[cctalk] Replacements for DEC 3639

It seems that nowadays you can't get *any* replacement for failing DEC 3639 aka 2N3639 transistors. All parts are obsolete and unobtanium, e.g. 2N3640, PN3640, MMBT3640 and so on. So, what can be used instead? The most important electrical parameter is the storage time. It needs to be *very* low, around 20-30 ns. Does it mean that a failing PDP-8 will stay a dead PDP-8 from now on? Christian