2204 Redux... rebuilding JCM > JMP

[greatmutah]

FFS just get a regular Marshall headshell

When I lived in CA the local GGF chapter was in order.

I move out of there, one turns into Joe Rogan Jr, another wants purple Marshalls...

Goddammit!

[Tortuga]

FFS just get a regular Marshall headshell

When I lived in CA the local GGF chapter was in order.

I move out of there, one turns into Joe Rogan Jr, another wants purple Marshalls...

Goddammit!

hahahahaha - totally missed this one

Dude... you know we have differing priorities... this is a learning project for me - not trying to build something to tour with.

...not yet, at least




...and, besides - those two moved out of state, too

[Holy-diver]

Damn looking great steve! cant wait to hear it then you can build me one

[Tortuga]

Damn looking great steve! cant wait to hear it then you can build me one

Thanks Chris! Good to see you, brother!

Still gotta sort some issues out on this one before it can be heard, but hoping things will improve as I work on it throughout this weekend

You still in Murietta/Temecula area? Gotta have you out to the new digs in Long Beach sometime

[Tortuga]

Fucking bias is driving me batshit… been getting input from some builders on my implementation (external bias pot, which mojotone used on the original kit and has since moved to an internal trim pot), and am now redesigning/rebuilding. Not sure if that’s a contributor to the problems I’m still having (works ok for a bit, then not), and am starting to wonder if I’ve managed to fuck up tubes. Gonna try these in my DSL head today to make sure they all work, and then continue the rework over the weekend.

This. Will. Improve.

Gatdammit

[Holy-diver]

Damn looking great steve! cant wait to hear it then you can build me one

Thanks Chris! Good to see you, brother!

Still gotta sort some issues out on this one before it can be heard, but hoping things will improve as I work on it throughout this weekend

You still in Murietta/Temecula area? Gotta have you out to the new digs in Long Beach sometime

Yeah I still live over here lol and you know I’m down bro, any time our schedules line up :cheers:

[Tortuga]

Had access to my old DSL today, so did some tube testing from the 2204. Power tubes seem fine. But, I determined that two of the 12AX7s are definitely bad. I’m hoping they were somehow damaged in the socket I replaced, but it appears I have more inspection to do.

Meanwhile, I got a couple spare 12AX7s (thanks to a lucky donation by maggotspawn that happened to wind up in my hands), and found them to be fine in the DSL.

After inspecting/resolving any sound issues, next up will be running through the whole startup process again, paying close attention to the bias modifications I’ve done (and will be described tomorrow, hopefully), and getting the bias set properly and various voltages checked.

[Tortuga]

Wow this thread got behind

Got a bunch of updates coming today... Shit got real last night

[Tortuga]

As I was troubleshooting the low output issues with some guys on the Mojotone facebook group, I had some feedback about the "strange" bias pot wiring I seemed to be using. I told them I got the wiring from the original Mojotone schematic/wiring diagram. Bruce Egnator was particularly helpful in getting me to switch the wiring to a safer circuit that didn't rely on the wiper to keep the bias current from runaway if (or rather, when) that pot ever went bad. Took me a while to grasp exactly what he and others were getting at, but it finally clicked for me enough to work out how to make the change without having to pull the board (did NOT want to do that) to change or remove the wires I'd soldered underneath that fed the bias. Instead, I was able to reuse them in a new way, and it's near-perfect now.

Old diagrams:



Updated bias:



Different locking pot (with higher value), different load resistor (also higher value), and tied the two blue wires together so the current defaults to the coolest bias value independent of the pot wiper.

Only thing not so great about this is relying on the pot-to-chassis grounding, and I may change that to a wired ground lug later. For now, that pot is installed with a good star washer, so it'll be a LONG time with a LOT of corrosion / loosening before that's an issue.

I'm able to dial in bias in a VERY controlled manner, with proper range - the old values and wiring implementation were simply shite.

[greatmutah]

I love Bruce. That guy is so helpful. He has a wealth of knowledge and loves to share and help.

[Tortuga]

I love Bruce. That guy is so helpful. He has a wealth of knowledge and loves to share and help.

Oh yeah - that dude is awesome. Super helpful.

