Anyone looking for a replacement tip for a P6139A:
Cal Test Electronics CT3667 is a perfect alternate.
$22.50 on Mouser.
I just got two, compared performance (rise time and ability to compensate)
and it is a perfect match. I used my PG506 to measure rise-time, and when I
superimposed an original 6139A with the replacement version they are dead on
top of each other. (using a 1GHz scope).
This is great new for use P6139A fans!