What is "fixed" bias? If you make it adjustable, is it no longer "fixed", or is it "broken"?

"Fixed bias" refers to the bias condition with respect to the signal cycle and means that the bias condition of the tube does not change over the signal cycle. To understand this, we must first look at cathode bias. Cathode bias is also called "self" bias. A resistor is placed between the tube cathode and ground. The grid of the tube is also tied to ground through a resistor, which for practical purposes looks like a direct connection to the bottom of the cathode bias resistor. As we said in the TUT-series of books, the tube is "cathode-centric". This means that the center of the tube's universe is its cathode. It measures every influence with respect to the cathode. If the grid is negative compared to the cathode, the tube will conduct less current from its cathode to its plate. If the grid is at the same voltage as the cathode - or more positive than the cathode - then the tube will conduct as much current as possible.  more