Help me with JFET preamp Rd and Rs value

[Agung Kurniawan]

Hi everyone...
Now I working on JFET preamp since they are sound close to vacuum tube, I love it.
the fetzer valve sounds good in my ear. but I'm using 2SK30A instead of J201.

should I replace the Rs and Rd since I subtitute the transistor?
and also what is the Vp and Idss value for SK30A the pdf and people say different

[blackieNYC]

Try 1.5V for Vp and 2ma for IDss.
These are his figures from a sample of ten, as ROG states, and it seems to jive with the curve in the data sheet.  I messed with a few fetzers and got gains similar to those listed. Someone around here has done them a lot.
Are you doing this on a breadboard? If it sounds good, and you tried trimpots for both resistor positions and ended up close to the values ROG offers you are probably there.  Not a great deal of gain to these - yours should give you about 7dB, as described. That's like the difference between rhythm loud (bypass) and lead loud (engaged).  He's pretty much done the homework.  You could measure those specs on your own Fet.

[whoisalhedges]

and also what is the Vp and Idss value for SK30A the pdf and people say different

You have to measure the transistor. Instructions are right there on the Fetzer Valve page.

The datasheet doesn't give "values" for Vgs(off)/Vp and Idss - it gives ranges. JFETs are notoriously variable in their parameters, and there's just no way to design a circuit like this around a part number and have it work right (other than pure luck). You need to measure the FET you're using; and/or use a trimpot - that is to say, measure the values using ROG's method at the bottom of the Fetzer Valve page and use the resistances they suggest; or use trimpots for Rd and Rs, and bias it by ear. But you can't just throw in resistors based on what they "should" be; because JFETs have such wide tolerances that there is no such thing as "should."