Questions about 68k grid stopper

Larry F
May 18th, 2007, 11:06 PM
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?

May 19th, 2007, 07:34 AM
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.

May 19th, 2007, 08:40 AM
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.

May 19th, 2007, 01:52 PM
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"...