Posted by
Frank_NH on
URL: http://guitar-fx-layouts.238.s1.nabble.com/J201-and-2n5457-replacements-tp24940p24960.html
Great that you built a JFET tester/matcher. The most important parameters associated with any JFET are the idle current (Idss) and the "pinch-off" voltage (Vp) that you measure with this device. The stage gain and required biasing are directly tied to them.
To answer your questions:
1) Can this circuit be used with MOSFETs? (example: 2n7000)?
No. MOSFETs do not work like JFETs.
2) What is the expected values in a J201?
First read this:
http://www.runoffgroove.com/fetzervalve.htmlThen look at the bottom of the post. You'll see "average" values for a variety of JFETs.
Out of ten (10) J201
average Vp: -0.83V (spanning from -0.57 to -0.97)
average Idss: 0.64mA (spanning 0.30mA to 0.83mA)
A couple of additional points:
* There are many substitute JFETs out there if read through the datasheets. For example, I found an eBay dealer selling J202 for a great price. J202s are a near drop in replacement for 2N5457s.
* There are two cases where the JFET's Idss, Vp are not as critical so a range of JFETs can be used:
- JFET buffers (source followers)
- Mu-amp gain stages such as you see in BSIABII and many others (including the Catalinbread Galileo II shown on the main page). Again, a range of JFETs can often be substituted
* Do test ALL of your JFETs just to make sure their Idss, Vp fall within the datasheet specs (I have had a bad JFET that didn't meet spec and tossed it). If you want to get fancy, take some white masking tape and attach it to the leads and mark a label on it (like A1, A2, A3...). Then log the label and the measured Idss, Vp in a spreadsheet. When you go to use your JFETs you can pick ones that are close. An alternative would be to put ones that are "close" in their specs in a bag, then mark the bag with the approximate Idss, Vp for the group.
* You can predict the performance of a JFET common source stage if you want to get into the formulas from JFET theory. That is for a given JFET, source resistor, and supply voltage, you can determine the required drain resistance and gain. This allows you to determine the right sized drain trimmer (100K is usually too large) for circuits like Dr. Boogie. Or you can determine the appropriate JFET Idss, Vp for given source, drain resistors (this is where your measurements and labeling of JFETs come in VERY handy!). I made a spreadsheet that I shared in a previous post which does this, if you're interested in this sort of thing.