68k's needed?

[MacGwyn]

I've noticed that when amp designers want to combine two input jacks to one side of a preamp tube, they often combine them through 68k resistors before hitting pin 2 or 7 (12ax7).   What do those resistors do... and can you get more gain by reducing their rating or eliminating them?

[SILVERGUN]

http://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

[HotBluePlates]

Do a search on the forum. There's at least 3 different function those resistors serve, and acting as grid stoppers (as described in the Aiken article) is but one of the reasons.