Modify an MXR M133 Micro Amp to 18v

[adco]

New to the forum, and relatively new to pedal building/modding.  I was thinking of trying to mod my MXR Micro Amp to 18v so I could get a little more clean headroom out of it.  I've searched the forum and the internet at large, and haven't found anything exactly on topic.  As far as I can tell, all I need to do is upgrade the capacitors to have higher voltage ratings and maybe upgrade the op amp chip (which I think is a TL061). I opened up my Micro Amp, and I have the red PCB version, labeled "ASSY REV D.4," which I think may mean I have the 4th assembly revision version?  I can operate a soldering iron pretty well, but lack theory knowledge so any help anyone could provide would be greatly appreciated.

[Seljer]

Electrolytic capacitors are probably the only thing you're going to have to worry about. And maybe the resistor that limits the current to the LED will have to be increased if it's too bright.

The TL061 can survive +-18V according to the datasheet so it's good up to 36V on a non-bipolar power supply.

[adco]

Thanks Seljer! Apologies if these questions seem simple, but my theory is very limited (I'm always looking to learn more). The electrolytic capacitors are the canister-shaped ones, right?  My micro amp's PCB looks like this: 



Based on that picture, there's only one electrolytic capacitor, but there are 4 other capacitors in the circuit (C1 - which looks like a polyester capacitor and C2, C4, and C5 which look like tantalum capacitors). Should I be concerned about replacing those capacitors with ones rated at something over 18 volts as well?

Also, is there a way to determine what value of resistor I should consider to replace the one that limits current to the LED?

[Mark Hammer]

The voltage rating should be written on them.  If it's 25v or more, you're fine.

The TL071 is actually a step up, noise-wise.  The 61 uses less current, but if you're powering it externally, you're more concerned about noise.

[anchovie]

Also, is there a way to determine what value of resistor I should consider to replace the one that limits current to the LED?

http://en.wikipedia.org/wiki/Ohm's_law

If you're doubling the voltage, double the resistor value to retain the same current.

[Elijah-Baley]

The voltage rating should be written on them.  If it's 25v or more, you're fine.

The TL071 is actually a step up, noise-wise.  The 61 uses less current, but if you're powering it externally, you're more concerned about noise.

I think so, exaclty, adco. I built a Micro Amp following the general guitar gadget project, (not using tantalum caps).

I'm sorry for the slghlty off topic, but on my solid state amp I noticed a bass boost with this pedal. Another guy here, with a stock pedal, notices the same thing. But other peaople notice, with original pedal, a slighlty treble boost.

My input capacitor, C1, (the red one on your pcb, probably) is 0.1uF, I used a film box. Usually 0.1uF is what diyers advice to have more bass for a booster/overdrive/distortion pedal.
Before to try other caps I'd like to know your opinion about frequency boost of this pedal.

Thank you!

[adco]

The voltage rating should be written on them.  If it's 25v or more, you're fine.

The TL071 is actually a step up, noise-wise.  The 61 uses less current, but if you're powering it externally, you're more concerned about noise.

Thanks Mark!

http://en.wikipedia.org/wiki/Ohm's_law

If you're doubling the voltage, double the resistor value to retain the same current.

Thanks anchovie!

My input capacitor, C1, (the red one on your pcb, probably) is 0.1uF, I used a film box. Usually 0.1uF is what diyers advice to have more bass for a booster/overdrive/distortion pedal.
Before to try other caps I'd like to know your opinion about frequency boost of this pedal.

Thanks Elijah-Baley!  The C1 capacitor in my Micro Amp appears to be a 0.1uF.  My digital multimeter doesn't have the ability to test capacitance so I am not 100% certain but the code on the capacitor is 104J which I've been told means it's a 0.1uF capacitor with +/- 5% tolerance.  I notice a slight treble boost with my pedal, but I've only ever tried it with one amp, which tends to be a brighter, more treble-y amp to begin with. 

[Mark Hammer]

I have serious doubts as to whether the tantalums used in many of the original-issue (but since reissued) MXR pedals needed to be tantalums.  The earliest MXR pedals  had big Carling stompswitches, and equally big pots, with the circuit board simply hanging from the wires, and stuffed behind the pots and switches.  The only way to fit them in there, given the size of electrolytics at that time, was to use tantalums that could be "folded over", to lie semi-flat.

Maybe they contribute to the sound, maybe not.  My guess is: not.