NAD - 50w JMP clone

[syscokid]

Sweet progress!

 

[fitz]

Got some changes to the board done tonight while it was out and easier to get to.
Here's a before pic of how it looked when it came to me in the old chassis.



Here's the stuff I changed to return some values to Marshall stock, and to add the EL34 / 6550 switching.



1. Changed the 150k to stock 220k and moved to a new turret (A) for connection to power before the standby switch.
2. Added a 270k resistor to new turret (B) - A/B will connect via switch for parallel 120k value for 6550 tube spec.
3. Changed the bias trim from 22k resistor & 50K pot to 47K resistor & 25k pot (trim pot mounted to rear chassis panel).
4. Swapped out the ugly 10k ceramic block resistor for same value - matches the 10k going to the cap above the board...
5. Added 470k resistors to new turret (B) that will be parallel to the 220K for a 150k value with switch connecting A/B for 6550 tube spec.
6 Changed the 100nF JCM800 cap spec to 22nF JMP spec.
7. Pulled the cap/resistor gain mod for a spec 10k.

Tomorrow I'll start on rewiring the tube sockets, power, pots & fuses.
If all goes well, I'll move on to reconnecting all the board leads and see if it bursts into flames...

 

[T-Rex]

Good work !!

 

[BFT Gibson]

Are you setting up for Greenbacks, 65's. t75's or V30's? Amazing what those 4 speakers will do with those circuits in end result tone.

 

[fitz]

Are you setting up for Greenbacks, 65's. t75's or V30's? Amazing what those 4 speakers will do with those circuits in end result tone.

Well actually, none of the above.
Tone Tubby alnico red 50w speakers are in the cab set that this amp is going with.

 

[BFT Gibson]

Well actually, none of the above.
Tone Tubby alnico red 50w speakers are in the cab set that this amp is going with.

Cool, love alnico's

 

[syscokid]

6 Changed the 100nF JCM800 cap spec to 22nF JMP spec.

When you finally get this amp up and running, and with a proper bias, maybe you should try adding a temporary switch that adds a 100nF in parallel with that 22nF. Depending on the rest of your setup, you might prefer the larger value.

 

[fitz]

When you finally get this amp up and running, and with a proper bias, maybe you should try adding a temporary switch that adds a 100nF in parallel with that 22nF. Depending on the rest of your setup, you might prefer the larger value.

How do the different values here change the tone?

 

[fitz]

Making headway.

Started with the heater wiring.



I think I got everything hooked up except the connections to the board.



DPST toggle switch with blue & yellow leads are for the EL34 / 6550 switching to the parallel resistors I added.



Stole the ground buss bar idea from a build by an MF'er instead of running a ground across the pot backs.
Long piece of Romex ground wire connected to the chassis at each end.
Seemed like a good idea if a pot needs changed.



Spliced in the pre-standby power wire to the bias supply.

 

[fitz]

OK, I ran out of wires to connect.
Now what?

 

[fitz]

So I hooked it up to my light bulb limiter and put some power to it.
Getting about 70v at the mains, and the voltages all seemed reasonable compared to about 60% of what's on the Mojotone diagram.
Went full power and every reading was within a few volts of the diagram.
Had 6550's in it and getting 440v plate voltage on the Eurotubes probes.

Flipped it over and hooked up a speaker.
Low input seems fine, but I'm getting nothing on the high input.
Gotta check the input wiring to see if something got knocked loose.

I'm going to recheck voltages with EL34's, but I'm done for today.

 

[Far Rider]

OK, I ran out of wires to connect.
Now what?

View attachment 110750

 

[syscokid]

How do the different values here change the tone?

The larger value should improve the low end response. The AC shunt cap of 100nF is the standard for these circuits with a long tail phase inverter. Using a 22nF as the AC shunt cap was probably another example of the Marshall/Unicord era doing things a little different.

Since I’ve never bothered with trying different values for the AC shunt cap in a 2204, it would be interesting to hear from someone who has tried it.

 

[T-Rex]

As far as the PI, I've not heard much of a difference, if any.

 

[fitz]

Got the hi input working.
Connections all look correct, and when I tried it again today, it was working fine.

Tried the EL34 setting with some 6CA7's and the bias trimmer and voltages were all fine.



Gave it some volume to check controls functionality and then switched to the 6550's.
Subjective, but I think I like the big bottles better.



I have no other 2204 to compare, although I think everything is working as intended.
Can't say it sounds drastically different than it did in the ugly chassis, but it sure looks better...
With catastrophic failure as a possible outcome, I'm calling it a win!

Looking forward to getting this in wrapped up, so the FX loop install is next.

 

[JohnnyGoo]

Wow very nice work.

 

[fitz]

Got the loop installed.

Loop board "In" to the MV pot wiper lug, "Out" to the turret where the MV wiper was connected.
"B+" to the turret connected to the cap above the board, "GRD" to the speaker jack ground.




There's a significant volume drop with the loop on.
Even with the loop trim on max.

Per the MJT instructions:

Step Six: Initial Startup / Adjust the return level of the loop.

Now that the loop is wired into the amplifier circuit, turn on
the amplifier. Measure the DC voltage on the 10k 2W
resistor on the board. If the side closest to the 10uF
capacitor is 400V or below, it can be assumed that the initial
installation is successful.

There should be a voltage drop
due to the load the loop imposes on the amplifier’s power
supply. If the B+ drops too low, towards 200V, find another
source for the B+ higher up in the amplifier’s power supply
filtering stages.

The voltage dropped from 323v to about 175v.
I moved the power over to the turret connected to the cap by the mains fuse (366v).
That dropped the voltage there to about 275.
Still have the volume issue with the loop engaged.

Did I hook something up wrong, or should I suspect something else is the issue?

 

[67plexi]

My guess your power transformer is under rated the best sounding Marshall's have 470 volts B+1

 

[syscokid]

Did I hook something up wrong, or should I suspect something else is the issue?

The wiring looks different from what I'm used to, and I think it might be wrong. Even though my two Marshally 2204's have the Metro fx loop installed, the Mojotone should be pretty similar. Using the Mojotone amp layout with the Mojotone fx loop pic superimposed, this is how I have mine wired...:
Notice the jumper wire that is marked for removal between the treble and master volume pot. The treble pot's wiper is wired to the fx loop board's "IN" eyelet. The master volume pot's input lug is wired to the loop board's "OUT" eyelet. And that's it! I think it's a good idea that you're tapping the voltage supply for the loop board from the phase inverter node.


The following video is a good example how the wiring should be done:

And here's a pic of one of my 2204's showing the connections between the amp's control pots and the Metro fx loop board:

Notice that I'm using a 2-conductor shielded cable to keep it clean and quiet. You can easily do this with yours, too!

Good luck...

 

[fitz]

Thanks!
I'll redo the in-out as you have shown (& in the vid).

Was going off this from the instructions.
I'm no schematics wizard, but it looks like they are just replacing the wire from the 22nF cap turret to the MV wiper.