New build: 2204 Mojotone kit

[Tortuga]

Squeezed in a bit more... this bitch gonna start up later tonight, methinks

You didn't think I'd leave well-enough alone, did you? Simply had to tuck that green wire under the board, as Jim Marshall intended



Got the loop wiring taken care of



Got all the tubes wired



Just needs the bias wired-in



Have to finish up the output jacks, and the loop still needs it's jumpers (from bypass switch to board) along with the fore-mentioned (amen, Cale!) power that'll need to be determined after I get voltages off the HT power lines, but I can put it in bypass for now and everything should be right with the gawds



After Bias wiring, I think the only thing remaining before I run through the startup procedures is to give it a good cleanout and final check of all wiring and connections. She's about to ROAR

[IAmTheKurgan]

Squeezed in a bit more... this bitch gonna start up later tonight, methinks

You didn't think I'd leave well-enough alone, did you? Simply had to tuck that green wire under the board, as Jim Marshall intended



Got the loop wiring taken care of



Got all the tubes wired



Just needs the bias wired-in



Have to finish up the output jacks, and the loop still needs it's jumpers (from bypass switch to board) along with the fore-mentioned (amen, Cale!) power that'll need to be determined after I get voltages off the HT power lines, but I can put it in bypass for now and everything should be right with the gawds



After Bias wiring, I think the only thing remaining before I run through the startup procedures is to give it a good cleanout and final check of all wiring and connections. She's about to ROAR

Dude... HELL YES!!! CAN'T WAIT!!

[Tortuga]

Last bits before power-up testing...

Loop wired in (still need B+ wiring & resistor TBD after amp is up & running)



Got the bias pot wired up



Found more errors on the Mojotone wiring diagram - presence pot was a fustercluck... made some changes...



Realized I'd missed the bright cap



Everything finalized (except for output jacks and a minor issue in the hi-sensitivity input jack, which was subsequently found and fixed)





Guess I didn't get a pic after those changes

So, cleaned up my area and got things ready for final testing / startup. Variac and current limiter are ready to go



Last pic of the night - things looking up - heaters running!

[Tortuga]

And, this is where the story turns south (at least for the night)...

Started going through the startup procedures in the MetroAmp build guide. Got as far as trying to see voltages on the power tube sockets, and didn't see what was expected. Tried a few other things, and since fuses weren't blowing thought I'd take a shot with the power tubes. Installed them. Got some MASSIVEZLY LOUD high-pitch squeals (oscillation, apparently) going on through the speakers, and shut things down. That's it - it's too fucking late for this... gonna regroup and finish this fucking thing in the morning.

Zozobra / Cale / anyone else with the know-how - I tried bypassing the loop and the oscillations persisted, so pretty sure the problem is after the Treble pot, where it feeds into the MV pot. Any thoughts?

[Zozobra]

Swap your OT primaries - your NFB loop is a PFB loop if they are the wrong way around!

[Hipster Salad]

Swap your OT primaries - your NFB loop is a PFB loop if they are the wrong way around!

This. I had a similar experience with an aftermarket OT that was incorrectly color coded. I have since learned that wiring in an OT is not a nighttime activity

[fretless]

Did you smell the magic smoke ?

[Tortuga]

No magic smoke or other weirdness, and that's the 2nd time I've seen something about OT primaries being backwards

Thanks!

[Tortuga]

I'm afraid to ask, but pretty sure I know the answer... OT Primary wires connect to pin 3 (plates) of the power tubes, correct?

Assuming that to be true, it looks like the red and white wires that I connected to my Plate measurement points need to be swapped, and that's not going to be easy






...and now, looking at the older wiring diagram that came with the amp and comparing it to the updated one, they do appear to be swapped

[Zozobra]

Yep, pin 3 is the plates. A quick n dirty way to see if you have a positive feedback loop by having the primaries wired the wrong way is to disconnect the nfb wire at the PI. It should stop squealing.

[Tortuga]

I'd like to check that... didn't catch the NFB wire until I reviewed the schematic after seeing your post. Do I have the right one? (see purple arrows)

[Hipster Salad]

I'd like to check that... didn't catch the NFB wire until I reviewed the schematic after seeing your post. Do I have the right one? (see purple arrows)

Bingo. You can also experiment with a 47k at that spot (replace the 100k), or parallel another 100k on top of that to get in the ballpark at 50k. Could also try moving it to the 4 of 8 ohm tap on the impedence selector if you want a little more aggressiveness, but it depends on how you like it as is first.

[Tortuga]

Will definitely check on those mods - thanks man

[Tortuga]

PSA - this amp is officially alive, muthafuggas!!!!!!!!!!!

[Tortuga]

Still have to hook power to the loop (already about 1/2 done with that), button things up a bit, set bias, and get it ready to mount into the headshell, but it's already sounding pretty damn good. It's super-quiet (well... noise-wise - yes, it can get loud in a hurry), and seems solid AF. Can't wait to get this thing set up and running. Will definitely be my main amp at this weekend's garage jam

Pics of the rewiring and final issue I found will be posted soon, but yeah - Zozobra and Cale (with an assist from Fretless) called it dead-on. Swapping the wire took care of all the oscillation issue I was having.

