Distortion + build problems

[dan5150]

I just finished building the Distortion + based on the PCB from generalguitargadgets.com and as documented in Brian's book DIY how to build effects pedals.

My only changes are that I left C5 out. (directions said it was optional) and I used a DPDT switch. (so I don't have an led).

So far the pedal seems to work fine, except the gain knob must be cranked to get any distortion. If I back it off to about "8" it totally cleans up. (and it remains clean from 1-8)

Since other than that, the pedal seems to work just fine.

Can anyone suggest a good place to start looking at why it behavies this way?

Has anyone successfully built this pedal based on GGG PCB and parts list?

Thanks,

-Dan-

[Mark Hammer]

My only changes are that I left C5 out. (directions said it was optional) and I used a DPDT switch. (so I don't have an led).

So far the pedal seems to work fine, except the gain knob must be cranked to get any distortion. If I back it off to about "8" it totally cleans up. (and it remains clean from 1-8)

Normally, there are only noticeable changes in gain with settings of the gain pot below about 50k or so.  That's why the original used an exotic taper on the drive pot, so that you could speed past the first 450k (if a 500k pot) or 950k (if a 1M pot) of resistance as quickly as possible, and have that remaining resistance distributed across more of the pot and the control's visual range.  What I recommend to people is simply to use a 50k or 100k pot for drive/gain.  Yeah it won't be an "exact replica", but much more of the pot's range will actually be useful to you and you won't have to drive yourself crazy getting the "right" taper.

You can do the math for yourself and realize why it is like this.  The gain of that stage is given by the sum of the feedback resistance plus the resistance to ground, divided by the resistance to ground.  So, that's 1M plus 4k7, plus whatever the pot is set to, divided by 4k7 plus whatever the pot is set to.  If the pot is set to 500k, that's a gain of [1M+500k+4k7] / [500k+4k7] = 2.98.  If the pot is set to 250k, that's a gain of [1M+250k+4k7] / [250k+4k7] = 4.93.  A little louder but nowhere near the gain needed to produce clipping from those diodes.  Down to 100k, and we have a gain of  10.55.  Down to 50k and we have a gain of 19.28.  Down to 20k and we have a gain of 41.49.  You can see that the big changes in gain setting really only occur within the last 50k or so, and huge changes in resistance above that produce negligible differences in gain.

So, your pedal is behaving absolutely normally....for a design that needed improvements in the first place.

[dan5150]

Mark,

Thanks for the great explaination!

Last night I swapped in a 250k (I had an old one in my parts drawer) and I noticed the difference, it is much closer to what I was looking for. Now I am off to find a 50k and a 100k to see which I like better.

Thanks again!

-Dan-