The link coupler is tuned to your plasma tube and transfers the energy from the PA3 amplifier to the electrodes on the plasma tube.
For PT tube: LC31S, $ 85
For ST tube, LC31P, $ 85
For BAT tube, LC31TCBA, $ 85
It is required to cool the LC31 coupler with a fan on power levels of 100 watts or more. Which means a fan is recommended for ST plasma tube for more than 75 volts, and required for the BAT tube, regardless of voltage setting.