Since the grid is negative relative to the cathode thus bias is achieved. In fixed bias a separate circuit provides negative voltage to the grid of the power tube and the cathode is simply grounded. This gives cathode biased amp their awesome sustain. As you hit a note the tube calls for more power and negative voltage on the cathode decreases briefly changing the tube's bias. In cathode bias a small value resistor is placed between the cathode and the ground so it develops negative voltage on the cathode thus achieving bias. When using the terms cathode or fixed bias we are discussing the power tubes of an amp (pre-amp tubes in instrument amps are nearly always cathode bias or grid-leak biased but that is a discussion for another time). A poorly biased tube can either be biased too cold making it sound dull and lifeless or biased too hot where the tube is destroying itself. You could think of it as a train without breaks. Without biasing a tube would just burn up. Biasing controls the amount of power a tube dissipates relative to the voltage applied to it. Technical Explanation (feel free to skip ahead) :Īll power tubes needs to be biased in some fashion to function. I will briefly provide a technical explanation and then provide some bullet points of the major differences in tone, maintenance and power.

The terms cathode bias and fixed bias are often used to describe amps but lots of folks are not really sure what they mean.

How to bias guitar amp tubes