The Ruby Amp is a great little battery powered solidstate practice amp. The design is based on the Little Gem design which in turn comes from the Smokey Amp designed by Dave Stork on www.blueguitar.org. The design uses relatively few components and is a good DIY project. Cigar box guitar maker J.K. built a Ruby Amp but found himself hunting for a suitable enclosure for it.

This is a sample project from MAKE magazine, which shows you how to build a cracker box guitar amplifier for around $5 . It only shows the first few pages of the project, but it gives the schematic for the circuit and you should be able to work out how to finish it yourself.