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
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.
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
Thanks Guys