These are some links that I had collected some time back for a friend. Check if they are useful:
hxxp://www.muzique.com/schem/projects.htm
hxxp://users.chariot.net.au/~gmarts/ampsmain.htm
hxxp://www.montagar.com/~patj/gindex.htm
hxxp://geofex.com/effxfaq/schemptr.htm
hxxp://www.muzique.com/schem/louis.gif
hxxp://ultramike.caltech.edu/projects/guitar/Electric%20Guitar%20Mirror/
hxxp://audible.transient.net/archive/
hxxp://geofex.com/
hxxp://www.buildyourguitar.com/resources/lemme/
If in case you are designing on your own you need to design a f to v converter.
Also search google for pic and avr based guitar tuner
check the april 2002 EPE magazine