R39 provides the DC ground-return path for the gate of JFET J6. Note that the crystal resonator is an open circuit for DC.
R37 is the source resistor for JFET J6. R37 sets the DC operating current for J6, and hence J6's transconductance Gm or Gfs. Watch the voltage across R37 as you vary R37, and think about J6's DC operating point.
R13 is the source resistor for JFET J3. R13 sets the DC operating current for J3, and hence J3's transconductance Gm or Gfs.
I am suspicious that 100 ohms is too small a value for R13. R42 and R35 together put +6 volts DC on the gate of J3. If the gate-source voltage Vgs of J3 is about zero, this puts 6 volts across R13. Divided by 100 ohms, this gives 60 mA DC source current in J3. This is a high zero-Vgs current Idss for a small-signal JFET, but is reasonable for low-on-resistance small switching JFET.
Check the J310 datasheet for its Idss rating and characteristic curves of Id versus Vds and Vgs. Manually derive the DC operating points of J6 and J3 for various values of R37 and R13.
I used small switching JFETs as very-low-noise audio amplifiers fifty years ago.
