The importance of bias

The bias current determines the amplifier class and is a critical parameter. A good value and good balance is mandatory to get the best sound experience.

 
The effects of bias imbalance in transformer

The firsts effects of bias imbalance are seen with the output transformer, which will not give optimal sound.

A typical output transformer is composed of :

  1. A metal casingThis is the mechanical protection of the transformer.

  2. FastenersThreads, nuts and other fasteners are used to assemble all pieces.

  3. Magnetic platesThese plates guides the magnetic flux generated by the bobbins.

  4. A bobbin assemblyThe bobbin assembly contains speaker bobbin and tubes bobbin(s). It consist of many windings and is a critical part of the transformer

  5. ConnectionsThese are used to connect the bobbin's wire to external electrical circuits.

The currents in the tubes's bobbins generates a magnetic flux, this magnetic flux is guided by the magnetic plates and generates current in the loudspeaker bobbin. the purpose of the transformer is to adapt the high-voltage low current on the tube side to the low-voltage high current on the speaker side.

Because perfect magnetic plates and bobbins doesn't exists, the behavior of the transformer is not purely linear. They are others reasons but they are way more too complex to be exposed here.

The magnetic behavior can be seen through a magnetization curve that represents somewhat how the magnetic circuit will deform the electrical signals.

 Description of the curves:

  1. Ideal curveThis is the ideal curve, when input and output are linear. No matter how much magnetic field is present, the output current will be proportional of the input current. This curve does'nt exists at all because transformers are not perfects.

  2. Real curve without saturationThis is the curve from reality, when the bias are in balance. The currents in tube's bobbins are equals, creates two opposites fields, and they cancel each others. The curve is then centered. Each positive increase in the magnetic field gives a shape that is the exact opposite to a similar but negative increase in the magnetic field. Thus, the negative and positive parts of the signals are treated the same, this lead to a very beautiful song.

  3. Bias imbalanceWhen the bias are not in balance, the curve is displaced because there is a permanent current in the transformer, thus a permanent magnetic field. The negative and positive parts of the signal are not treated the same, this causes bad sound.

It is clear from above that we need a good bias balance to bet the better sound.

The effects of bias imbalance in tubes

From the tube standpoint, there are many effects.
For one, the tube working load will be different, leading to differences in amplifying the positive and negative parts of the sound, thus degrading the sound.
 
Another side effect is that the larger the current imbalance, the sooner the tube will become used. Because the idle current in the tubes are not equal, one tube will be more stressed because it will have more current. Then, this tube will need to be replaced sooner, thus reducing the tube's life. We have seen that in the section "filament heating" in the page "how tube work".
 
Lastly, when a tube heats more, it will draw more current. Because one tube has more current than the other, it will heat more, and the current will rise again in this tube. This means than even a small current mismatch will lead to a significative current (bias) mismatch, with all effects described above.

 

 

Biasing hot or cold

The bias value is a critical parameter of the amplifier affecting not only the tube's life but also the tone, the amplifier working class and the distortions. A High value of bias current is said to be hot, and a low bias current value is said to be cold.
 
The tone will vary accordingly to the bias value and to the music. This can often be described as follow:

  • Biasing too hotThe amplifier will a little bit harsh, distort too quickly and will lose focus at high volume,

  • Biasing hotThe sound is warmer sound but has less headroom,

  • Biasing coldThe sound is cleaner and has more headroom but present distortions sooner,

  • Biasing too cold: the amplifier will sound cold, fragile and will have excessive distortions at high volume.

 

There are no good and bad biases, but an optimal one which depends on the played music and the listener. You will see in the next pages how the biasing value affects the amplifier working class 

 

 

Final remarks

Bias imbalance causes severe degradation in sound quality, thus it’s an important matter. Bias value sets the working class of the amplifier and improper value can lead to a class B amplifier with a poor sound quality. Some people prefer running with hot bias, others with cold bias, it depends on preferences and music type.
 
The Smart Bias technology overcome all these issues and permits the user to set the bias hottest or coldest, easily, depending on the feeling, the music and the use of the tubes.