Author Topic: mxr distorsion Plus gain Pot  (Read 248 times)

stomp bomb

mxr distorsion Plus gain Pot
« on: September 21, 2019, 03:13:38 PM »
Hi guys, I'm trying to rebuild the Mxr distortion + following electro smash instructions, but I have 2 big problems:

1) until now I just breadboarded the circuit, without soldering, when I plug my guitar the distortion works, but I get an extreme noise too (especially when I increase the gain of course), do you know what the possible reason could be? do you think the reason could be that I did not solder anything? but just wrapped the wire around the jacks hole ;D?

2) in the shematic it's written to use a 1Mohm rv-log Pot, I tried, but in my opinion it's not the right one because since it's logarithmic, it feels like:
no gain- no gain- no gain- no gain- BOOM :icon_twisted:! all the gain
if you know what I mean.
what can you advice me? do you think I should replace it with a linear pot?

thanks in advance


Mark Hammer

Re: mxr distorsion Plus gain Pot
« Reply #1 on: September 21, 2019, 03:55:21 PM »
The Distortion+ requires a reverse-log gain pot.  Do not confuse this with a regular log pot.
What is the difference?

As the pot resistance gets smaller, the gain of the circuit is increased.  The resistance of that leg ranges from 4k7 when the pot is set to max gain, down to 504.7k at minimum gain.  At maximum gain, small differences in pot resistance make a bit difference in gain.  At zero ohms (plus the 4k7 from the fixed resistor) the gain is 214x.  Add in 1k from the pot, and the gain drops down to 176x.  Add in another 1k (total of 4k7 +2k), and the gain drops down to 150x.

Now let's go to the other end of the pot.  With pot resistance set to max (500k+4k7), gain is roughly 3x.  Shave 20k off the maximum resistance of the gain pot (down to 480k from 500k) and gain barely budges (3.1x).  Take off another 50k (430k), and the gain skyrockets up to 3.3x.

Putting all of that together, if one wants a gain pot that increases gain as you turn clockwise, and offers good "dialability", then you want something that moves through very big changes in resistance at the bottom end (between 7:00 and 11:00), but only very small ones near the 2:00 to 5:00 area.  And that, my friend is what a reverse-log (C taper) pot will do.  A regular log pot will be the opposite: smaller changes going counterclockwise, and bigger ones going clockwise.

Welcome on board.  You'll learn a lot here.  Good people.  Multi-lingual.  Friendly, and helpful.

stomp bomb

Re: mxr distorsion Plus gain Pot
« Reply #2 on: September 21, 2019, 04:24:33 PM »
thanks a lot, for the helpful reply.
the fact is:

I actually used a 1Mohm reverse log pot, and the thing I probably don't like is the log, because it kind of feels wrong.
I mean I only start getting gain after I turned the pot a lot


Fancy Lime

Re: mxr distorsion Plus gain Pot
« Reply #3 on: September 21, 2019, 04:36:14 PM »
Hi stomp bomb,

welcome to the mad house!

1) Very difficult to tell where noise on the breadboard comes from. There are many possibilities but for me it is most often a problem that has to do with either a mistake on my part or that some part has a bad connection to the breadboard. The fastest way to find or eliminate the problem us usually to carefully check if every part is connected to what it should be connected to and nothing else. If you are sure that that is the case, take out the parts and put them back, one by one. Like so: Take out C1, put C1 back. Noise gone? No? Take out C2, put C2 back. Noise gone? ... and so on. Works for me about 90% of the time when I have that kind of problem.

2) Mark said it already. I would add that you can use your regular 1M log pot. But you need to wire it backwards and the maximum gain will be all the way anticlockwise and minimum all the way clockwise. Probably not a good solution for the final product but works well enough for the breadboard while you wait for the delivery of the 1M rev-log pot.
EDIT: you posted while I was writing. If you used a rev-log and it behaves the way you describe, then you probably connected the wiper (middle pin of the pot) to the wrong end. Try connecting it to the other one. Common mistake, still happens to me all the time after all these years.

