The Balanced ECC83S is located in V2 which is the middle of the three pre amp tube positions.

Combo amps are very hard on tubes because of vibrations which can cause tube rattle with any brand of tube. If your amp is the combo version and you play by yourself in a quiet environment or record with your amp you may want to consider using the EuroDamper tube damper rings for the EZ81 and the two power tubes. These are available in the "Related Products” tab as a set of three. These will greatly decrease tube rattle.