7f33ac1a_ecp_cc__Value_Notes
[Part] R3L, R3R [Size/#] Bourns 3296W [Value/Notes] 
These are used to set the tube's bias. How to do this is explained elsewhere, but what I suggest is that you set the trimmer to about where you think it should be (by doing the math), then adjust from there. The Bourns 3296W series fits, I would think that 2K would be a decent value to try. Better to be too high here than too low as you can always adjust down.
ecp.cc
Value/Notes