NAD - 50w JMP clone

[fitz]

Picked up a homemade hand wired 50w Marshall clone.
Threw out a lowball offer and a quick counter offer later, it was inbound.

Kinda ugly for now, but I have plans...
Here's some listing pics.



Supposedly a Mojotone JCM800 preamp layout, but looks to be TAD caps and various tidbits on a Mojotone turret board.
PT from an old Hammond organ with a Hammond 50w guitar OT, and a Pacific Audio choke



Rear panel is set up with external bias test points and a bias pot between the fuse holders.



I've ordered a proper JMP50w chassis with front & back plates, along with a loop kit from Mojotone.



My current plan is to put the amp in these cabs that currently house my Origin20 #2.

 

[fitz]

Amp arrived today.
Well packaged and nothing looked amiss, so I popped the tubes in and checked the bias.
Played it a bit and it runs fairly quiet unless the gain is cranked.
I'd imagine some neater wiring will do no harm.
Hammond iron sound fine to me.



One of the Mesa El34's is running a little hotter than the other.
Swapped them and the results followed the tubes.



Put in a set of JJ 83's and a pair of JJ 6CA7's.
Much tighter on the bias and sounds more to my liking.



Some quick gut shots.





I'm going to check the schematics and decide if anything needs changed for a '78 JMP 2204 spec.
Chassis and other stuff from Mojotone won't be here for a few days.

 

[C-Grin]

Cool

 

[Robb]

Very interesting project.

 

[BFT Gibson]

Get that bad boy set up to specs you like & it is probably "the" circuit in my experience & SL100 that is rock ! Enjoy maximus !

 

[T-Rex]

Amp arrived today.
Well packaged and nothing looked amiss, so I popped the tubes in and checked the bias.
Played it a bit and it runs fairly quiet unless the gain is cranked.
I'd imagine some neater wiring will do no harm.
Hammond iron sound fine to me.

View attachment 110588

One of the Mesa El34's is running a little hotter than the other.
Swapped them and the results followed the tubes.

View attachment 110589

Put in a set of JJ 83's and a pair of JJ 6CA7's.
Much tighter on the bias and sounds more to my liking.

View attachment 110590

Some quick gut shots.

View attachment 110591

View attachment 110592

I'm going to check the schematics and decide if anything needs changed for a '78 JMP 2204 spec.
Chassis and other stuff from Mojotone won't be here for a few days.

I built a few amps with rescued Hammond iron, Baldwin too. They are fine to use if you get the voltages needed.

 

[Mitch Pearrow SJMP]

Congratulations on a stellar amp @fitz .
HNAD

 

[fitz]

Deceived by ambiguous tracking info - the chassis & face plates showed up today, FedEx.

Front hole placement isn't even close.
Glad I went with the chassis purchase.





Back looks to already be punched for the loop.



2 holes between the fuses and impedance selector.
I'm assuming one would be for a voltage selector?
Not sure about the other, but either could be used for the bias trim pot.
I'll need to add some holes for the probe sockets.



Top will need some work for the lay down PT.
OT holes should work, but I'll need to drill for the 2-point choke mount.
Tubes and caps connections I'm hoping to be drop in.



Too mentally spent from work on a Friday night to start anything.
Circuit comparison to schematics is scheduled for tomorrow after a few mugs of coffee.
Lots of photos before anything comes apart.
Parts list starts with chassis bolts and cage nuts...

 

[DirtySteve]

Looks like fun!

 

[fitz]

Got some chassis mods out of the way.
(stuff I know how to do...)

Added the rectangular opening for the lay down PT and 2 new mounting bolt holes.
The holes closest to the back side will be reusable.
Also drilled 2 holes for the choke mount.



If I abandon the external bias pot and probe sockets and go with a standard board mount bias trim pot, that should be it for chassis mods.
With the Eurotubes probes I have, I don't really have a need for the external stuff.

The back is already punched for the loop, and I'm assuming the Mojotone loop is going to fit the Mojotone chassis holes...
Just gotta drill the plastic rear cover for the loop holes.

Less stuff to mess with for the guts transplant.

 

[fitz]

I've gone through the amp and compared it to the Mojotone wiring diagram for the '78 JMP 2204 & JCM 800 versions.
It looks closer to the JCM800 specs, so I'm going to make a few changes to get closer to the JMP version.
There're a few other minor value variances here and there that aren't worth changing.



