MXR Distortion Plus Questions

[john1056]

Hello,

I just completed the MXR Distortion Plus circuit from Tonepad.  Everything works, but the distortion only seems to really kick in at the extreme range of the pot.  I've actually never played through a "real" Distortion Plus.  Is this the way the distortion should behave?  If not, what could I have screwed up to cause this?  

Also, I was wondering if anyone has built this project with germanium diodes.  I noticed that Small bear carries the 1N270 diodes, and the mods section of diystomp says that the germs are less harsh than the diodes Tonepad calls for.  I was able to take the harshness out with a cap over the output pot terminals, but I thought it would be interesting to try to germ diodes to see if there is any difference.

Thanks,

John

[RDV]

The reason for the distortion occuring in the last bit of the pot is because of the taper of the pot that you used. A reverse audio taper pot will give a more gradual climb into distortion, or you can wire a regular audio pot backwards(you'll just have to think backwards when you turn it) to get the same thing. I left mine as is because it's not too big a deal to me.

The diodes are purely a matter of preference. Some are quite happy with Si diodes in the Dist+. Others(like me) prefer Ge diodes for their smooth sound. Socket them and have fun figuring it out for yourself.

Regards

RDV

[petemoore]

I'm at the xact same point of Dist+ building.
 I used a 'regular' 1 meg [not the reverse taper specified, and the Dist level can be adjusted through it's proper range [It would seem, because it gets extreme] but most of the change in the range is at the very end on the pots travel...N/P as far as I'm concerned, a different taper pot would certainly provide a different taper to the knob.
 Yes diodes...Ge types are probably going to cut output level and provide harder [closer to the center of the linear swing] clipping.
 Well mine of course has a 8pin IC used as diode/cap to ground socket. Also the board is well secured in the shallow box, I'll be trying out different diodes for another day or so I imagine...I like to play and retune after each diode swap that doesn't immediately sound like S.
 I just posted a general question about what specific diodes are like in the Dist+ by others...titles Dist + Diodes...it should still be on the BYOSB forums page 1...

[Jim Jones]

If you use a 100K linear taper pot for gain you don't notice the taper issues at all.  The 1M isn't necessary in my opinion, maximum gain is achieved when the pot is reading 0 ohms resistance.  The only thing you're gaining by using a 1M is more "clean" room on the gain pot - now why would you want that on a distortion pedal?  

Jim

[RDV]

The only thing you're gaining by using a 1M is more "clean" room on the gain pot - now why would you want that on a distortion pedal?  

The way a lot of people use them is for a semi-clean boost(even though I don't) which is why you would want to be able to reduce the gain that far.

RDV

[Jim Jones]

I can't imagine a Distortion + sounding very good in that application, RDV?  Although, as with anything music-related "to each is own" I guess.  

Jim

[petemoore]

The main reason for Dist+ IMO is it's internally generated distortion.
 I tried it with the gain pot all the way down, part way down etc. just seemed like a dull sounding booster. Cool, usable [I guess] but not all that desirable to 'go after'.

[RDV]

I can't imagine a Distortion + sounding very good in that application, RDV?  Although, as with anything music-related "to each is own" I guess.  

Maybe the term semi-clean booster is not right, what people do is use it as a distortion booster or lead booster, with the gain down & volume up. Randy Rhodes, & Yngwie Malmsteen are two of the more famous users who do(did) this. It works well, actually.

RDV

[Jim Jones]

I just don't see it working very in that capacity at all - it's not like the Distortion + is a "loud" pedal - there just doesn't seem to be enough output.

Randy Rhoads - what a killer talent, but one of the worst guitar tones in history...

Jim

[Fret Wire]

The most common pot values were 500k rev. log and 50k log. Mxr did put some out with 1m and 10k, though you won't get much volume with the 10k. You will definately hear a difference between ge and si diodes. When they're at spec, you'll even hear a slight difference between the 1N34a's and the 1N270 Ge's. Even an MXR reissue will benefit from replacing the si diodes with the above Ge's. I guess I'm one of the few that likes the original circuit values (including the 741 IC). I just sub metal film for the resistors, tants, and ceramics, but keep all vaues original.

If you want to really want to experiment, get the duel layout board (Dist.+/MicroAmp) Tonepad has. It uses the 4558 dual, so you can to town subbing IC's and diodes.

Jameco has 1N270's very reasonable:
http://www.jameco.com/cgi-bin/ncommerce3/ProductDisplay?prmenbr=91&prrfnbr=1340&cgrfnbr=697&ctgys=503;520;697;

[RDV]

I just don't see it working very in that capacity at all - it's not like the Distortion + is a "loud" pedal - there just doesn't seem to be enough output.

