Gain pedal JFET selection calculator tool

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microbailey
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Post by microbailey »

Hi, I'm new to this forum so hopefully I don't break any of the rules :lol:

I thought this spreadsheet might help others who are building reverse engineered JFET based boutique pedals.
I'm in the process of building my first DIY stompbox (for a few years anyway), a Wampler Plexi Drive, and I wanted to sort through my J201s for the ones closest in spec to the ones in the production pedal.

Elsewhere, members have posted the measured Vdrain for each JFET gain stage in the production (or clone) pedal, to assist other builders.
I created an Excel calculator which allowed me to enter my measured Vgs(off) (= Vp pinchoff voltage) and Idss for each of my J201 JFETs, and then calculate the actual Id (drain current) I would get if I put it in the pedal circuit.
This is important since these type of pedals (as Mr Wampler has confirmed) have a tone which depends on using JFETs which give the correct Vdrain when biased in the circuit without changing the Rd (drain resistor) value.

So here's how it works (currents are in mA, resistors in Ohms, voltages in Volts):
1) enter the voltage the pedal runs off as Vdd
2) enter the voltage measured for a production pedal for Vdrain (relative to the supply ground)
3) enter the component values for the gain stage, Rd (drain resistor) and Rs (source resistor) from the pedal schematic
4) measure each JFET in your bunch for Vgs(off) and Idss (see elsewhere in these forums for how to do this with a multimeter and a couple of resistors)

Now the spreadsheet will show, for each JFET, what drain current is expected (Id actual) versus the value which was designed (Id wanted) - and calculate the difference between them (Id diff).
This difference should be as small as possible to get close to the Vdrain which was measured, keeping the original Rd and Rs values.

I made the spreadsheet show the result green if the difference was within 60uA of what was wanted (an arbitrary choice of mine).
In other words the smallest numbers in the green shaded results indicated the best JFETs for my pedal.
It also quickly told me I had no JFETs which were of any use in building a Wampler Black 65.

Here's the Excel spreadsheet:
Jfet_reveng_calculator.xlsx
Excel spreadsheet for selecting from measured JFETs
(11.65 KiB) Downloaded 327 times
Feel free to modify the spreadsheet if you need something else from it.
Anyway I found it pretty useful.

cheers Microbailey

Note: If you're interested, the calculator uses the standard JFET Id formula approximation Id=Idss(1 - Vgs/Vgsoff)^2
Also, I wanted originally to calculate the voltage directly but that involves a recursive calculation which was out of scope for a spreadsheet.
"There's something about a Gucci loafer kicking on a fuzz pedal" Alex Turner, Arctic Monkeys

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artsmiles
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Post by artsmiles »

so good. I will operate it via what you post above. :applause:

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julessouren
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Post by julessouren »

Great job and idea, I like it! I could clean it up a little if anyone's interested, add units to the cells, add explanations inline etc. Or perhaps I'll turn it into a Python script, generate a nice output format... Oh the possiblities :horsey: Now, time to build a new jfet matcher first...

A 'database' of all the parameters of the different pedals would be useful too, so you can immediately check if your jfet perhaps specs correctly for a different pedal.

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