Proposition Distortion J201 Measurement
- Thread starter Fingolfen
- Start date
Fingolfen
Well-known member
More than "a few" - less than "a lot" - let's call it... "several"How many 201's do you have?
Robert
Reverse Engineer
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
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.
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
Thanks for the explanation! I'll just go with whatever the highest I have and go from there.
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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?
If I've got a couple scopes, I don't need to get the DCA75, right?
Bricksnbeatles
Member known well
Bricksnbeatles
Member known well
Then it shall be done. Guess this’ll be one for a 1590BB2
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?
Would bypassing that portion of the circuit have any effect on the sound of the pedal?
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Cybercow
Well-known member
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