Cheers and rock on,
Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

anotherjim

Re: mxr distorsion Plus gain Pot
« Reply #4 on: September 21, 2019, 04:44:44 PM »
Following old advice I've seen here, I found 47k linear pot worked fine for the gain and 100k audio taper for the output volume.
The point of these changes is that the gain control is only most effective in the low resistance ranges and the linear pot becomes more usable although reverse-log would still be ideal, the ordinary linear pot works well enough.
With the original 10k volume control, the signal at the clipping diodes is reduced and the effective sustain becomes shortened. Raising it to 100k brings an audible improvement.
More umph can be got by raising C3. Try 100nF.
"Designing with PLLs in production circuits is reserved to those with the courage of the thoroughly uninformed."
Nuts & Volts
"Turtles have the short legs, not for the walking."
-Kenji "Damo" Suzuki

stomp bomb

Re: mxr distorsion Plus gain Pot
« Reply #5 on: September 21, 2019, 04:49:47 PM »
thank you all for the great advices, and Fancy lime thanks too I think I wired the pot to the wrong pin :icon_biggrin:

Mark Hammer

Re: mxr distorsion Plus gain Pot
« Reply #6 on: September 21, 2019, 05:30:04 PM »
Given that you ARE using a C-taper pot, a wiring error would be my guess as well.

The Dist+ lacked the things that would curb hiss and fizz.  It needs a small value cap in parallel with the 1M feedback resistor. 47pf is about right.  The .001uf cap in parallel with the diodes, rolls off highs starting around 16khz, which doesn't do much for hiss.  Kicking that up to .0027 to .0033 will killa lot of the hiss and yield a warmer tone.

willienillie

Re: mxr distorsion Plus gain Pot
« Reply #7 on: September 21, 2019, 06:58:34 PM »
With the original 10k volume control...

After seeing discrepancies among various schematics, I spent way too many hours one night looking into the pot values for the D+.  Seems the earlier "script" models used 500K for gain and 50K for volume, but I could never 100% verify that because the pots they used had part numbers without resistance values stamped on them.  Maybe starting around 1980 or so, the pots were changed to 1M and 10K.  The value of the gain pot doesn't really matter, the 1M is just able to get a little cleaner than 500K, but like you said the volume pot value does make a difference.

FWIW, the 50K volume pot actually measures right around 10K in circuit, this may have added to the confusion over the years, from people who own originals but don't want to unsolder anything.

(Edit:  It's probably not just a coincidence that the "script" MXR Dyna Comp also used C500K and A50K pots.)

(Edit 2:  Looking at some pics online, seems for at least a period in the mid- to late-70s, the part numbers stamped on the pots were MXR9471 for A50K and MXR9479 for C500K.  You can see these part numbers in gut shots of both Dyna Comp and Dist+ pedals.)
« Last Edit: September 21, 2019, 07:31:04 PM by willienillie »

Rob Strand

Re: mxr distorsion Plus gain Pot
« Reply #8 on: September 21, 2019, 07:40:58 PM »
FWIW, the original pots were 500kC (reverse log taper) and 50kA (audio taper).

The 1MC pot is part of the problem why you unit is harder to adjust.

The problem originates from the markings on the  pots from 1980 onward.
100-1013-103    is 50kA  not 10k
100-1013-105     is 500kC not 1MEG

These numbers are MXR part numbers; 100- means an electronic part.  If you look at MXR's original schematics you can see the same format part numbers for PCBs, drawings etc.   The confusing thing is the suffix.  The 3 digit code looks like a resistor value but it's just a number without a meaning.

For earlier models, pre-1980, MXR used a different part number scheme.  The Distortion pot was marked "MXR 9469" which is the same pot used on the Phase 45 and Phase 90, which we know is 500kC.  It's also used on the MXR envelope filter (in fact you will find 100-1013-105 in the later Envelope filters)
« Last Edit: September 21, 2019, 07:43:24 PM by Rob Strand »
The mind often distorts without gain.

Mark Hammer

Re: mxr distorsion Plus gain Pot
« Reply #9 on: September 21, 2019, 08:12:26 PM »
These days, if I build one, I just use a 100k gain pot.  When they first came out, I suppose some might have considered the ability to get gains below 10x, with negligible clipping, a sort of perk; like a Micro-Amp for free.  But honestly, the LM741 never did clean boost particularly well, so why chase after it? A 100k pot (again, reverse-log preferred) starts you off at a gain of 10.5x, and doesn't waste pot rotation on things that don't matter or that you'll never use.  I understand the desire to make something authentic, but I see little value in replicating the shortcomings of the original.  Make use of what it does well, and leave the rest on the bench.
One man's opinion.
A 50k Volume pot will get you more output, and 100k even more, although I think there will be some loading issues with 100k.
Note that the 10k series resistor before the volume pot works in conjunction with it, such that - with a 10k pot - the two behave like a 20k pot that can never be turned up beyond halfway.  10k + 50k pot behaves like a 60k pot that can never be turned up beyond 5/6 of maximum.  Not perfect, but noticeably better.
« Last Edit: September 21, 2019, 08:17:33 PM by Mark Hammer »

willienillie

Re: mxr distorsion Plus gain Pot
« Reply #10 on: September 21, 2019, 09:12:39 PM »
100-1013-103    is 50kA  not 10k
100-1013-105     is 500kC not 1MEG

...

