NAD JMP Superbass

[Longo]

I think it's from 74
it is missing the back plate and it does not have a serial number. it looks quite original, apart from the master volume mod. It sounds quite good, very big,
what do you think?

 

[LP Freak]

Looks great, congrats ??

 

[SpiderWars]

It looks more like a Super Lead without a bright cap. The coupling caps are Super Lead values and I think that's a 33k slope resistor. It looks orange-orange-orange but I can't really tell. Those two red dogbone caps are the same size so I'm assuming they are the same value which would agree with Super Lead Treble cap value.

The master volume is pretty well done but almost useless in a Super Lead circuit. I'd pull that out and put a 4700pF bright cap in it and any other maintenance/upkeep it needs and rock it.

Very cool amp!

 

[Longo]

thanks....I don't understand much of the technical details. but I will study it.
I have to replace the dangerous bulgin plug and hardwire the voltage selector. I also see the bias caps replaced by a wrong value 22uf..?

 

[harddriver]

It looks more like a Super Lead without a bright cap. The coupling caps are Super Lead values and I think that's a 33k slope resistor. It looks orange-orange-orange but I can't really tell. Those two red dogbone caps are the same size so I'm assuming they are the same value which would agree with Super Lead Treble cap value.

The master volume is pretty well done but almost useless in a Super Lead circuit. I'd pull that out and put a 4700pF bright cap in it and any other maintenance/upkeep it needs and rock it.

Very cool amp!

Yep.... it's a Superlead.....

Does that back panel actually say Superlead. I think all the panels were silkscreened with Superlead in black and they used to use a sticker that said BASS over the Lead letters and those tended to fall off.

The amp looks very clean not butchered and well maintained and original IRON worth the price of admission IMHO. I would not worry about those 22uf bias supply caps 10uf to 22uf is just filtering a bit more efficiently and really does not affect the audible tone in any way in that part of the circuit.

I can't tell if the master is a PPIMV version using a single 1 meg pot, usually those were a 1 Meg pot added after the treble wiper before the driver circuit. I have not used this design but it is in the book How to Service Your Own Tube Amp by Tom Mitchell. It does look like a wire is coming off of the treble pot center wiper and going to the 1 Meg pot through the red wire then back trough the white wire. So with that interrupt path it almost has to be feeding the driver circuit but the brown wire from the same center Treble pot lug is still connect to the driver circuit(PI) .022uf cap at the circuit board edge so I guess it is some kind of variant to the Tom Mitchell design. The 220K bias splitter resistors are still on the board so it's not any type of PPIMV.

Tom Mitchell did alot of great mods so I imagine the master would work well as long as you are not going for bedroom volumes and if you don't like it dime it and it is out of the circuit and then use the amp with out it.

As Spider suggested you could add the 5000pf to the bright channel volume, it will make the amp more aggressive with brighter attack that Superleads from the 70's were known for. I like the 5000pf cap but alot of people don't either way the amp will sound good from the looks of things.

Congrats on the amp, it looks killer.

 

[blackba]

Congrats on the super lead. Oldest Marshall I have seen is a 79 jmp 2203, what a great amp that was. Wish I got more vintage marshalls in for service.

 

[glpg80]

Drop the 22uf bias caps to 10uf or 8uf. The tau charge time is too large on initial fire up with 22uf causing the tubes to run wide open until negative grid voltage catches up to throttle them down. You could get away with those values back in the day but tube quality isn’t what it used to be.

Congrats and killer score. I have a 74 chassis with a jcm800 2203 swapped preamp board.

 

[Stradazone]

A few months ago I took one of DLR's Marshalls to Friedman, was a 69 Superbass and had it converted to Superlead spec, bias to 90v..Friedman said it had the same power transformer as EVH's #1 amp, sounded killer after the conversion, will be used as my backup amp once we get gigging!

