Strange Distortion+ behaviour

[otis_soulman]

Hallo everybody!
First of all, sorry for my english! I'm an italian diyer. 
I have built my first pedal, a MXR Distortion+ using veroboard layout.
I used 1N4001 diodes, because I had only that kind. Moreover, I used a 500k log pot for the "Drive" control instead of 1M log pot, as suggested by Tonepad.
It works, but I experience a strange "distortion gain jump". I mean that the distortion degree raise (very) slowly when I turn the "Dist" pot (as it should be!), but, when the pot is almost turned fully clockwise, the gain jumps up suddenly. It's a very big jump, something like passing from a TS9 to a Metal Zone in 2 o 3 millimeters of pot rotation! 
I have never played through a Distortion+, but I don't believe that this is a normal behaviour...
Is it due to the diodes I used? Or to the different "Drive" pot?
Thanks!
    Otis

[comfortably_numb]

I imagine what you have is a linear taper "gain" pot.  What this does is bunch up all of the gain change at one end of the pot's rotation.  What you want is a reverse audio, or reverse log pot (same thing, different name).  This will have a more even distribution of apparent gain change.

[GibsonGM]

This is very common, Otis.  Like Comfortably Numb said, a reverse log taper pot would solve your problem.  This happens in many of my circuits -- I usually just live with it (I use my Dist+ very close to 100% gain  )  So, it doesn't bother me very much.  But for a sensitive circuit, you may need some more control.  Small Bear Electronics carries the correct type of pot, I believe.  You could also visit the GeoFX site in the links and read "The Secret Life of Pots".  There is a way to simulate that kind of pot.
Your English is very good )

[Cardboard Tube Samurai]

Nice work on the English by the way!!!

You could always try and use a 500k log pot instead of the 1m. That way you would get the effect starting earlier in the turn. Give it a try before you go looking for the reverse log pot. All I know is, I have had a hell of a time trying to find a reverse log pot anywhere here.

[petemoore]

  MAX gain is O resistance through that pot.
  There are plenty of options that are 'usable'
  I found a 100k pot to deliver plenty of gain control.
  Then I found no gain control able to deliver the gain settings I want, guitar volume and preboost volumes are still used...and the pot space now holds a simple LPFilter tone control pot.

[cortezthekiller]

I read an old Craig Anderton article in Guitar Player magazine on using pots to pan between pickups instead of selecting them.  what does this have to do with anything? well, he also explained how to set up linear pots as audio.  Simply connect a resistor to lug 1 & 2 (reverse log) or 2 & 3 (log). the resistor should be about 20-25% of the pot value. ie: 1M log pot with a 220k resistor between 1 & 2 = rev log.  i think.

[GibsonGM]

You should read up on that Geo article, it lays it all out.   I also used a 100K linear taper on my Dist+, works fine for me for the rare times I want to bring it back a little.  Not a ton of control, tho.  I'm going to actually TRY putting the resistor on lugs 2&3, lol - I've only read about it but didn't explore that possibility...

[billings]

Note that instead of using a reverse log pot you can also wire a log pot backwards, so that you get max volume with the knob to the left and min with it to the right. 

And when I tried using the parallel resistor trick to get a reverse log pot out of a linear pot, it looked like it would not work unless the pot were wired as a variable resistor with zero resistance with the knob at zero.  In the Dist+ you have maximum resistance with the knob at zero, so this actually does the exact opposite of what you want to do. 

I don't think there's any way around buying a reverse log pot if you want it to sound smooth and you don't want a reversed control.

[Mark Hammer]

The gain of a non-inverting op-amp (Like the Distortion+ uses) is calculated by  Feedback-plus-ground-leg-resistance / ground-leg-resistance.

In this case the feedback resistance is 1meg, and the ground leg resistance is 4k7 plus whatever the gain pot is set to.  If the gain pot is a value of 1meg, then gain = [1meg+(1meg+4k7)] / [1meg+4k7] = approximately 2.

Reduce the pot resistance to 500k (half).  Gain is now (1meg+500k+4k7)/(500k+4k7) = 2.98

Reduce the pot resistance to 250k (cut by half again).  Gain is now (1meg+250k+4k7) / (250k+4k7) = 4.92

Reduce the pot resistance even more to 100k (1/10 of original value).  Gain is now 1104k7/104k7 = 10.55

Here we have covered 90% of the possible range of resistance values on a 1meg pot and we have only changed the gain slightly.  With a gain of 10.5 there is still not enough gain to make the guitar signal clip (although the LM741 clips a little bit on its own).  Because so much of the pot does not do very much, MXR used a special "tapered" pot called reverse log.  This taper results in very rapid changes in resistance for a little bit of clockwise rotation, so that the user gets from 1meg resistance to 100k resistance very quickly.

As petemoore correctly pointed out, most of the useful part of the pot is in the last 10%.  If you simply use a 100k pot, you will not be able to achieve lower gain "clean boost" settings like the original did, but the 9:00, 10:00, and 11:00 settings on the gain control will be useful and different from each other.  You will have access to ALL the distortion settings, just not the clean settings.

My theory is that when MXR came out with the pedal, very few players had a booster.  So, even though it only had 2 knobs and one switch, the Dist+ could be a sort of clean booster (although, again, it never really cleans up) and distortion in one pedal, and more attractive to consumers because of that.  Although there was not a huge delay between them, the Microamp did not come out until some time after the Distortion+ was released (well, at least I did not see any advertisements for the Microamp until at least 18 months after the Dist+ came out, and maybe even longer than that....it was much easier to keep track of the industry at that time because there were very few places to advertise).  So, I think that MXR attempted to design the Dist+ to perform two functions by using a gain pot with a taper that allowed it to use the low gain range and high gain range without pushing one or the other into a small arc of rotation.  I believe that if the Dist+ and the Microamp had been released at the same time, MXR probably would have just allowed the Dist+ to be only a distortion pedal, and used a 100k pot.

[343 Salty Beans]

and mark comes to the rescue with an answer that boggles us all

[burnt fingers]

Mark you are a tollerant man.  I know I've seen you post the reply to this question a dozen times.  Maybe we can get this one stickied so it's always there for the reading.

Scott

[Mark Hammer]

In view of the number of queries regarding both the Dist+ and the TS-808/9, there ought to be an FAQ created specifically for each of those.

[MartyMart]

I had all kinds of similar problems with Dist+/Liquid Drive and Ross Dist.

I put up with it for waaay too long !

Ordered a bunch of rev Log pots from smallbear and changed all of the circuits
to include these ( P.I.T.A )

Perfect, all have a lovely gain range now 

Like "Nike" -  JUST DO IT !

( BTW  I agree , Mark you have the patience of a saint !! )