The only two things I can think of off the top of my head. For stability purposes we bias fixed bias amps colder than cathode bypass amps. Now biasing the tubes hotter may give a different harmonic content but if you biased them hotter in fixed bias you should get the same effect. If you have the tubes biased hotter so the amp does not go into Class B then you will not get the added distortion. If the bias is set so that the voltage shift puts the amp in Class B rather than Class AB you will get crossover distortion adding harmonic content. In overdrive when the grids are driven positive there is a voltage shift on the capacitor that changes the bias voltage. The capacitor bypasses the AC around the resistor so the resistor only effects the DC current through the tube. What can happen though is related to the thing that goes with the cathode resistor, namely the capacitor. It should not under undistorted operation. A replacement set might be bang on but then again it might not. The bias circuit does not change with a change in current (it is oblivious to what is going on in the amp unlike the resistor in the cathode bias circuit) so if it is set for -20V on one tube giving X amount of current you may have to change it to -18 for the next tube to get X amount of current.Ĭathode biased amps generally do not need to be looked at but fixed bias amps probably should. In the end things balance out and unless the tube is way out of whack it finds its happy place.Ī fixed bias amp does not have this resistor, the bottom of the tube is connected to ground, so one tube may pass X amount of current while another may pass Y given the same bias voltage on the tube grid. If one tube draws more current than another in the same circuit you end up with a greater voltage drop (more current more of a voltage drop across a resistor) the tube gets biased more negative and this has the effect of reducing the current through the tube (which then reduces the voltage drop). This voltage drop is used to bias the input of the tube (I won't go into how).

When you have current running through a resistor you get a voltage drop across it. Cathode bias has the current through the tube run through a resistor before it gets to the ground and returns to the power supply.

Cathode vs uberlayer