Its the DIY 5W Compact Tube Audio Amp. On Minute 3:00 you can see the pot and the ground wiring. I have these suggestion from an old book....this describes the groundpath from the V72 Telefunken studiomixer Falsch means wrong and richtig means right....in this case could v....(v means verstaerker or amp) .....be an amp or an Potentiometer! I tried it....for tube gear its better! The printdate of this book is 1959 Hey....... and thank you lovley skunkie for your meritorious work!