...The 3 digit code looks like a resistor value...

Yeah that got me too.  Thanks for clarifying.

Quote
The Distortion pot was marked "MXR 9469" which is the same pot used on the Phase 45 and Phase 90

Okay, now I'm confused again.  I see MXR9469 on an old Phase 90 pot, but MXR9479 in pics of D+ and Dyna Comps.

examples (large pics):

1975 Dist+
https://ams3.digitaloceanspaces.com/soundgas/2019/02/mxr_dist_1-1-2000x1125.jpg

1976 Dyna Comp
https://auctions.c.yimg.jp/images.auctions.yahoo.co.jp/image/dr000/auc0508/users/da61c76ea123133da9a0b2abbaea143852e44d70/i-img900x1200-1566862856aswkcs25265.jpg

Rob Strand

Re: mxr distorsion Plus gain Pot
« Reply #11 on: September 21, 2019, 09:48:27 PM »
Quote
Okay, now I'm confused again.  I see MXR9469 on an old Phase 90 pot, but MXR9479 in pics of D+ and Dyna Comps.
It's very confusing.  I've spent many hours trying to resolve it and there's a couple of holes that still need plugging.

The very early Phase 45, Phase 90, and in fact Distortion+ used the MXR9469.
It later changed to MXR9479 I'm not sure of the exact year but it was around 1977.
AFAIK, the Micro Amp, Envelope Filter, Dynacomp, Noise gate, Phase 100 only used the  MXR9479.

I don't know what the difference is between MXR9469 and MXR9479.   I suspect the taper was changed (maybe the percentage of resistance at 12:00)  since a lot of early Phase 90's are slower at the 12 O'Clock position.   There's a small possibility the MXR9469 was 1M but I have my doubts.

If anyone can fill in this gap please post it!

There's actually quite a few variants of the Phase 90 (maybe two variants of the Phase 45).  The PCBs actually have different version numbers.   One Phase 90 variant has the colors of the wires swapped on the Speed pot.

The other one to be resolved is the  "MXR 9470" which is the Blend on the Blue Box.  I'm not convinced the value and taper have been resolved despite the values on web schematic and the original schematic.  AFIK there's no "100-1013-xxx" pot version of the Blue Box since I think they stopped making it after 1980.
-------------------
Ah, another one.   The era of Distortion + that used the  MXR9469 for the Distortion pot used the "MXR9741" pot for the Level pot.   So this make me think it *is* possible for 1MEG + 10k version of the Distortion+ to have existed.   My other thought on this is the "MXR9741" was a stuffed up numbering of the "MXR9471" which is the 50kA pot used on later models.
« Last Edit: September 21, 2019, 10:57:04 PM by Rob Strand »
The mind often distorts without gain.

willienillie

Re: mxr distorsion Plus gain Pot
« Reply #12 on: September 22, 2019, 12:19:03 AM »
I suspect the taper was changed (maybe the percentage of resistance at 12:00)

This would make sense.  We have 10% and 30% audio taper, I guess we could have 70% and 90% reverse audio too.

Quote
MXR9741...MXR9471

Now you have me thinking I saw both of those numbers too when looking at pics earlier.  Maybe 10% vs 30% audio taper?  I guess we'll never know for sure.

Rob Strand

Re: mxr distorsion Plus gain Pot
« Reply #13 on: September 22, 2019, 01:25:14 AM »
Quote
I guess we'll never know for sure.
Yes it's impossible to tell without someone measuring it.

I've only found one post which implies the older output pot, MXR9741, is also 50kA.

Back in 2003 DanN said he had unit but it might have been built up from parts.
However he measured the  MXR9741 as a 50kA pot
"Dist+,  1975
pot #1 (50K A) says MXR9741 1377**4 (*'s covered by solder blob)
pot #2 (500K Reverse A) says MXR9479 1377551
"
So even though the pedal might be built-up the pot itself is 50kA.

I went through some stuff and I've narrowed down the dates for the change in pot numbers
to early to mid 1975.   A bit earlier than I thought.

Appears on: Scripts and Bud boxes
MXR 9741         1377451
MXR 9469         1377451   

Appears on:  Scripts and some Bud boxes, and anything after
MXR 9 471    1377524      50kA
MXR 9479       1377537      500kC

So the MXR 9469 is the only one we have no information about.
« Last Edit: September 22, 2019, 06:08:31 PM by Rob Strand »
The mind often distorts without gain.