2204 Redux... rebuilding JCM > JMP

[Tortuga]

Best shop practices always wear eye protection.

Oh shit... you're absolutely right

...busted

You made some massive progress this week!! Getting close to the finish line

Can't wait to hear the amp when it's done. I have a weakness for 2204s!!!

It's been almost a year since I heard the thing for that 1 hour, and I've never been able to get that sound out of my head. I CAN'T wait to hear it again!

[Tortuga]

Lots of work this morning...



Dylan saluting the glasses request...



Input jacks just about done



Under-board filter cap wired to the circuit board - extra service loops added so that the filter can be changed down the line w/o having to remove the board



Having to stop for a graduation party now, but got most of the front controls and standby wired up



Overhead of where we're at



Hoping to be back with a couple hours to work on it before we go to a show later tonight. Have a few grounds and the wiring to the input jacks on the front of the chassis, plus all the preamp tube connections and speaker outputs to do. Then it's time for the final QC and on to startup

[GuitarBilly]

I like Dylan's salute lollol

[fretless]

Safety first!

[Tortuga]

HAHAHAHA - Fuck YEAH!!!!

Fret's comment on my Marshall Tolex thread has me thinking... purple?

[GuitarBilly]

HAHAHAHA - Fuck YEAH!!!!

Fret's comment on my Marshall Tolex thread has me thinking... purple?

Noooooooooo

[Tortuga]

Gonna have to Duncan this one for now… it’s alive!!!!


Off to dinner for Father’s Day, then pics will be sent shortly after.

Clips will have to wait a day or two, so I can sort out some problems with the input jacks, but everything else appears solid.

Fuck yes

[Tortuga]

Here it is in a (nearly) completed state. I plan to rebuild the input jacks and possibly install the effects loop board tomorrow, and then will get the bias set and everything readied for installation in the head shell.

Because of the input jack situation, I wasn't able to get a clip of the sound, but will do so as soon as it's up and stable.

[Tortuga]

Well… gonna have to start reading up on troubleshooting procedures… I completely rebuilt the input jacks today, and results were not good. Amp still makes sound (primarily through the hi input), but the volume is extremely low and thin. I guessed input jacks yesterday, because I was able to get the amp sporadically louder and more full-sounding by playing with the wires and components around the inputs with wooden sticks. Now it’s more consistent, but in the bad way.

I started measuring plate voltages for fun, and most of them seem really low (some more than others).

Bias is set coldest for startup, and I haven’t adjusted it.

Also got going on the loop board, but that’s on hold for now.

<edit - forgot to mention, the amp will occasionally emit pops and scratches, almost like an old filter cap. Hoping I’m not going to have to prepare filter cap(s).>

Any (constructive) input is most welcome

[ajaxlepinski]

Looks like a work of art!!! Beautiful!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Tortuga]

Thanks man - now I just gotta get it running right

[GuitarBilly]

The amp is sounding bad because it doesn't want to go into a purple headshell. I've tried to warn you

Seriously, it's a cold solder joint somewhere. Did you put an effects loop in it? Try to go into the return and see if it sounds normal. If does, the issue is in the preamp section

[Tortuga]

No loop yet - didn't want to complicate this thing further - not to mention that the loop board is going to cover over V1 & V2, and I don't want to have to remove that board to do this troubleshooting.

Planning to start going through the wiring & circuit board to see if there's a bad connection

It ain't purple yet

[Tortuga]

So... I'm working with some helpful/knowledgeable guys on the Mojotone FaceBook group on diagnosing the problems.

Have rebuilt the input jack wiring, and seem to have gotten past the intermittent issues I was having on Sunday. That seems like progress, but the volume is now consistently low and only when I use the Hi input jack. Since the signal goes through the Lo input on a 2204, that seems to confirm that I'm at least on the right track with respect to the inputs, so I think I'm moving past that now.

Did some voltage checking on the B+ power distribution and plates/cathodes, and it seems to be universally low, especially on the first half of V2 (front of the cathode follower for the tone stack):



A lot of the advice I'm getting is to double-check all the wiring, components, and solder joints, which I've been doing. I started with V2 last night (specifically, the components shown in the pic below), and still having no luck. Really not understanding how voltage can drop from 288.1 to 5.08 on the plates with the only difference being that 100K plate load resistor, but that's exactly what seems to be happening. I think I'm going to pull the tubes and re-measure the voltages to see if they stays same (especially on V2).

[Tortuga]

Pulled the tubes and got voltages closer to what I'd expect - especially with that V2 half. Tried putting in another tube and suddenly got volume out of the amp... for a minute...

Really not liking the idea of using tubes as fuses, but I think I'm at least onto something

[Tortuga]

After looking carefully at the V2 tube socket and locating what looked like a scarred socket on the plate input (pin 6 - the one that's become suspect), I decided to pull the whole damn socket and rebuilt it. Took some doing, but it seems to have brought the damn thing to life!!!

Not as tidy as it was, but working is better than bricked

Not working:



...in progress:



...working:

[Tortuga]

Now its off to set bias and install the MetroAmp loop before putting the chassis into the (temporary) headshell and learning how to play the damn thing...

[GuitarBilly]

Nice!!! Looking forward to hear it.

Why is the headshell temporary?

[Tortuga]

Because… purple?

[GuitarBilly]

Because… purple?

Happy pride month to you

[Tortuga]

Lol

Right now, I’m either going to mod one of the JCM cabs I have to mimic the JMP style, or just scratch-build a new one. I’d then cover that using the Marshall tolex that mojotone sells. I absolutely love the 70s era JMP look.

I’ve got another idea for a totally custom headshell using hardwoods (like a hardwood boogie shell, but with a very different finish), but I’m not quite ready to build that yet.

[GuitarBilly]

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]

In other news, I managed to get the bias set to a reasonable level (more on that later) and the loop installed. Fired it back up at 11:30pm, and… nope…

A couple “fucks” were uttered as I shut down for the night.

I’m pretty sure the amp voltages (along the main B+ rail) are still too low, which means I’ve either got an overly aggressive dropping resistor or something is stealing power somewhere. Regardless, I’m pretty sure the loop board isn’t running because it’s not getting enough power to operate.

Hoping to resolve this soon

[GuitarBilly]

How are the voltages without tubes installed?

[Tortuga]

Haven't measured them that way since startup... that's a good point - will definitely try that