Hi, A few years agoI would like to make a preamp based on this diagram: https://www.talkbass.com/attachment...e2889d805d-5000-0000074733c4bbba-gif.2309486/ This is an old Wal bass preamplifier. At first I wanted to build this preamp using modern parts. Instead of the double U406 transistor I chose two J201 as a similar replacement. It was a mistake... When I used J201, the circuit was unstable, immediately after switching on the battery, the operating point was shifting and the preamplifier did not carry the signal... Then I realized that I needed paired J201 transistors. I bought ~100pcs J201 transistors, I measured each of them and chose a few pairs with almost identical parameters. I swapped pairs of transistors in my preamp and... WOW! It works! Unfortunately, after some time it turned out that this preamplifier was also unstable and the operating point was also shifting. After an hour of operation, the preamplifier stopped working. I was able to turn off the battery, turn it on and it worked again an hour. I analyzed the chematic, took measurements and compared it with simulations. I figured this preamp can only work with the original dual transistors, but where to get them?! I gave up the idea of building this preamplifier. It doesn't make sense, I waste a lot of hours finding out I'm doing something that can't work because there are no original parts... But now a miracle has happened! I bought a few pieces of NOS U406 on sale in the original Siliconix packaging! The fun starts all over again! I will build a new preamplifier based on original parts. I'll make it as a stompbox. I'll let you know how it turned out, keep your fingers crossed! Cheers!