The Balanced High Gain ECC83S is located in V7 farthest from the input jack, the standard ECC83S is located in V6 and the ECC81 is located in V3.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 four power tubes. These are available under the "Related Products” tab as a quad and will greatly reduce tube rattle.