DIY 12AU7 Tube preamp - Shine7 Audio DIY Page The design is exactly the same as the 12AX7 preamp, it is a single gain stage triode tube with cathode follower. Schematics : DIY Vacuum Tube (Valve) RIAA Phono Preamp - DIYAudioProjects Figure 3: Updated Schematic - Groovewatt Tube (Valve) RIAA Phono Preamp. Figure 4: Updated Power Supply Schematic - Groovewatt Tube (Valve) RIAA Phono Preamp. Two adjustments can be made. First while the preamp has nearly 45 dB gain you can get a little more by using quality 100 uF / 16 V capacitors on the cathodes of both gain stages.