John writes -
The Firefly is a tube-based guitar amplifier popularized on ax84.com (a tube amp community and forum). The Firefly PCB was created so even the beginner to tube amps can succeed. The PCB is “open hardware” meaning that the PCB file is free to download and modify as well. Otherwise the PCB’s are sold for $19 each. There’s a detailed assembly guide as well for download at the site.
Firefly PCB – Link.