My 5th great-grandfather immigrated to Russia in 1803 under the name Karl Ivanovich Jungfer. We have a good deal of records of his time in Russia, but nothing of his time before entering the country. Where would be the best places to look for records of his time in Germany (especially his birth record)?
Let me provide some context on his German name. In Russian naming convention, the middle name is a patronym. In this case, his father's name would have been Ivan. This is likely derived from the German Johann or Johannes. A possible alternative is that his name was actually Karl Johann and he adopted the middle name as a patronym.
From reviews done of him at his places of work as an instructor of German in Russia, we know he was the son of an evangelical pastor and attended the gymnasium in Bunzlau (then part of Prussian Silesia, now Bolesławiec in Poland) after his father's death. He then studied as a non-degree student at the University of Halle. Following his education, he entered the Prussian civil service and worked as a translator in Lomza (now Łomża in Poland) before becoming the polizeimeister and head of the customs department.