Edit: I should have had another cup of coffee before asking this. The AC impedance is 12k parallel with 12k + some impedance from the bypass cap. Roughly half of 12k.
I found a schematic for the JHS Morning Glory (Marshall Bluesbreaker based OD) online but this part of the schematic is a head scratcher. It's a JFET common source gain stage but as drawn there is no reason for any current to pass through the circled part short of a lightning strike. The bypass/AC path has a greater impedance than the DC path.
I'm *guessing* that one of two things has happened. Either the DC JFET source resistor had to be reduced to properly bias the JFET and the bypass resistor wasn't changed to match, or whoever traced the board messed up on the values. Not that it matters, I would re-bias the JFET anyway but I'm curious about the intended ratio of resistance between the DC path and the bypass path.
(http://i.imgur.com/Cv1TNvM.jpg)
R18 simply limits the effect of c14 while still allowing all frequencies to be amplified.
It adds about 3dB and it's pretty pointless to include those two components ... You could redo the FET's source resistor instead if you want to add gain.
Can i sub 2N5457 to J112/J113/BF245C?