A lot of the guys here are, too

…more on that in a bit

[Tortuga]

So, after getting the bias sussed out, I set to work on the low output issue. I'd had the thing in an intermittent state earlier (normal volume for a few seconds, then super-low volume and almost no overdrive), which made me think I probably have all (or at least most of) the wires and components where they needed to be - if something were REALLY wrong, the thing would be blowing fuses, smoking, or otherwise NOT putting sound to the speaker.

At that point, I started focusing on making sure all the components were still in spec and checking voltages on the plates, cathodes, and other areas of the circuit. I'd been pretty concentrated on a problem I was picking up with the PI (V3) and the cathode follower (V2), and was convinced that some component must be completely out of whack. I reached out to the facebook group, Mojotone, and finally started a new thread here (viewtopic.php?f=5&t=54520) and found out that the voltages on the Mojotone schematic were incorrect. Zozobra pointed me to a better diagram with correct voltages, and was able to confirm that my amp is working a lot better than I was thinking.

...and yet, the amp was still not functioning. I got my meter and went through every component I could test, and nothing looked wrong. Grabbed the soldering iron and reflowed every connection on the circuit board, and a lot of the wired connections to the various tubes, etc., and still nothing.

Had someone question my input jacks again, so I decided to wire up an entirely new set of jacks, using new components, and connected it in using jumper wires. The symptoms remained EXACTLY the same, so I determined the jacks I'd been using must be ok, and put them back.

I'd had a lot of people speculating that there must be a short or ground issue somewhere, but I couldn't find anything to confirm that. Finally, after Mort told me that he's sometimes found problems with control pots, I decided to start poking ("chopsticking") all the wire and component connections with a pointy dowel. Fired it back up and still nothing. It was only when I started REALLY focusing on the pots that I managed to hit the one wire that was the culprit, and was able to get the volume to suddenly jump just by moving that wire back and forth.

Played the amp for a few minutes. Kinda sounded like shite, tbh - but, the speaker was turned away from me, the amp was still running on the current limiter, and I had the bias set to the coldest possible setting, so I knew it could only get better from here. It faded out a couple times, but came back by just poking that wire with the chopstick, so I was pretty sure I had the root cause found.

Spent a couple hours replacing that shielded wire (this is the one that goes from the preamp gain to the 2nd half of V1), cleaning up the various wires, etc that I'd moved around during diagnosis, set the bias properly, and got the loop board installed with a new dropping resistor after determining I had better PI plate voltage than before.

And the thing fired up immediately.

Some pics of the final work:

This is the problem wire (see arrows) - turns out I reused a shielded wire from my prior build, which turned out to be a mistake



Closeup of the POS



MUCH better prep of the new shielded wire



Installed



Loop board installed

[Tortuga]

This is the amp in it's near-final state. Still safed at that point (green wire in the lower-left that's grounding out the high voltage B+ lines and keeping the caps from regenerating themselves)



Final testing



One last gut shot



Loading into the cab



Almost done



The money shot

[Tortuga]

...and, the obligatory first "official" clip made in the very early morning hours - so low volume, shit playing, fiddle out of tune, and finally getting kinda high af



...yeah, I need to get to work on my playing

Signal path is Squier wannastrat > TS9 > 2204 preamp > Carbon Copy (in the loop) > 2204 power > 4x12 (greenbacks)

The sound is everything I remember hearing for that one hour last year, and it's fucking awesome. Soft clipping, killer touch & dynamics, unbelievably sweet harmonics & squeelies, etc. I can't wait to get some time with this thing, and jam with my friends soon.

[greatmutah]

That sounds great! Glad you got the output issue figured out and the wiring figured out. That’s a proper JMP build.

[Tortuga]

That sounds great! Glad you got the output issue figured out and the wiring figured out. That’s a proper JMP build.

Love the metro loop board you recommended, Eric. Glad I got it when I did - apparently, they’re having supply issues with the transistors they use in their design.

[GuitarBilly]

Yep, it's sounding like a 2204

[greatmutah]

That sounds great! Glad you got the output issue figured out and the wiring figured out. That’s a proper JMP build.

Love the metro loop board you recommended, Eric. Glad I got it when I did - apparently, they’re having supply issues with the transistors they use in their design.

That’s true. They ship them out as soon as they complete them but there’s been some delays and boards have been a bit limited. Speaking of George, I just checked his Facebook page and have something to post in another thread.