HGi Alex. I finally managed to build this, but sadly it doesn't work.
Here there are some pictures of the board etc.
https://drive.google.com/folderview?id=0B0zMctdC-Kv-TFBBdWZiZTRwZFkI used some subs.
I didn't have 12n caps (made 56n + 15n series, or 10n+2,2n parallel).
I didn't have original JFets so I used 2N5457.
I didn't have the LM358 (here in Italy it's kind hard to find this, don't ask me why
) and so I replaced it with LM1458.
That's all.
Ps: should I ask this in debugging?
Pps: Could you check the P1 and P2 connections? Because inside the original only legs 1 and 2 are used on both the 50k and 500k)