Questions about 68k grid stopper - Telecaster Guitar Forum
The Number 1 Fender Telecaster Guitar authority in the world.
   

Go Back   Telecaster Guitar Forum > Amplifier Discussion Forums > Amp Central Station
Forgot Username/Password? Join Us!
Notices

Amp Central Station Amps, tubes, speakers & everything AMP related.


Wilde Pickups by Bill & Becky Lawrence WD Music Products Amplified Parts Mod Kits DIY Amps, Mods, Pedals dallenpickups.com Warmoth.com seymourduncan.com


Forum Jump


Reply
 
Thread Tools
Old May 18th, 2007, 11:06 PM   #1 (permalink)
Doctor of Teleocity
 
Larry F's Avatar
 
Join Date: Nov 2006
Location: Iowa City, IA
Posts: 10,837
Questions about 68k grid stopper

I'm new to amp building. I see that a 68k resistor is commonly used between the input and pin 2 of the preamp 12AX7. It is described as a "grid stopper" and is said to act as a low-pass filter. What does "grid stopper" mean in this context? What is the approximate cut-off frequency? What differences are there between carbon comp, carbon film, and metal film for this resistor? Additionally, if an amp has two inputs, each going to a 68k resistor, would the sound be improved if the second input (and resistor) was removed from the circuit?

__________________
Check out my new book on Amazon: 2000 Blues Licks That Rock!
Larry F is offline   Reply With Quote

Old May 19th, 2007, 07:34 AM   #2 (permalink)
Tele-Afflicted
 
DrewB's Avatar
 
Join Date: Feb 2005
Location: Independence, MO
Posts: 1,598
Grid stopper resistors are intended to suppress parasitic oscillations in tubes. The frequency cutoff is extremely high and is above the fundamental frequencies of a guitar. As to the differences between the various resistor compositions, carbon comp is the noisiest but, it certain parts of the circuit, can lend some "musical distortion" to the overall sound. Carbon films are quieter and a good compromise between the noisier carbon comps and the quietest, metal film. I can't speak to resistor composition's effect in this position in the circuit.
__________________
There is no substitute for Sound Pressure Level
DrewB is offline   Reply With Quote

Old May 19th, 2007, 08:40 AM   #3 (permalink)
Tele-Meister
 
Join Date: Oct 2006
Location: Wake Forest, NC
Age: 53
Posts: 408
The 68k resistor on the input stage grid is there to filter out radio signals, this in combination with the grid input capacitance makes a low pass filter above the amp's audio range, but below the AM band. It helps if you happen to be close to a AM antenna (My wife's brother use to have a house near the WOWO antenna in Fort Wayne, IN and he would get radio on various bits of electronics in his house amps, phones, etc).

On later stages grid stoppers are used to prevent oscillations in the tube or to prevent blocking distortion. Aiken has a good page on all of this: http://www.aikenamps.com/InputRes.htm

Now, since you are talking about building your own amp you should also know that the 68k input resistor is also a source of 60Hz hum/ 120Hz buzz. It is a fairly high resistance in a section with low signal currents so it is susceptible to induced noise from your power sections (a little induced current over this resistance makes a large voltage change compared to your guitar signal). Sometimes you will find that you can reduce hum if you remove this resistor or reduce it to say 10k, if your not sitting under an AM tower your probably OK without it, if your Fender and want to sell as many amps as possible you probably want it. There is quite a bit of discussion on this on the AX84.com forum.

If your making a kit and the resistor is in the design the layout probably minimizes the hum effects if your doing it all from scratch its something you should watch.
BottyGuy is offline   Reply With Quote

Sponsored Links   #
Sponsored posting
 

Old May 19th, 2007, 01:52 PM   #4 (permalink)
Poster Extraordinaire
 
jhundt's Avatar
 
Join Date: Mar 2003
Location: Netherlands
Age: 60
Posts: 7,548
great answer BottyGuy. I have put together several amps from old parts and dumpster finds. I never really understood the purpose of this resistor. Sometimes I left it out, sometimes I did 1/2 value. Neither variation seemed to affect my sound... but then again, my friends always referred to my home-builds with cute little names like "Buzz-Bomb", and "Three-Mile Island"...
jhundt is offline   Reply With Quote

Reply

Bookmarks

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Forum Jump


» Random Photo for Guests
Day of the Dead Tele
Untitled Document



 


Powered by vBadvanced CMPS v3.2.2



IMPORTANT:Treat everyone here with respect, no matter how difficult! No sex, drug, political, religion or hate discussion permitted here.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Content Relevant URLs by vBSEO 3.6.0 RC 2
© TDPRI.COM 1999 - 2014 All rights reserved.