The Balanced ECC83S is located in V7 behind the front row of preamp tubes closest to the power tubes.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 mount using the same screws as the stock retainers and are available under the "Related Products" tab.