JMP50 clone converted to a Dumble ODS type of clone :D

[turbo5speed]

Hi. This is my first post so i'd like to congratulate ya all for this great forum.

The thing is, about 3 or 4 years ago i built a Marshall JMP 50 clone but now i play a lot of Robben Ford stuff so i thought i changed the amp to sound like an ODS. I kept the iron, PI and power supply and changed to the Dumble preamp with internal trimmers to adjust the tone of the overdrive. Guess what? I nailed it!! With one single but... I have a Gibson Les Paul Studio and a 1991 Fender American Standard Stratocaster. With the Les Paul it sounds freaking awesome but the Fender sounds like crap, i mean, really crap. The clean sound is ok but the overdrive sounds like a mixture of a chain saw with a fuzz face

I think my amp doesnt like single coils... is that normal in this type of amp?

Anyone has any ideas on how to fix this?

Thx

[Structo]

What schematic or layout did you follow for your ODS?

These amps can be a bit finicky about guitar pickups.

Some component choices sound great with a humbucker equipped guitar while a single coil sounds thin and bad.

Not sure what tone stack you used (Skyliner?) but many of us put a switch on the amp to be able to pick the mid cap used.
The mid cap seems to play an important role in how the amp sounds with pickups.

This can be a simple On/ On switch or you can go crazy and have three choices of mid caps on a switch.

Right now on my amp I have a .01uF and a .05uF mid cap that I can select.
Labeled FAT and THIN.
The FAT is the .05uF and the THIN is the .01uF cap.
The .01uF seems to sound better with humbuckers and the .05uF sounds best on the single coil guitars like Strats and Teles.
I also have a 250KA mid pot installed.

[IMG:799:261]http://i23.photobucket.com/albums/b392/ ... G_0298.jpg[/img]


You also might consider building a Dumbleator FX Loop.
It really compliments the ODS type amp and many feel it is a required bit of gear for the amp.
If you use any time based effects such as reverb, delay and even chorus sound great with the D' Lator which also has level controls to dial in the right balance of the effects.
Here is the one I recently completed.

[IMG:796:212]http://i23.photobucket.com/albums/b392/ ... tPanel.jpg[/img]
[IMG:800:600]http://i23.photobucket.com/albums/b392/ ... G_0271.jpg[/img]

[turbo5speed]

Wow! Nice reply. Thx

I used a schematic i found on schematicheaven.com named "1997 Humble Overdraft Specimen".

The only difference is that i used the typical Marshall tone stack on the clean channel (also 250k on the treble) and it works very well.

The mid cap selection is a good idea. I will do that.

The sound i want to get rid off is a fuzz box kind of sound that i dont like while i am using the strat. Also i am using a 2x12" speaker box loaded with Vintage 30's that has little bit too much bass. I guess that with a little more pacience and experimenting i will get there.

I was also thinking about a tone stack bypass switch on the overdrive channel. What do you think about that? Is it worth it?

[ChrisM]

I'd stay away from that schem.

Use the #124 schem found in the files section. Replace the clean channel with what #124 has then tweak the OD as needed.

[Structo]

The tone bypass is the preamp boost circuit or PAB.

Yes the V30 speakers may not be the best choice for this amp.

It provides a volume boost by lifting the breaking the link between the treble and bass pot, effectively gaining the lost volume that this type of plate loaded tone stack imposes.
Another way is the way the boost is achieved on the Bluesmaster amp.
It lifts the mid and bass pots for the boost and some prefer this over the normal PAB.

Another thing to do is instead of connecting the PAB straight to a ground reference is to "pad" this with a resistance.
This allows you to sort of control how much of a boost you get when the PAB is turned on.
Some use 22M resistors while others prefer smaller resistors like 470K or even 270K.

Not sure if you have put relays into your amp but it is the best way to implement the boost and OD selection.
Then you have a foot pedal that can switch the OD and PAB and even a mid boost if you desire as well as the front panel toggle switches.
Look in the Dumble file section for the schematics and info.

Now if you are talking about a high end fizz on the OD channel then there are some things you can do to address that.
It seems to be a somewhat common complaint and I as well have battled it.
Some of what you are experiencing may be due to the Marshall tone stack you are using.
Probably best to change it to the Dumble type so you have a baseline to adjust from.
It sounds like you have the HRM section in your amp which is the tone control trimmers for the OD channel in your amp.

From my notes:
The series resistance after the OD2 coupling cap has a very drastic effect on the overall sound of the OD section. With no resistance at all, the OD section can sound very harsh and brittle. Adding in a 150k resistance for a 0.01uF coupling cap, or a 330k resistance for a 0.005uF coupling cap seems to work really well in shaving off some of the high frequencies while leaving plenty of definition in the sound. But with the 500k trim pot in there (at least temporarily) you can determine the "sweet spot" for your own amp. Once you find the value you prefer you can hard wire in a
fixed resistor (or combination of resistors) for that particular resistance.

The series resistance after the OD1 coupling cap has a more subtle effect on the sound of the OD section, and when I dialed it in for the "sweet spot" it worked out to around 105k (very close to the 100k resistor used in both the 70's and 90's designs). However, you are encouraged to try the 500k trim pot here since your own amp may be tuned a bit differently. I noticed that as I set the trimmer to 91k and 82k the sound was a bit brighter, and would improve the blues tones. In seeking an
"ultra-smooth" sound, I ended up setting the trimmer to roughly 220k.

[Bob-I]

I used a schematic i found on schematicheaven.com named "1997 Humble Overdraft Specimen".

I had the same experience with that schematic, buzzy thin on SCs but ok on hums.

Go to the files section and pull down the ODS101HRM. I've used this for several amps and they love both hums and all type of SCs. I don't even change settings when switching guitars!!

[turbo5speed]

Priceless information.

Thanks