The Balanced ECC83S is located closest to the power tubes in V4 and the two ECC803S's are located in V1 and V2.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 two power tubes. These are available under the "Related Products” tab as a pair and will greatly reduce tube rattle.