Building 250 Distortion. Question about cap value (47nf)

[D1chotomy]

Hi Guys,

I am building the DOD 250 clone and ran across an issue, I thought I had all the parts but it turns out that I am lacking the 47nf capacitor for C3. What else could I get by using here, or do I just need to wait until I have the right cap? I really love this circuit and still have the GGG kit I bought over 10 years ago on my board so I am excited to build another one to see if they (hopefully) sound the same.

Also, I socketed the diodes so I can play around with those. Could LED's work there?

After I get a few more builds under my belt, I'm gonna try two drive effects in one box. Not sure which ones yet, but I love stacking drives so having a low gain drive and then being able to kick on a hotter circuit, then turning the effect off with one button instead of tap dancing is appealing.

Sorry, I got off topic. Thanks in advance for any help you can provide.

 

[Willybomb]

You can stick whatever diodes you like in it. I liked BAT41 (might have been 46?) to ground with back to back red LEDs across pins 1 and 3 of the gain pot. I'm mostly going off the Dist+ here though, but it's much the same thing. I found that some worked better than others.

 

[D1chotomy]

You can stick whatever diodes you like in it. I liked BAT41 (might have been 46?) to ground with back to back red LEDs across pins 1 and 3 of the gain pot. I'm mostly going off the Dist+ here though, but it's much the same thing. I found that some worked better than others.

Cool. Good to know about the diodes. What about the cap question? Any thoughts there?

 

[benny_profane]

You’ll alter the HPF formed with the gain knob. You can change it, just double check the frequency response to see if it works for you:

ElectroSmash - MXR Distortion + Circuit Analysis.

The M-104 MXR Distortion + aka Distortion Plus aka D+ is a distortion guitar pedal designed by MXR and released between 1978 and 1979. The original stompbox did not have external power jack or indicator LED. Jim Dunlop bought the MXR licensing

www.electrosmash.com

 

[D1chotomy]

You’ll alter the HPF formed with the gain knob. You can change it, just double check the frequency response to see if it works for you:

ElectroSmash - MXR Distortion + Circuit Analysis.

The M-104 MXR Distortion + aka Distortion Plus aka D+ is a distortion guitar pedal designed by MXR and released between 1978 and 1979. The original stompbox did not have external power jack or indicator LED. Jim Dunlop bought the MXR licensing

www.electrosmash.com

Okay cool. Thank you sir. I don't remember what the closet value I had laying around was, I'll double check tonight.

 

[D1chotomy]

Okay, so update on this. I ended up throwing a .082uF cap in C3 and used 1N4148 diodes. The pedal works, but the gain range sucks. You get almost nothing until the very end of the taper and then you get the full amount of gain. Is this caused by my choice of cap in C3? Really wish I would have socketed that now...

Any help would be appreciated and I can post this in the troubleshooting forum, as well.

 

[benny_profane]

Increasing C3 from 47nF to 82nF reduces the HPF corner from 720Hz to 413Hz with gain at max and 1.9Hz at gain min (assuming you have a 1M potentiometer).

Fc_max = 1/(2*π*(4700+0)*(.082)) = 413Hz
Fc_min = 1/(2*π*(4700+1000000)*(.082)) = 1.9Hz

Since the purpose of that is to mitigate op amp instability, I don't think that's where your problem is. You might consider looking at a C-taper pot (the DOD 250 uses a C500k pot instead of a B1M used in the Distortion+)

Also, if you want to preserve the original frequency response and you have more 82nF caps, try a series arrangement:

C_total = (C1*C2)/(C1+C2)
C_total = (.082*.082)/(.082+.082)
C_total = .041 => 41nF

It won't be quite the same, but it'll be closer.