The Biasing Methods

The biasing method is the way to set up the bias current in amplifier. There are many kind of biasing, from the most basic to the most evolved.

 
The cathode bias

This is also called "self-biasing stage" or "automatic bias" because it doesn't require biasing tuning. The tube's current is used to create the negative voltage on the control grid.  When the tube's current flows through the cathode resistor, it creates a voltage. Then, the voltage on the control grid is lower than the voltage on the cathode, thus creating the negative voltage.  The system regulates itself to an equilibrium point: the more the current through the resistor, the more the negative voltage on the control grid, and the less the current on the tube.

  1. Cathode voltageCreated by the cathode current flowing through the cathode resistor.

  2. Control grid voltageNegative voltage regarding the cathode.

  3. Input capacitorLet only audio signal pass and blocks grid voltage.

 

This methods works pretty well on a wide range of tubes without any adjustment, but has many disadvantages:

  • Lost of powerHigh power tubes needs high voltage on their control grid. As a consequence many power is lost on the cathode resistor and not used to produce sound, because the voltage and currents will be high.

  • Bad audio soundThe cathode resistor must have a huge parallel capacitor (not represented) to maintain constant bias voltage. The characteristics of this huge capacitor are not constant neither in frequency. nor from capacitor to capacitor As a result, the sound is affected.

  • Unstable bias valueThe bias current value is regulated but his value is not precise and depends upon the tube and it's aging, which is a big problem for push-pull topologies.

These are the main reasons why this system is not used for high power tubes.

 

The fixed bias

This method is generally referred to as "fixed bias" because the control grid is not floating like in the previous biasing method, but fixed by a voltage generator.  The bias voltage will be adjusted manually accordingly to the bias current measurements while the amplifier is running. It is not a difficult task but it can be very dangerous if the amplifier must be opened. This method has the advantage of not losing power in a cathode resistor, but this came at a price of other disadvantages:

  • Regular tuningTuning must be regular to accommodate for the tube use.

  • Risks of failureA serious tube failure can happened if the bias is not correctly set.

  • Not stableThere are no compensations for other factors affecting the bias such as the warming up and the line voltage.

 

The following figure shows a typical fixed-bias circuit:

  1. Cathode voltageDirectly connected to the ground, the voltage is null with respect to the ground

  2. Control grid voltageNegative thanks to the adjustable voltage source

  3. Input capacitorLet only audio signal pass

  4. Adjustable voltage sourceCreates the negative voltage on control grid, and is used to tune the bias value

  5. High-value resistorInsulates the voltage source from audio signals

The adaptive bias

These types of bias are little more evolved than the others.  These circuits replace the voltage source in the fixed bias, and regulates the bias current accordingly to a target current. These are all pure analog-electronics, some of them can cut off the high voltage if the current is too high.

To achieve this, the current is measured through the cathode resistor. The targeted current value is then subtracted to the measured current, with the help of analog circuits. The result is the amount of excess current if the value is positive, or to the less current if the value is negative. This signal is passed through an amplifier to convert the positive or negative result to a negative voltage, driving the control grid.

These circuits has the advantage of freeing the user from tuning the bias, but still have disadvantages as some types of tube failure cannot be detected and the bias cannot be adjusted. Moreover, the bias is also adjusted in real time, which rises a big problem because it increases the distortion.

Find out how The Smart Bias technology solve these problems and adds new functionalities !

  1. High-voltage relayCuts off the high voltage supply to the tube and can be controlled by the adaptive bias.

  2. Current inputThis signal comes from the cathode resistor and is a measurement of the current flowing through the tube.

  3. Reference currentThe reference current is the targeted current value, to which the measured current is compared.

  4. SubstractorThe measured current is substracted to the reference current.

  5. Substractor resultThe difference between measured current and targeted current gives the amount of current in excess or in lack, also known as "error".

  6. AmplifierThe difference between the measured current and the targeted current is amplified, other mathematical laws can be applied such as accumulation or derivation.

  7. Output commandThe output is used to control the control grid voltage.

Factors influencing the bias

There are many factors influencing the bias, the most known of them is the tube aging, but the line voltage and the ambiant temperature are only some of many factors influencing tube bias.

Many of them cannot be controlled easely, leading to a bias imbalance in circuits based on traditional biasing methods.

 

A word on matched tubes

Some amplifiers imposes you to change your tubes with matched sets, because of their biasing methods.
Having a matched set gives you tubes with similar but not identical static characteristics. With the proper biasing, matched sets gives similar, but still different, bias current. This cannot ensure correct biasing, especially with Push-Pull topology, as the characteristics will always differ more and more with aging.
Hence, we recommend when you change your power tubes that you do so in matched sets, because the dynamic characteristics will be more similar, and the sound could be better.

Final remarks

There are many different biasing methods, but all suffers from major drawbacks: either the bias is not in balance, or you need to adjust it very often, or the sound is degraded. Moreover, many factors influences the bias, there are no way to control them, this explain why traditional biasing fails to maintain perfect bias balance even if they are finely tuned.
 
The The Smart Bias technology is able to maintain the bias balance between tubes and counters every factor affecting it. As an exclusive feature, it allows to change the bias value in live, thus ensuring the best sound for all musics.