Proposition Distortion J201 Measurement

Fingolfen

Well-known member
So.... will my Peak Atlas DCA55 get me the info I need to test the J201's before they go into a Proposition Distortion or do I need to upgrade to a 75?
 

Fingolfen

Well-known member
So... I need the 75? (Please say yes... ;) )

I basically need to verify:

Vgs(off)-1.0V@5uA
Rds(on)~700Ω
 

jeffwhitfield

Well-known member
Ok, so what if you have a good dozen J201's and none of them reach the spec needed? All the ones I have don't reach a VGS of 1.0v. Highest I get is around 0.75v with an RDS that's still short of 700. I would assume that you would use the one that has the highest VGS and then use the bias resistors to get it to spec when on the board. If that's the case, what should the voltage measurements be?
 

Robert

Reverse Engineer
Staff member
The two originals measured were -0.84V and -0.95V, both used the same values for RB1 and RB2 (6M2, 5M1 respectively).

I've gotten acceptable results with anywhere between -0.65V to up to -1.3V without needing to change the resistor values at all. If you get too far out of that range you'd need to start compensating the voltage drops of the diodes / LEDs in the compression circuit.

The Ultimatum isn't quite as critical as the X100 because it doesn't have a clean tone. Biasing the X100 is a bit more involved because you have to find the sweet spot where you get the right amount of compression with no clipping.

If you want to get extremely precise you'll need a scope, signal generator, and to hand select the forward voltages of the LEDs. I personally feel like this is overkill for this one.
 

jeffwhitfield

Well-known member
The two originals measured were -0.84V and -0.95V, both used the same values for RB1 and RB2 (6M2, 5M1 respectively).

I've gotten acceptable results with anywhere between -0.65V to up to -1.3V without needing to change the resistor values at all. If you get too far out of that range you'd need to start compensating the voltage drops of the diodes / LEDs in the compression circuit.

The Ultimatum isn't quite as critical as the X100 because it doesn't have a clean tone. Biasing the X100 is a bit more involved because you have to find the sweet spot where you get the right amount of compression with no clipping.

If you want to get extremely precise you'll need a scope, signal generator, and to hand select the forward voltages of the LEDs. I personally feel like this is overkill for this one.
Thanks for the explanation! I'll just go with whatever the highest I have and go from there. :)
 
Last edited by a moderator:

Feral Feline

Well-known member
Hmm I guess it's time I finally plug in a scope and figure out how to use it.

If I've got a couple scopes, I don't need to get the DCA75, right?
 

Robert

Reverse Engineer
Staff member
The easiest thing to do is to find a transistor with specs as close to those listed in the doc as possible, then you won't need the scope.

You don't need the DCA75 if you have some other method of measuring Vgs(off), but it certainly makes things easier.
 
  • Like
Reactions: fig

Bricksnbeatles

Member known well
on the same topic.... @Robert in the preliminary docs it says that you can just jumper q1 to bypass the compression circuit. Big enough difference to warrant putting it on a switch in my build?
 

Billyhank

Member
My tester measures Id and @Vg I’m not sure what uA it uses to measure. Would this be something I can use for this? So far nothing I’ve measured has gotten close to 1.0v they’re mostly close to what’s shown in the picture.
Would bypassing that portion of the circuit have any effect on the sound of the pedal?
 

Attachments

  • image.jpg
    image.jpg
    1.3 MB · Views: 14
  • Like
Reactions: fig

Cybercow

Well-known member
My tester measures Id and @Vg I’m not sure what uA it uses to measure. Would this be something I can use for this? So far nothing I’ve measured has gotten close to 1.0v they’re mostly close to what’s shown in the picture.
Would bypassing that portion of the circuit have any effect on the sound of the pedal?
Unfortunately, those testers do not adequately measure JFETs. Those testers will not display the Vgs(Off), Vgs(on), R(On) and several other specs. The Vgs(Off) spec value is what needs to be targeted for the JFET. If you want to get serious about testing JFETs, a Peak Atlas DCA75 is the way to go. It will also display all the important Si & Ge BJT specs - distinguishing between Si & Ge and exposes any measurable "Leakage" values.

The only difference in the bypassing the compressor part of the circuit is that without it, the Gain control will bring the compressor to bear when it is turned up. The compression portion of the circuit is tied directly to the "Gain" control section. That's how Tom originally designed it.
 

Causefarley

New member
I looked inside my X-100 and the jfet he used in that part of the circuit was a 2n4339. Pretty close to the specs. It’s $12.00 at mouser
 
Top