You can use this little board to adapt a Bourns 3296W (or Vishay T93YA) trimpot as a bias adjustment control in a tube amplifier. The bias supply from the amp goes to “B.IN” pad, bias supply out comes from the “B.OUT” pad, and the “G” pad goes to the main bias resistor to ground depending on the circuit. The resistor in parallel is an emergency pull-down resistor. I took this idea from the Valve Wizard. They write: “This ensures that if the pot wiper fails to make good contact with the rack (because it is dirty or worn out, say) the grid leaks will effectively be connected directly to the raw bias supply through R1 instead, biasing the valves safely cold. R1 should have a value of about ten times [the trim pot] so it has negligible effect on the rest of the design.”