.Measures the current of 4x any standard base octal power tubes (6L6, EL34, 6550, etc. The BiasMaster(TM) allows to monitor the bias current savely and easily. While adjusting the Bias control pot of your amp the BiasMaster(TM) does show the actual result. This lets you set and evaluate all reasonable biasing range your amp and tubes offers to find and set your personal perfect choice. The Bias setting has a dramatic impact on the amps respsonse, break-up and dynamic character.