Those shorting input jack wiring always confuse me... still. They're actually summing grid resistors for the 12AX7 tube's grid. Here's something you might enjoy reading: http://www.aikenamps.com/InputRes.htm I've gone with different values, other than 68k and had some interesting results. It's also fun to try the 5F1 circuit without the 25uF cathode bypass on the 12AX7 preamp tube. It's much darker, but the original 5F1 had no bypass cap. I sometimes put a 120pF bright cap across the Volume pot to compensate when clean. When overdriven the cap sounds like it's not even there, but the amp is much cleaner without the cathode cap.