So i recently got interested in using tubes on lower voltages and incorporating into designs. I found out that there are some designs primarily with 12AU7(called ECC82 in europe), for example a link to the most popular one, The valve caster:
*document with summary of the original design and some user modifications
*a more beginner friendly tutorial with a valve caster x2 in one circuit

The circuit is for guitars and i think its really great project for beginners, the components are easy to find and its not too many parts. And you don’t even need a pcb because you can solder everything between the jack and tube pins like this:

Some information and the more in-depth schematic:
The 12A*7 tubes are all designed for 12v operation. the valvecaster circuit works “best” (according to the online documentation) with 12AU7 tubes but they also work with 12AT7 tubes (i tried this and it sounded nice). For a more clean tone and less distortion you would try to get 12volt instead of 9v if you use 12A*7 (12AU7, 12AT7, 12AX7 etc.) tubes OR you could look into 6A*7 tubes which run on 6volt.