1 - This 100nF cap is the JCM spec, changing it to 22nF for JMP spec.
2 - I'm told his parallel cap/resistor contraption is a gain mod, should just be a 10k resistor in either version.
3 - Both specs show an optional 10pF cap across pins 1 & 3 on V1 to "subdue high frequency noise" so I'm going to add it.
4 - JMP specs a 5k presence pot, JCM is 25k - guessing it's a 25k, so I ordered a 5k pot
5 - Decided to abandon the external bias stuff, so I ordered a board mount trimmer.

Got a few other items in the cart.
A multi color pack of wire (just in case).
Some M6 cage nuts for the chassis bolts.
A pair of red speaker jack nuts, since I'm adding a loop kit to add a little visual difference to the jacks.

Gonna wait for the loop kit & new parts to arrive before I get into the surgery phase.

 

[BFT Gibson]

I smell some Judas priest tones a cookin ! JMP 50 has got to be almost the ultimate amp the Plexi morphing into the JCM 800.

 

[SG John]

Cool! Congrats on the new amp.

 

[ibmorjamn]

Killer deal , wow . What a journey you are on Fitz.

 

[fitz]

Killer deal , wow . What a journey you are on Fitz.

I love finding bargains on gear.
I've kinda always wanted a 2204, but the coin required for those is hard to justify or even rationalize.
I've looked into the kits from Mojotone & Ceriatone a few times.

Got the amp for $270 shipped.
Mojotone for the chassis & front / back plates was $150.
Another $70 for the loop kit.
Amplified Parts for the misc. stuff was $35.
So, I'm into this for about $525 (so far... ).

The complete British 50 kit from Mojotone with the loop added would be $910 plus tubes.

 

[syscokid]

I've gone through the amp and compared it to the Mojotone wiring diagram for the '78 JMP 2204 & JCM 800 versions.

When I look at the Mojotone wiring layout and the schematic, it seems that Mojotone has copied the Marshall mistake of the 70’s Unicord era. In the 70’s, Marshall moved the bias supply circuit after the standby switch. In the 60’s and then the 80’s, the bias supply is before the standby switch, which means that the bias supply is energized, along with the heaters, when the power switch is on. Supposedly this helps minimize the surge going through the power tubes at full startup for less stress. This is a semi-easy fix by disconnecting the AC voltage dropping resistor from the SS rectifier and adding a wire from one leg of the PT's HT supply to the standby switch back to the AC voltage dropping resistor...

Mojotone original:


IMO, improved:


You don't really need to add a turret, but it would look prettier that way! Hopefully someone else could chime in on this?

 

[fitz]

When I look at the Mojotone wiring layout and the schematic, it seems that Mojotone has copied the Marshall mistake of the 70’s Unicord era. In the 70’s, Marshall moved the bias supply circuit after the standby switch. In the 60’s and then the 80’s, the bias supply is before the standby switch, which means that the bias supply is energized, along with the heaters, when the power switch is on. Supposedly this helps minimize the surge going through the power tubes at full startup for less stress. This is a semi-easy fix by disconnecting the AC voltage dropping resistor from the SS rectifier and adding a wire from one leg of the PT's HT supply to the standby switch back to the AC voltage dropping resistor...

Mojotone original:
View attachment 110626

IMO, improved:
View attachment 110627

You don't really need to add a turret, but it would look prettier that way! Hopefully someone else could chime in on this?

Thanks,
Actually, had the same advice from my thread on MF.
I made this to see if I understood what they were talking about.
I don't have a spare turret, but I will be incorporating this basic change.
Thanks for reaffirming the issue with the Mojotone diagram.



Here's an example pic from another build.

 

[fitz]

Right now I'm looking into adding some switchable parallel resistors to the 220k bias resistors so I can switch to 6550 power tubes.
If I piggyback some 470K to the 220k, I should get close to the 150k spec for 6550's.
Run the parallel 470k's to a DPST switch on the back panel.



Marshall schematic below lists the values for each tube type - 220k for EL34, 150k for 6550.

They have the dropping resistor (R8) at 220k, 120k respectively.
Mojotone has that at 160k for their (EL34) version .
Mine is actually a 150k, so I may leave it as is.

 

[fitz]

Took lots of reference pics before disassembly, but here's one last overall "as-was".



Disconnected a little as possible to remove everything from the old chassis.
It's a little tighter fit in the Mojotone chassis, so I think the existing wiring can be mostly reused.
None of the wiring is reconnected - just mounted everything to see how it all fits.
Cutout for the laydown Hammond was just about perfect.
Just needed to tweak the 2 drilled mounting bolt holes.



Going to reuse the external bias trim but abandon the test points.



Definitely redoing the heater wiring.
Got a pack of 6 color 22 ga. wires (10ft each).
Going to think about reuse vs. replace on a lot of the other connections.