If you want to look for a cheaper/easier-to-find substitute, the important datasheet specs are cutoff voltage (VGSoff) & Drain-Source Saturation Current (Idss):
2SK170GR: Vp: -0.2-1.5V, Idss: 2.6-6.5mA (for the -GR variant)
Here are some similar (but not guaranteed to work) substitutes: - Through-Hole: 2SK30-GR (slightly higher Vp range), MPF4393 (higher Idss range), PF5102 (higher Idss range), J113 (higher Idss range)
- SMD: BF862 (higher Idss), 2SK3557-6 (higher Idss), CPH3910 (higher Idss)
Forgot to mention one more SMD option that I recently used in place of a 2SK170BL, the 2SK932-22: Vp: -0.2-1.4V, Idss: 7.3-12mA (for the -22 variant)
As an aside, you'll notice that a lot of low-noise JFETs have multiple suffixes, like GR/BL/V for 2SK170 or -22/-23/-24 for 2SK932. These suffixes denote different part numbers that share 99% of their specs, but vary in 1 or 2 key specs (often Vp or Idss for JFETs). To create these "bins", each individual semiconductor is tested after fabrication and sorted into spec range "bins".
As an example, for 2SK170, the different bins are low, medium, & high ranges for Idss spec:
- GR: 2.6-6.5 mA
- BL: 6.0-12 mA
- V: 10-20 mA
The other specs (Vp, Vgds, Pd, etc.) are generally the same, so a circuit designer can more reliably choose a device with a spec range that works in their application.
The requirement for spec "bins" are a side-effect of how difficult JFETs are to fab with tight/consistent spec parameters, and can make sourcing parts a bit more difficult. Depending on the circuit, you may be able to sub one similar spec range "bin" for another (like BL for GR), but there is no hard & fast rule for when it will work and when it won't. If you're using a substitute part, I'd suggest you buy 2-3x the quantity you need for your build and test/sort them for Vp & Idss. That will allow you to select individual transistors that are closest to the range of the original part.