The two Balanced ECC83S's are located in V7 and V8 which are both in the last two positions farthest from the input jack.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 10.00 a quad or set of four and mount using the same screws as the stock retainers. These are also available under the "Related Products" tab.