7f33ac1a_ecp_cc__Value_Notes
[Part] R1L, R1R [Size/#] 1/8W is fine [Value/Notes] 
These are the so-called grid leak resistors. However, in this design the pot already does that. These really offer a connection for the grid to ground in case something goes wrong with the pot. Should be 10X the pot value, so probably 500K is a good value.
ecp.cc
Value/Notes