Circuit Mod is necessary. Comes with resistor modification kit. The Sovtek 7591XYZ is an improved and reliable replacement for expensive and hard to find 7591 tubes. The Sovtek 7591XYZ has increased screen grid current capability for more reliable performance and can be installed in any amp which uses 7591's by performing a simple bias modification (instructions and required cathode resistor included). Get those vintage Ampeg, Gibson, and Harmony amps working again!