The Balanced ECC83S is located in V6 which is the sixth preamp tube away from the input jack. The ECC81 is located in V7.If your amp is the combo version and you have chosen to use the JJ 6L6GC's then please let us know in the comments section and we will ship your kit with the special Double Mica 6L6GC’s at no extra charge which will greatly reduce any tube rattle. JJ Electronic started building us a proprietary version of their 6L6GC (the 6L6GC-DM) in 2005 with extra layers of mica. This reinforces the plate structure and all but eliminates tube rattle symptoms. For other tubes such as E34L's, EL34II's, EL34’s, KT77's and 6CA7's, we recommend using our EuroDamper rings. These are available under the "Related Products" tab as pairs and will greatly decrease tube rattle.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.