The same amp in fixed bias would be about 40 Watts. Example: a cathode bias Tweed Bassman type amp with two 6L6’s would be about 25 to 30 Watts. Fixed biased amps require biasing to change power tubes as well as periodic bias checks to ensure they are working properly.Ĭathode biased amps are less efficient than fixed bias amps. Fixed biased amps are cleaner.Ĭathode Biased amp do not require biasing to change power tubes. With all other things equal: Cathode biased amps are more easily overdriven. One thing to note is that properly set up cathode bias can be quite firm but still singing.
Fixed Biased amps have a firmer feel but less dynamics and sustain. So what is the difference?Ĭathode bias amps have more sustain and dynamics but a spongier response particularly with bass notes (more sag). This is what give fixed biased amp a crisper stiffer response. Since the voltage applied to the grid does not change the bias is “fixed”. 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.
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