There are two types of biasing, fixed and self. Fixed biasing applies power supplied by the power supply to the control grids of the output tubes. Self biasing voltages, on the other hand, are derived from current conducting through the tube. Fixed bias is adjustable and self bias is not (short of changing components). Bias determines the amount of current that flows through the output tubes at idle, that is with no input signal present. Set too cold, it will make your tone too thin and brittle. Set too hot, your amp may sound grainy and have background noise or crackling. Too hot a bias will shorten the life of your output tubes and when extreme can cause costly component failure. We check the bias of every amp that comes through the shop. You are welcome to come to the shop and listen to your amp as the ideal bias point is "dialed in". We use several factors in determining the best bias point. These include tube current draw at idle, plate voltage, notch distortion and listening to your amp. For more information click on the link "How bias is determined" on our home page.