I used 500K/100K.
Mine boosts like crazy if you crank both knobs! :wink:
For boost mode just take the volume ALL the way up, and bring up the gain until something starts smoking and there you are. :shock:

RDV

[john1056]

Thanks so much for the responses, I have many options it seems...

I think the first thing I'll try is putting a 100,000 K resistor in parallel with the gain pot, this should make the range from 0 to ~90000K which should give me the behavor Jim described.  If this isn't what I'm looking for, I'll try a 500K reverse audio pot.  

One thing I didn't understand from RDV's post was using a regular audio pot.  Wouldn't this make things worse?  Isn't the change in resistance fastest (per amount turned) near 0 ohms (max gain in this case)?

I'm going to check out the germ diodes too.

Thanks,
John

[RDV]

One thing I didn't understand from RDV's post was using a regular audio pot. Wouldn't this make things worse?

I'll see if I can clarify.
If you wire a audio taper pot Backwards, then you will have a reverse audio taper, only the pot will be backwards, it will go down as you turn it up(and visa-versa).

Also read this:
http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

RDV

[john1056]

RDV,

Thanks for your post, I'm definitely going to read that article when I get a chance.  Looking at the diagram of the 4 different pot styles, it looks like the audio taper would be the one that would fit this application.  From 0 - 1/2 turn the resistance increases very slowly, so if this pot was put in backwards, as you suggest, it would give good sensitivity to the small area where distortion begins (near 0 ohms).

The reverse audio looks like in would always be fast near 0 ohms, so it would never provide good sensitivity right?  Or maybe my brain's just not working properly  

Anyway thanks, this will probably be clearer to me when I get a chance to read the article!

John

[john1056]

Nevermind, I think I understand.  I was thinking "backwards" in terms of which way the dial would turn on the pedal, but I think you mean that wiring from the wiper to lug 3 will always be the inverse of lug1 to the wiper.

[Fret Wire]

The reverse audio looks like in would always be fast near 0 ohms, so it would never provide good sensitivity right?  Or maybe my brain's just not working properly  
John

No, the reverse log was the original spec (500K). Remember, this is a simple circuit, so it's been modded to death. Now, many of the personal mods are now listed as original. 500k rev. log and 50k log are the original spec values. Someone reversed engineered a unit with 1m and 10k pots, and now, most every schematic shows those values. In other words, they were just re-copied. They (1m & 10k) were not the most consistant value used. A small company like MXR used a few different values because of what was available at the time.

[john1056]

I tried the 100K resister in parallel with the 1M pot last night by sticking the resister leads in the little holes in the pot lugs of the 1M pot (to adjust the pot range to ~100K).  The result was a surprising high pitch squeal.  I can't figure out why this would happen since the 100K resister should only be able to alter the circuit by lowering the resistance.  Since the pot can go to 0, it's strange that this is a problem.  Maybe I shouldn't be sticking things in those little holes?  I used the little holes a few nights ago to experiment with cap values to cut the high end in the volume pot (as described in the mods section).  That seemed ok.

[john1056]

No, the reverse log was the original spec (500K). Remember, this is a simple circuit, so it's been modded to death. Now, many of the personal mods are now listed as original. 500k rev. log and 50k log are the original spec values. Someone reversed engineered a unit with 1m and 10k pots, and now, most every schematic shows those values. In other words, they were just re-copied. They (1m & 10k) were not the most consistant value used. A small company like MXR used a few different values because of what was available at the time.

I think I understand why reverse is right for this circuit.  If you take a reverse log pot and wire it between the wiper and lug3 you get the desired behavior.  With the knob turned full left you have max resistance (500K).  In the middle (12:00) you are already past 90% of the total resistance (based on the diagram in the secret life of pots), so the resistance would be < 50K.  From this point to full right the resistance would move very quickly to 0.

[nightingale]

just a tip~
try red LED's for clippers... (fairchild i beleive)... it brushes on tht subject @ GGG, in the OD 250 page...

it is actually alot smoother than SI diodes in my opinion... i am a low gain tone wizard type..? and i went with the red leds in mine.

also~
do a search mark hammer shed some pretty good light on the dist+ pot taper about a year ago..
hth,

[Davide]

I've used a 500K linear pot for gain... and i hear something only at maximum gain :/...  from 1% to 99% of the pot = *nothing*
The 100k log pot i've used for volume seems to work.. but at max the volume is veeery low... i have to turn drastically up amp volume to hear something...
please.. help me
when i bypass it.. it works perfectly.. so i think jacks wiring is right..

oh i've used DOD250 schematic with YJM308 variations..