JCN-9K - Tube test

Well, here is my tube PCB. All the MPSA42 are soldered underneath the Tube PCB. I would like to test my circuit in this fase to see if its everything ok. This is my first time using SMD components....
My doubt is What is the voltage that I can apply on the MPSA42 Base to close the circuit? Indicated on the picture as "?V"