I used steel cores with a diameter of 5 mm. Of these, I built coils with a height of 11 mm and a diameter of 13 mm. He wound a wire with a diameter of 0.05 mm, about 10,000 turns. As a result, I completely filled the coil, and reached the resistance of 2k5. The final resistance of the pickup when connecting the coils in series-in parallel came out 1k2. The capacitance was about 80 pf, and the inductance was about 1.8 Henry. This is ideal for preamp, it remains only to adjust the peak attack, which is also done in the original scheme. I used ceramic magnets. Since the standard length of the steel rod is 15 mm, taking into account the coil and the board, the gap between the magnet and the coil was 3 mm. This is more than acceptable. Although, even if the magnet was lying close, it wouldn't bother me