Easy questions need answered please

[whomeno]

I am building an mxr GT-OD from here http://tagboardeffects.blogspot.com/2012/08/mxr-gt-od.html
It calls for a 1m liner for gain. Could I use a 1m audio?
Also it calls for a 18n cap. Could I use a 15n or a 22n with out much differance?
Thanks

[Mark Hammer]

Hard to provide a confident answer in the absence of a schematic, but I'll try.

I'm assuming that the Gain control either sits within the feedback loop of an op-amp, or in the "ground leg".  A linear pot will work in either location, but will alter the dialability of the pedal.  Whether that provides a nuisance or benefit depends on which end of the Gain range is your preference.  So a log pot may spread the higher-gain settings out a little more, rather then bunching them up.  If you're picky about your high gain settings, you may find it a little trickier to consistently nail "your" pleasing sound.

I don't know exactly what the 18nf cap is for, although I gather the GT-OD is essentially a ZW-44 with a few part changes.  A ZW-44 drawing show the DC-blocking cap from the input buffer as being 18nf.  A change to either 15nf or 22nf will alter the bass rolloff, but only negligibly so.  If it's a struggle to retain bass, use 22nf.  If your rig delivers too much bass, go for 15nf.  Again, it won't make a huge difference, but sometimes every little bit adds up.

This is why I don't like stripboard layouts.  Too labor-intensive to reverse engineer the function of any given component.

[Fancy Lime]

I have traced so many schematics from stripboard layouts, I have gotten quite efficient at it. The layout you linked has a Tube Screamer type clipping stage. So I would recommend an audio taper aka log pot over a linear one. The 18nF cap is indeed the input coupling cap of the opamp. I would go with 22nF here.

Andy

[whomeno]

Thanks Guys