These petite C3m metal valves are quite inexpensive. After making a 6N7 (also metal valves) preamp which sounded positively excellent, I decided I would make a preamp with the C3m(s) in a very attractive 1UR 19" enclosure. I could find no preamp schematics incorporating them but a fellow DIYer in the States designed one for me.
This preamp uses the same power supply I have used in the last dozen preamp builds. An external 24VAC 1A wall wart is stepped up to 250VAC by using a step down transformer backwards. Double filtration and a DC filament supply ensure a dead quiet preamp. Wire wrap wire is used for signal hookup wire and heavy duty gold plated RCAs are employed. No shielded wire or PCB is used. An Alps blue velvet pot is on the output for attenuation. Jensen Copper, paper and oil caps with silver leads are used on the output. Three sets of input RCAs are selected by two small power switches.
The sound stage is extremely deep and voice and instruments have a uncanny natural sound and feel about them. With symphonic pieces you get the unnerving feeling you are in row twelve and right in the center. I would highly recommend the construction of this preamp.