2N6027/6028 is the current generation of parts and available from
Mouser, Digikey, etc.
2N4870/4871 and 2N2646/2647's are previous generations (but still
available from Newark, etc.)
Tim.
2N4870/1 are not PUTs, they're UJTs. There was an even older one that
was popular, but I can't remember the part number.. 2N6246? Ah,
2N2646. UJTs have a chunk of intrinsic Silicon between the two bases,
wheras the PUTs are essentially an upside-down SCR. The application
circuits are almost the same except the PUT needs two resistors to set
the trigger point (and the parallel resistance of the two resistors
sets the valley current).
Best regards,
Spehro Pefhany