MXR distortion + clone.....which value for the volume pot?

[pupina]

Hi folks,
I just built my second distortion +clone...(I don't remember what i did in the first one because I sold it) using the Tonepad layout, two 1N277 germanium diodes, an Ua 741 CN, a 1mega pot for drive and a 10 K pot for the volume.
After I built it  I saw that many people in this forum advise to use a 100K pot for the volume...maybe to increase the drive and the overall volume, so I did this in my pedal, but as far as I can hear, nothing has changed.
In conclusion: i can't hear any difference from a 100K pot and a 10K pot in the volume....what's wrong with my pedal?
maybe it's the wrong type of diodes of something else....

Let me know if you have any ideas
Thanks a lot for any previous and future replies from this forum

[Mark Hammer]

The 100k pot does not increase volume, it only prevents the pedal from losing volume.

Notice that there is a 10k resistor in series with the input of the volume pot.  With a 10k pot turned up full, what you have is something that works the same as a 20k pot turned down halfway.  Replace that 10k pot with a 50k unit turned up full, and you have something equivalent to a 60k pot (50k+10k) turned down 1/6 of the way.  Replace it with a 100k pot and you have the equivalent of a 110k pot (100k+10k), turned down 1/11th of the way.  Clearly the larger the pot value, the less loss you have at the max volume setting.

Why not just change the 10k resistor?  Because the diodes come immediately after the 10k resistor and that would alter their behaviour.  Think of the diodes as being a "tap" off a larger-value volume pot.  Because the diodes set the absolute maximum level of the output, there are limits to how much louder a 100k pot can make things.  Germanium diodes will set the maximum output level somewhere in the vicinity of 450-500mv peak-to-peak.  If you have a 10k resistor + 10k pot, then even at max volume, the level is never going to be higher than half that (i.e., max 250mv). Raise the pot value to 100k, and you get to keep most (though not all) of that 500mv at max volume.  So, it will never be more than twice the output level of a 10k pot.  For many of us, though, that makes a big difference.  If you need even more volume, then what you do is switch to silicon diodes (1N914, 1N4148) and use that in tandem with a 100k pot.  That will give you nearly 4x the maximum output level of the original 10k volume + Ge diodes.

[pupina]

Thanks Mark, as always you get right to the point even though you give me some problems with the translation....eheh ...as I have to try to understand both the technical meaning as well as the language, but it's always like that when you speak to a guru.....you try to learn something everyday....
Only one more thing: what do you think of 1N277 diodes? should I change them for 1N34 or does any germanium diodes work well?
Thanks again Mr. Hammer

[petemoore]

Only one more thing: what do you think of 1N277 diodes? should I change them for 1N34 or does any germanium diodes work well?
  Short answer yes.
  Long answer is I couln't tell much any difference between the widest and shortest tolerance differences I could find to match/mismatch in Ge diodes [measuring the threshold voltage and matching closest match BTB ge Diodes for clipping, then finding the two GE diodes with the lowest and highest Thresholds]..very slight difference if any. Some report part #X sounds different...different ok...better?...
  Now put an Si on one side and a GE on the other of BTB set, or two si's VS. two BTB Ge's..and the differences start to become quite apparent in the sound.
  See FAQ and AMZ labs notebook

[pupina]

Thanks Petemoore,
what do you mean by BTB?
and what do you think is the best mod i could do to my pretty new Distortion + clone, re GE or SI diodes?

[Mark Hammer]

There is so much discussion here about the MXR Distortion+, you should probably do a search instead of asking.  I don't say that to be cruel or insulting.  I say that because there have been at least 10,000 good and informative words written in this forum about good mods to the Dist+, and you would do yourself a disservice if you overlooked them.