How to use Jfet?

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I've bought 2sk30Y, I intended to use it for differential pair.
In the original sch, the ccs feeding the differential is about 5.5mA.
But when I look at datasheet and check the device (by connecting D-S together and measure the current with current meter with 12VDC supply) is only 2mA. This is Idss (max drain current) that I'm measuring.

If the mosfets are 2mA max, how many mA should be in the differential's CCS? I read that with positive VGS Jfet acts like bipolars (sucking current from Gate?)

Many shematics write Jfets without specification about the type of Jfet (Y, GR, BL, V)? How to choose which one for certain schematic?
 
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