Morley Little Alligator - Reverse Audio Taper Mod

I bought one of these volume pedals a while ago - actually I bought two, to the delight of the clerks ("haha, that's gonna get loud!") - because they were cheap, I needed a volume pedal, and I had plans to make a stereo pedalboard.

tl;dr - do this.

After a while, the taper on these started to bother me. I tend to play weird shit(tm) and these weren't so great for volume swells. Morley is happy to explain that these are linear taper volume pedals, but in practice they function more like audio taper pedals - not much change over the first half of the pedal travel, then a sudden jump near the end of it. Overall I'm happy using these - they're completely noisefree and don't seem to load down the signal at all (no "tone sucking.") If I were playing guitar for a living, I probably would've bought something else, but it's just a hobby/passion so I've just dealt with it.

Fast forward, oh, a year probably, and today I find this thread about Ernie Ball volume pedal tapers. About halfway down the page there's a comparison of "old" vs. "new" potentiometers used in these things. You'll notice a distinct lack of a linear taper, only audio on the new ones and reverse audio on the old ones.

Well, shoot! RG Keen shows how easy it is to change the taper of a linear pot! The only problem is, of course, that the Morley Little Alligator doesn't use a potentiometer, it uses a clever arrangement of photoresistors to form a voltage divider, which is how a pot is used as a volume control. Check the schematic here.

This is where it got a little trickier. To add a taper resistor you need to know (roughly) the maximum resistance of the pot/voltage divider you're using. Try as I might, I can't get an accurate read of the Morley photoresistors. It doesn't help that their maximum resistance happens when they're dark. Okay, I could've hooked up a wheatstone bridge, but accuracy isn't important here.

Seriously, that's it.

With a multimeter, the highest value I could get consistently out of either photoresistor was in the ballpark of 5M. This means the total resistance is somewhere in the ballpark of 10M (or higher).

LDR 1 is hiding under L3.

Referencing the schematic and RG Keen's site, for a reverse audio taper we need to solder a taper resistor across LDR1. For a decent taper we might choose something in the ballpark of 2M, but since this was more of a "proof of concept" mod I just grabbed a 1M and soldered it in there.
 

I suppose I should mention that I did try to take the board out but in addition to the screws and jacks it seems to be glued somewhere in the upper half. Maybe it just needs more convincing.

Putting the resistor over the LED works better than under.

An added bonus to having a pair of these to mess with is someone already modded the other one - they put another 12k resistor in parallel with R3 which makes the range of the "minimum volume" control a little more useful, so I did that too.

I didn't have a 12k so I used a 10k. Again, this isn't the Manhattan Project.

 And that's it!

Deja vu!

The taper is much more useful now, though the 1M value isn't quite ideal. The volume used to start increasing about halfway through the pedal's travel, and now I can get it around the first quarter, which is nice because the "mute" function of a volume pedal is super handy. The bulk of the volume change now happens over the middle half of the pedal's travel, so you can get nice fade in, swell, and even tremolo if your ankle can take it.

1M isn't quite perfect though; there does seem to be a bit of a corner at about halfway through the pedal travel. Now that I know it works, I'll probably try a few values between 1M-3M.

I haven't contacted Morley about the specification of these LDRs - the 400 - 850 listed on the schematic seems to be the range of wavelengths over which the LDR is effective. It doesn't matter too much though, the tolerance on LDRs seems to be massive and every player is going to have their own preference anyway. To that end, if you wind up with a sharp "on and off" response you might want to try values lower than 1M. It should be relatively independent of what guitar you're using - that's what the minimum volume control is for - but if you're using active pickups or hitting this pedal with a really hot signal you might not want to bother with paralleling R3.

One last point is this will work on any volume pedal (or wah pedal, or volume/tone pot) with a linear taper. Read this RG Keen link again. This will actually be easier to dial in when you're using actual potentiometers because you'll know the total resistance you're starting with. Small Bear Electronics is a good source of replacement wah pots and they sell a few linear taper ones as well.

Alright, well, I was excited so I had to post this. I don't think anyone has posted on the webternets about how to do this, so I guess I could've done what that Ernie Ball guy did and tried to turn this into money, but what the hell. I get the feeling I won't make it far in this business if I don't believe in intellectual property, but really who'd go to a gear swap and say "oh, you have a Keeley modified compressor and a BillM Blues Jr? Well I've got a PaulP modded volume pedal!"

They'd probably say PP, which is fine. You like the 'brown sound,' just wait'll you hear the yellow sound! Okay, I have to go to bed.

There's a tube post coming soon.