I FUCKING LOVE this forum and the folks in it



And yeah - NAD / clips will be posted as soon as I can

[Tortuga]

Looks like Mojotone just issued the very kit I'd want this one to have been (smaller chassis, JMP cabinet), but that's just wish list stuff... maybe this is my next build



And yeah - they drilled the board for the leads like I did. Good shit right there



The documentation confirms what Bill said about the discrepancies I saw in the early JMP 2204 schematic (more of a plexi preamp w/o cold clipper), and the kit apparently lets you choose which version you want to build.

Not that any of this matters (for now) - I'm happy as fuck with this amp, and plan to use the hell out of it, and don't exactly have money to spend on another kit.

[itchyfingers]

Congrats man, stoked you got it fired up without too much troubleshooting needed. Lord knows I've built a few pedals I've never heard...

[Tortuga]

Congrats man, stoked you got it fired up without too much troubleshooting needed. Lord knows I've built a few pedals I've never heard...

Let's fix 'em!!!

[Tortuga]

Requisite catch-up post-animation update poast...


So, to review, where we started yesterday morning - after the 1am SQUEEEEEEEAAAAAAALLLLIIINNNNNGGGG oscillation, which I was only able to deduce was in the PI or poweramp section by kicking in the non-functioning loop (effectively split the amp at the MV control), and thanks again to Zozobra and Cale (Hipster - and I hope I'm using your correct name) for letting me know that I was dealing with a miswired OT before I'd even gotten my coffee

The reason I wasn't too keen on swapping those wires was because I'd dressed them in pretty tight, and knew there was a possibility I'd have to lift the board (which would NOT have been fun). You can see these wires (further complicated by the bias test points) indicated by green arrows below:





And, your honor, before you convict me of fucking things up, I consulted the original wiring diagram to see if I followed the directions as prescribed:

<yep - red wire on the left, white on the right>



...and see that Mojotone must have caught this at some point and updated it as such:

[Tortuga]

So, we started pulling the fucker apart...



...and we unwired (OT/BIAS/PLATE connections indicated with green arrows - orange arrow points to the other wire that connects to the HT fuse



Luckily, able to pull the wiring out from under the board without having to lift it



And then managed to fish it back through to their CORRECT locations and get everything locked down. Decided to use buss wire (pink arrow) instead of insulated jumpers from the bias test points to socket terminal, despite the high voltage wire being just a little more accessible to wandering fingers

<note: had to extend the white wire so that it could reach the more distant socket, but Fretless helped me out a bit>

[Tortuga]

Now it was time to start the power-up procedure, once again. Decided to follow Rob Robinette's guide this time. In it, he talks about the various components and more obvious general things to look for, like capacitor polarity (for the few electrolytics in this amp). It was there I noticed that my board-mounted electrolytics were mounted with the positive to ground

...not to mention, I had totally forgotten to wire the floating ground buss that I was so proud of to actual chassis ground - and then rectified it, as shown with the orange arrow



...and then, I read that this is correct, because bias voltage is actually negative with respect to ground. So all was good with the world, except for the missing jumper across the two caps



...that I then fixed

[GuitarBilly]

Thanks for all the detailed posts man. I'm learning a lot just from following this thread.

[Tortuga]

And then, GLORIOUS (out of tune) NOISE ERUPTED!!!

[ajaxlepinski]

Bottom of board

Added wire to capacitor leads so that the cap can be replaced w/o having to pull the board up

GENIUS IDEA!!!!


Ditto to what Billy said... thanks for the detailed posts, I'm learning a lot too!!!


Clip sounds SPECTACULAR!!!!! CONGRATS!!!!
Post a clip with POWER CHORDS, please!

[Tortuga]

Thanks for all the detailed posts man. I'm learning a lot just from following this thread.

Thanks Bill - that's exactly why I do these DIY threads - happy to return the MANY favors I've received by being part of this group

Bottom of board

Added wire to capacitor leads so that the cap can be replaced w/o having to pull the board up

GENIUS IDEA!!!!


Ditto to what Billy said... thanks for the detailed posts, I'm learning a lot too!!!


Clip sounds SPECTACULAR!!!!! CONGRATS!!!!
Post a clip with POWER CHORDS, please!

Wish I could claim credit, Brian, but I'd gotten the idea from my researching build threads and the really good instruction in the Metropoulos build guide (along with others), and Rob Robinette's (and others) phenomenal sites. This kind of readily available info is why the internet is so important, IMO.

This thing does Angus in spades - yes, big chords and power chords will definitely be recorded

More clips will be done, that's for sure. Gonna do some speaker comparisons using the ones I have, and I'm talking with Mike about doing a shootout against his Jube. Also gonna get my buddy to bring his Mrak IIC+ and see what kinda Hetfield tones we can dig up