 

[SpiderWars]

I can't tell if the master is a PPIMV version using a single 1 meg pot, usually those were a 1 Meg pot added after the treble wiper before the driver circuit. I have not used this design but it is in the book How to Service Your Own Tube Amp by Tom Mitchell. It does look like a wire is coming off of the treble pot center wiper and going to the 1 Meg pot through the red wire then back trough the white wire. So with that interrupt path it almost has to be feeding the driver circuit but the brown wire from the same center Treble pot lug is still connect to the driver circuit(PI) .022uf cap at the circuit board edge so I guess it is some kind of variant to the Tom Mitchell design. The 220K bias splitter resistors are still on the board so it's not any type of PPIMV.

Tom Mitchell did alot of great mods so I imagine the master would work well as long as you are not going for bedroom volumes and if you don't like it dime it and it is out of the circuit and then use the amp with out it.

It's just a simple pre PI MV. As you know those just neuter the tone as you turn them down and reduce gain a ton. But the reason I'm posting again is because it looks like there is a series resistor right there at that cluster on the board. So diming it wouldn't really be stock anymore, now there's a voltage divider in there.

 

[harddriver]

It's just a simple pre PI MV. As you know those just neuter the tone as you turn them down and reduce gain a ton. But the reason I'm posting again is because it looks like there is a series resistor right there at that cluster on the board. So diming it wouldn't really be stock anymore, now there's a voltage divider in there.

Yes it would be a Pre- PI master ala JCM800. Yeah... I see the resistor on the white wire too Spider.... I cant' see if the resistor goes to ground if it does then it is a voltage divider as you say or of it in series with the audio signal???

The fact that the original brown wire is still connected to the driver circuit from what I can see which is not in the Tom Mitchell 1 meg volume design at all, just a 1 meg audio pot in between the treble wiper and the driver circuit. The you have the smaller gauge black wire feeding the red shielded wire.

To me this looks like it would be bleeding audio signal for some reason maybe it just bleed enough volume off to make things bearable but that kinda defeats the purpose of the master volume. It would be interesting to hear this setup in action strange as it is.....

 

[SpiderWars]

I cant' see if the resistor goes to ground if it does then it is a voltage divider as you say or of it in series with the audio signal???

Look close, you can see the end of the brown wire PI input...just a sliver of brown visible. It looks to be in series to me.

EDIT: The brown wire connected to the Treble control is the Bass cap wire and is connected to the left lug, same as the pink wire going to the bass pot. The only thing connected to the Treble wiper is that little black wire.

 

[Nigel]

Why can’t we all be 18-22 in LA with these and sweet axes, circa 79-91, and get our starts with what we know now? Lol, middle age real talk. ?

 

[311boogieman]

Congrats on your Super! Enjoy

Why can’t we all be 18-22 in LA with these and sweet axes, circa 79-91, and get our starts with what we know now? Lol, middle age real talk. ?

Meaning we'd be able to make the music that we know will work and also to not fall into the same pitfalls as (most) of our heroes. What would suck is knowing about all the amps and gear that is coming out but that we can't use yet.

 

[harddriver]

Look close, you can see the end of the brown wire PI input...just a sliver of brown visible. It looks to be in series to me.

EDIT: The brown wire connected to the Treble control is the Bass cap wire and is connected to the left lug, same as the pink wire going to the bass pot. The only thing connected to the Treble wiper is that little black wire.

Yeah... I see that now....good eyes Spider.... I got those brown wires confused with one another. The one brown wire from the treble pot far left lug the goes to the .022uf cap in the tonestack circuit and the other brown wire is from the treble output wiper to the Red wire of the MV pot.

I wonder what the function of that resistor really is and why would it be needed since it's in series???

 

[glpg80]

Yeah... I see that now....good eyes Spider.... I got those brown wires confused with one another. The one brown wire from the treble pot far left lug the goes to the .022uf cap in the tonestack circuit and the other brown wire is from the treble output wiper to the Red wire of the MV pot.

I wonder what the function of that resistor really is and why would it be needed since it's in series???

Tonally it will remove flub, remove gain, and dampen highs.

 

[Longo]

a little help

 

[misteregent]

I think it's from 74
it is missing the back plate and it does not have a serial number. it looks quite original, apart from the master volume mod. It sounds quite good, very big,
what do you think?

Looks good, congrats