This is why I hate watching people spend on a specific part. On another forum I saw someone pay £30 for a basic TO3 pass transistor because it had a different number to 2n3055 written in it...
My Studer mixer has BC309b transistors, according to schematic. I don't. I only have BC550 and 560 in my stash. I reluctantly was prepared to substitute the suspect BC309b with a BC560, after reading up on it being possible (a bit more study than "it's the same symbol and the same casing shape" actually
). My hesitation swiftly disappeared when I discovered a Siemens BC560 already there, and no, that was no bodge, that was the original from factory.
With some careful design (like building and making it work on a breadboard first
) one can make many circuits pretty foolproof wrt component choice. And the Studer 169 input module is the stuff of dreams for some recording audiophools (they perform all kinds of ritualistic mods to it.), so it apparently works well too. I can confirm, it is a good mixer, although a bit limited in features.