The BiasMaster(TM) allows to monitor the bias current safely and easily. While adjusting the Bias control pot of your amp the BiasMaster(TM) does show the actual result. Fits all 8-pin tube types (6L6, 6V6, 6550, EL34, 5881, KT66, KT88, KT100, KT120, 7072A). Includes two test sockets.