2204 Redux... rebuilding JCM > JMP

[Tortuga]

Some of you may remember my first ground-up build thread from last year: Mojotone 2204 (JCM800-style) kit.

Here's a couple pics of it at the end of the build:

(note: this one was apparently just before hooking up the bias pot)






After a few weeks of working on it, I managed to get it fired up and running for all of an hour or so, before the amp suddenly died when the mains fuse blew. Took some time to troubleshoot and found out that the power transformer was no longer functioning correctly (one of the windings had become shorted). Mojotone told me they were having trouble with covid-era transformer supplies and sent me a replacement. When that one turned out to have an open heater winding, they sent me a third, which (after some basic testing) seems to be working correctly.

While all that swapping was going on, Mojotone released another version of the 2204 kit in a JMP style kit, which I like a lot more. Decided to buy the new chassis and faceplates (which look a LOT better, imo), and enough replacement components, wiring, etc to allow me to fully rebuild the amp. Yeah, it's a lot of work, but I figure that I might as well get the final amp I want, and essentially 2 builds' worth of experience out of the whole project.

Mike (Ruboto) also hooked me up with a couple legit Marshall head cabs (dummy cabs - complete with chassis, pots, switches, jacks, etc) that one of his relatives was getting rid of. One is a JCM-800 and the other a Jimi Hendrix Plexi.

So, here's the major new parts I'll be using for the swap:



The plan is to gut the rest of the components from the first mojo kit and move everything to the new chassis. I'd like to still find an actual JMP cab (or buy / build one), but for now will use the older Plexi-style head as it's suited for the small box I'll be building and is a much better match for the older 1960AX 4x12 I have.

Shit's still gotta match, right?

[Tortuga]

This is a pic of the new JMP kit that Mojotone put out. It's very similar to the JCM kit, just a lot more compact in the smaller chassis and some minor circuit differences (bias & presence control seem to be the bigger standouts).



Started by mocking up the new chassis in the plexi cab to see if everything fits ok... I still prefer the 70s JMP cabinets, but this thing will work perfectly and looks pretty killer, if you ask me







The idea is to use this cabinet for the time being, and eventually build a plexi-type amp for my son (who is a Hendrix freak) into the original chassis that came with that, and either build or buy a JMP cabinet for myself at that time (or might even consider modding the JCM cabinet, which wouldn't be all that difficult). Anyway, this is what we're going with for now.

[Tortuga]

Out with the old...





As far as the rebuild goes, I'll be doing a few changes to my board and wiring to incorporate the older-style (1978) design. Planning to keep the JCM bias control (primarly because it uses an external 50k pot, which I like), but will be using the smaller 5K presence control. I'm also going to do the same mods I did before: external bias and B+ test points, mounting grommets for the V1 tube, and installing the Metro effects loop.

[maggotspawn]

That's going to be killer. Wish I had the skills.

[Tortuga]

Mocking things up to see how things fit in the new chassis, and figuring out where I want to place all the holes I need to drill for bias test points...

Effects loop board will mount above V1 & V2




Rear & bottom parts placement





Getting ready to drill out holes for the effects loop



Effects loop and 3 bias test (V4 & v5 + ground) points drilled



Holes for V1 mounting grommets & B+ test points completed - starting the major component installs



And here's how far I got last night. Transformers, filter caps, bias pot, tube sockets, and front control pots (all harvested from the Hendrix plexi chassis - yes, I'm stealing those) have all been installed.





Today & tomorrow's plan is to get the rest of the front controls (switches and jacks) installed and start wiring the power circuit (including the heater wirings) so that I can fully test the 2nd replacement power transformer to make sure it's going to work right this time. If all goes well, I may be cranking this thing up in the next week or two.

[GuitarBilly]

Yeah man that looks absolutely killer!!!!

[Shask]

Looks good!

I have been considering this small chassis kit as well. I wonder if it will fit in their smaller headshell. So much wasted space in that giant headshell, lol.

[ajaxlepinski]

right on, Right On, RIGHT ON!!!!

Looks awesome!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[eyeball987]

Much respect! I have trouble getting the amp out of the heads hell let alone doing any of that.

[fretless]

Looking pro as fuck !

[Riffraff]

Love it! That's going to be a monster.

[GuitarBilly]

Much respect! I have trouble getting the amp out of the headshell let alone doing any of that.

This!

[Tortuga]

Thanks, dudes... glad to be back in action

Looks good!

I have been considering this small chassis kit as well. I wonder if it will fit in their smaller headshell. So much wasted space in that giant headshell, lol.

Honestly, I think the new Brit 50 is a much better kit than the Brit 800, overall. In addition to the minor circuit changes between JMP & JCM, there's a lot of refinements that I really like about it: the panels are much closer to the Marshall gold color (don't know why they use that orangish gold on all of their other british kits ), they eliminated the voltage selector (although the wires are still present on the PT - just use the one you need - and if you're really going on world tour, there's plenty of space to drill & install a selector switch), and the smaller chassis is going to make the transformer wiring a lot cleaner.

