean sounding amp. I made the mistake of buying a 76 2204 and it was a complete dog. Marshall changed this in 77 onward.

There’s a few transitional 1959HW chassis in JCM800 boxes. They look like jcm800 but with 4 holes and are actually 1959HW. I’ve actually plaid one jumpered and boosted with an SD-1 and it was straight Van Halen. I went back to buy it the next day and it was sold. To this day I still kick my ass for not buying it on the spot.

 

[BesaMoogie]

So according to MojoTone, the 100k NFB resistor is wired to the 8 ohm output tap of the OT. I was actually asking this question recently here. However, I did read on other places, that their NFB resistor was wired to the 4 ohm tap in their JCM800 RI. Maybe that`s a difference as well.

You could try different cap brands as coupling caps. I just finished a TAD Kit, that is basically a Mojotone Kit, and I`m planing to experiment with different caps. While doing so, I will as well pay attention to the "right" foil orientation when installing the caps. Haven`t done this in the first place.