You can replace any one or all three of the 12AX7 tubes that come standard in the Fender Blues Junior amp with these tubes that are designed to reduce feedback and create an even warmer, richer, fatter sound. The "feedback threshold" will be greatly improved with these tubes, meaning that you'll be able to turn your amp up louder before it'll feedback. These tubes can be ordered individually, but this 3 tube upgrade kit includes one ECC81, one ECC832 and one ECC83S. You position them as follows: The tube closest to your power tubes is sitting in the Position V3. The middle tube is in V2, and the tube farthest away from the power tubes is in the V1 position. Place the ECC832 in V3, the ECC83S in V2 and the ECC81 in V1.