Unison research simply two, what gridvoltage for EL34?

Hello Everyone,

I hope someone can tell me what gridvoltage is, used for the EL34 tubes in my Simply two Unison research
I have a guy that I can buy tubes from and he asked me what's the gridvoltage
Can't find this info or a schematic anywhere on the internet so thought asking the experts

Thanks very much in advance:)

Maikel
Holland