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Tone Control mods

8/6/1999 12:39 PM
Tone Control mods
While working on some mods for a typical tone control used in many effects boxes, I hit upon a design that allows response similar to the classic Fender amp tone stack but with one tone knob instead of 3. As you rotate the knob it rsponds similar to going from 0/10 to 10/0 on the treble/bass knobs. Obviously it cannot duplicate all of the 3-knob settings but it's a good tool to have in the ole lab book.  
It might find some use in amp circuits so I thought I'd post the link here (since many of you tube guys may not read my effects pages). I plan on trying to incorporate it into a tube preamp that I'm designing.  
Check it out on the Lab Notebook Page at  
regards, JACK
8/7/1999 2:49 AM
Chris Hurley

Thanks for that informative article. Looks to be real useful info.  
Now if I just had an easy setup to compare these things! :)  
8/7/1999 4:04 AM
Steve A.

    Look great! Are there any changes in values that you might suggest for use in a guitar amp (like right after the initial preamp tube or two, or after a dc-coupled pair/cathode follower)? Thanks!  
Steve Ahola
8/10/1999 11:53 PM

Hi Jack,  
About a year ago I made up this little four-stage tube preamp for experimental purposes and as a learning tool. I wanted some type of tone control but didn't have room for the typical three-knob stack. So, I decided to use a single pot with a bass cap on one side and a treble cap on the other. Maybe it was a one meg pot, don't remember at the moment, but I had to put a series resistor with the bass cap and even a small resistor to ground to balance it better. If not, the bass side lets through a lot more signal and it just won't be balanced with the treble side. Of course, you knew that anyway. I may have even used an audio taper pot favoring the lesser value towards the treble, that way as you turned it towards bass, it wouldn't loose the treble so fast as in a linear pot from what I experienced.  
In the diagram at your link, maybe changing the 39k resistor on the bass side to something considerably larger would help out or take a resistor parallel with the .01uf cap to ground to get rid of some of the lower bass.  
I can tell you this much,that I experienced some darn good tones with that single pot and I will get into it more down the road. As I turned from the treble side to the bass side, somewhere in between I started to get some of the prettiest blues tones I've ever had personally. I actually placed one pot in between the first and second stage and also one between the third and fourth stage. That gave me a bit more overall control. I just want to get away from the same ole stacks, Marshall or Fender types.  
I was pleased to read your post about the subject. Hope you keep us updated. I will try to get back to my project someday soon, first have to get a couple of amps tweaked to my satisfaction.  
Take Care, T.B.  
8/11/1999 1:28 PM

Glad to hear of your positive experiences with a single knob tone control. I think it has merit in certain applications.  
The tone control shown is careful designed to pass frequencies similar to those expected with a tube amp, and are fairly balanced. A 1M tone control pot will shift the frequencies away from the design parameters.  
BTW, the input to the tone control should be ac coupled via a fairly large cap (say 0.22uF or more) and the output can have a 1M volume control connected to it if you need it at that point.  
I have another variation that I want to post but my phone has been out of order for 2 days due to a storm (summer afternoon thundershowers). Supposed to be back online today and maybe I can get some of my updates done.  
Thanks for the comments, JACK
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