Fixed or Cathode bias?

Cathode Bias vs. Fixed Bias: What’s the difference?

Cathode bias and fixed bias are often used to describe amps but lots of folks are not really sure what they mean. I will briefly provide a technical explanation and then provide some bullet points of the major differences in tone, maintenance and power.

Technical Explanation  :

All Tower tubes needs to be biased in some fashion to function. Biasing controls the amount of power a tube dissipates relative to the voltage applied to it. Without biasing a tube would just burn up. You could think of it as a train without breaks.  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.  When using the terms cathode or fixed bias we are discussing the power tubes of an amp (pre-amp tubes are nearly always cathode bias or grid-leak biased but that is a discussion for another time).   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. As you hit a note the tube calls for more power and negative voltage on the cathode decreases briefly changing the tube’s bias. This gives cathode biased amp their awesome sustain. In fixed bias a separate circuit provides negative voltage to the grid of the power tube and the cathode is simply grounded. Since the grid is negative relative to the cathode thus bias is achieved. Since the voltage applied to the grid does not change the bias is “fixed” that what give fixed biased amp a crisper stiffer response.

So what is the difference?

  • Cathode bias amps have more sustain and dynamics but a spongier response particularly with bass notes (more sag). Fixed Biased amps have a firmer feel but less dynamics and sustain. One thing to note is that properly set up cathode bias can be quite firm but still singing.
  • With all other things equal: Cathode biased amps are more easily overdriven. Fixed biased amps are cleaner.
  • Cathode Biased amp do not require biasing to change power tubes. Fixed biased amps require biasing to change power tubes  as well as periodic bias checks to ensure they are working properly.
  • Cathode biased amps are less efficient than fixed bias amps. Example: a cathode bias Tweed Bassman type amp with two 6L6’s would be about 25 Watts. The same amp in fixed bias would be about 40 Watts.  It is important to note that the volume difference is actually quite minimal. Many tube amps are over powered for the average users volume needs.

Some Famous Cathode Biased Amps (with my versions model numbers there too):

  • All Fender Tweed Deluxe Amps ( CPC-15T is a 5E3 Style and the Octal Deluxe is a 5B3 Style)
  • All Fender Champs ( CPC-5T is my version of the 5F1)
  • Vox AC-30
  • Most Matchless amps
  •  Marshall 18 Watt

Some Famous Fixed Bias Amps:

  • Fender Narrow Panel Tweed Bassman, Super, Bandmaster and Pro (CPC-30T and CPC-40T)
  • All Fender Blackface Amps (except the Champ)
  • All 100Watt and 50 Watt Marshalls
  • All Hiwatt Amps