NAD - 50w JMP clone
- Thread starter fitz
- Start date
OP
fitz
Ambassador of DIY
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...
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...
Last edited:
BFT Gibson
Ambassador of Originality
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.
OP
fitz
Ambassador of DIY
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
Ambassador of Originality
Cool, love alnico's
syscokid
Ambassador of War & Peace
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.
OP
fitz
Ambassador of DIY
How do the different values here change the tone?
OP
fitz
Ambassador of DIY
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.
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.
Last edited:
OP
fitz
Ambassador of DIY
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.
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.
Last edited:
Far Rider
Ambassador Of The Cataverse
syscokid
Ambassador of War & Peace
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.