The Balanced High Gain ECC83S is located in V5 which is located farthest from the input jack. The two standard ECC83S's are located in V3 and V4.If you choose to use the KT66's, 6550's or KT88's and your amp has the "Bear Trap" style tube retainers you may want to swap these out for the "Spring and Cap" style retainers. These are 5.00 a pair and mount using the same screws as the stock retainers. These are also available under the "Related Products" tab as pairs.