Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MacGwyn on July 09, 2014, 03:27:26 pm

Title: 68k's needed?
Post by: MacGwyn on July 09, 2014, 03:27:26 pm

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?

Title: Re: 68k's needed?
Post by: SILVERGUN on July 09, 2014, 03:59:56 pm

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

Title: Re: 68k's needed?
Post by: HotBluePlates on July 09, 2014, 04:13:09 pm

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.