The other thing I like about the new circuit board is that there are holes drilled that let you route all the wires up from underneath the board that connect to the turrets, rather than just having them lay on top of the board on the 800 version. I noticed that when I was building my first board last year, and decided to drill my own, as you can see here



Here's the Mojotone pics, that show the differences in boards and panel colors

JMP



JCM



All that being said, I think I'm in a really fortunate position in this double-build - even though it's costing more in money and time, I'm getting two builds' worth of experience out of it, not to mention finding out how great Mojotone customer service is (they swapped the PT on their dime TWO times, even though I bought the original kit second-hand and it was WAY out of warranty by the time I initially contacted them). I'm also winding up with the chassis style I really wanted to begin with, although I'm definitely seeing the luxury of all that extra space in the longer 800-style pan. But, the smaller chassis is going to be more of a challenge; I am already seeing this making things a bit more difficult to keep the wiring neat, so this could probably be seen as a more "advanced" kit.





...anywho - back to overthinkus enormous

[Tortuga]

Mostly small detail work from yesterday... pointing out all my build flaws so that anyone interested can learn from my mistakes

Bias pot - I located the thing in the best place I could determine from the inside of the chassis - made sure it would properly clear the circuit board and mounting holes / screw hardware for the transformers, and used my best judgement (from the wrong side) to place it, said a small prayer to the mojo gawds, and drilled the fucker. It was only when I got the transformers in place that I had my "oh shit" moment, thinking I may have made it impossible to adjust around all the transformer hardware on the topside...

...I'd say I guessed pretty damn good



Then I found this mistake... I think those nuts work better when the teeth are on the bottom .... fix'd



The filter caps didn't have anything anywhere near them on the big ol' battleship chassis, so orientation wasn't an issue. But shit matters when it's on this chassis

...as you can see, the capture lug sticks out just beyond the side of the chassis, which wouldn't normally matter when installed in the head shell (the mounting flange extends a bit further), but it makes a clearance problem with my chassis stand, so it had to be flipped.



...lottamobetta



Bias test points installed



Chassis stand - had an issue with the new chassis, because they flipped the mounting flange from inside to outside, as indicated with the red arrow here



...that, along with the tall transformers (which turned out to be taller than the sides of my chassis stand), made hanging it by the flanges impossible



...and the components being so close to the sides made placing the chassis directly on the cradle impossible





...so, like any good diy maker, I got out a couple chunks of toobafour & some lag screws and fixeded it. Worked a treat





Decided to steal the bona-fide Marshall power & standby switches from the 800 dummy cab (Mojotone power switch shown for comparison). I guess Marshall figured out how to internalize the wiring for the indicator light, and it won't require me to wire it like Mojo's does.





The Marshall pots were another story, though. I like the knurled fit better (setscrews suck), but the hole diameter is different between Mojo & Marshall (the Mojos are bigger), meaning I'd have to fiddle with the placement of the Marshall pots in the larger holes on the Mojo chassis. Fuck that noise - I'll going back to the originals.



I think that's it for updates this morning. Here she is - almost ready for initial wiring later today.

[Tortuga]

Had some help yesterday - my sons are out for the weekend



Swapped out the pots for me, and got the old bits of wire & components all desoldered for reinstallation.



Decided to do my grounding and bias using some ideas from FourT6and2's incredible work - specifically this one for the grounds:



and this one for the bias test points:



both of which are in this thread: https://www.guitargearforum.com/forum/v ... as#p282064

Here's a picture of the ground bus I put on my prior build. The idea was that having it on the board (indicated by red arrows on the following picture) gave me a common place to tie the pots to, without creating potential ground loop problems by running a buss wire across the backs of all the control pots.



It worked fine, but because I hard-wired the pots that needed grounding, the buss wire would often get bent whenever I needed to lift or change things inside the chassis. Not a huge problem, but I like the way FourT6and2 did his, by running the buss wire across the backs of the pots, and tie the board to that using wire leads.

I decided to not run the wire through the eyelets of each pot that actually needs the ground connection, as he did. While this makes a good, strong mechanical connection, it will create a problem later if any of those pots (expecially the middle one) need to be changed out. Because of this, I'm going to just solder the lugs to the buss wire via direct contact (not through the eyelet), and expect the solder to hold the connection. Since there's three of them all in a row, I feel pretty certain that they'll help each other and the buss wire to bear the effect of any vibration trying to shake things apart later. The end of the wire will be connected directly to the chassis, and will further help keep the pot connections from shaking apart.

Got this far with it yesterday. Need to get a lug to tie that ground to the chassis, and that'll be the main grounding for all connections on the board. Seems a clean way to do it.

[Tortuga]

Quick one for this afternoon... my son helped me ground the buss and give it some presence

[GuitarBilly]

It should be fine without running the buss through the eyelets. Although running it through the eyelets does not cause a problem later. If you need to replace a pot you can use a small cutter to snip the eyelet open to remove the pot. I've replaced pots with these busses before and it's not a big deal. One extra snip.

But I don't see an issue with doing it this way either. The solder should hold it just fine.

[Tortuga]

That’s a good point. Problably going to leave as-is, but would definitely consider changing if I wind up having to redo anything.

[Ostinato Rubato]

That’s awesome. My favorite part is the child labor.

[Tortuga]

You forgot the part about the labor being “free” and “forced”

[greatmutah]

Oh I like this. I like this a lot.

[JiveTurkey]

This looks great! Skills, dude. Skills \m/

[Tortuga]

Don’t want to jinx it, but hoping to have this done in time for BOTR. 2023, that is

[GuitarBilly]

Several months out and this is his "reunion tour" post:

That’s awesome. My favorite part is the child labor.

[Tortuga]