The Advanced menu of the amp block has a parameter called "Pwr Tube Bias". This sets the quiescent operating point of the virtual power tubes. What is the quiescent operating point you ask? It is the amount of idle current flowing through the virtual tubes when no signal is present.
Power tubes are basically nonlinear controlled current sources. They can only sink current (current flow in one direction only) so to get an alternating signal you have to have some amount of idle current then you modulate that.
The push-pull power amp was invented to increase efficiency by allowing lower idle currents. The name "push-pull" refers to the fact that one tube is responsible for mainly the positive portion of the waveform and the other for the negative. Some overlap of the responsibility is required since power tubes are nonlinear and without that overlap crossover distortion occurs.
Setting the idle current, or bias as it's commonly known, affects the resulting transfer function of the power amp. Too little bias and there can be excessive crossover distortion. So-called "Class A" amps bias the tubes quite high.
The following graph depicts the transfer function of the Axe-Fx II for five different bias settings: 0, 0.2, 0.4, 0.6 and 0.8. These are the red, green, blue, cyan and black traces, respectively.
Note the severe crossover distortion at zero bias. At a bias of 0.4 the transfer function is almost a perfectly straight line. At 0.8 the response gets softer. Amps like AC-30s run the bias around 0.7 which gives them the soft distortion characteristics. Note that this applies to power amp distortion which only occurs when the power amp is driven hard (Master Volume set high).
Some amps are intentionally biased cold to generate crossover distortion. Small amounts add an aggressive distortion. Some amps (i.e. Boogies) are intentionally biased cold to avoid having to set the bias and thereby reducing maintenance and warranty costs.
Negative feedback around the power amp (Damping) further linearizes the power amp. So the transfer functions depicted are only accurate when Damping is zero.
In conclusion, Power Tube Bias is a powerful parameter that can allow you to fine-tune the power amp distortion characteristics to your particular style.
Power tubes are basically nonlinear controlled current sources. They can only sink current (current flow in one direction only) so to get an alternating signal you have to have some amount of idle current then you modulate that.
The push-pull power amp was invented to increase efficiency by allowing lower idle currents. The name "push-pull" refers to the fact that one tube is responsible for mainly the positive portion of the waveform and the other for the negative. Some overlap of the responsibility is required since power tubes are nonlinear and without that overlap crossover distortion occurs.
Setting the idle current, or bias as it's commonly known, affects the resulting transfer function of the power amp. Too little bias and there can be excessive crossover distortion. So-called "Class A" amps bias the tubes quite high.
The following graph depicts the transfer function of the Axe-Fx II for five different bias settings: 0, 0.2, 0.4, 0.6 and 0.8. These are the red, green, blue, cyan and black traces, respectively.
Note the severe crossover distortion at zero bias. At a bias of 0.4 the transfer function is almost a perfectly straight line. At 0.8 the response gets softer. Amps like AC-30s run the bias around 0.7 which gives them the soft distortion characteristics. Note that this applies to power amp distortion which only occurs when the power amp is driven hard (Master Volume set high).
Some amps are intentionally biased cold to generate crossover distortion. Small amounts add an aggressive distortion. Some amps (i.e. Boogies) are intentionally biased cold to avoid having to set the bias and thereby reducing maintenance and warranty costs.
Negative feedback around the power amp (Damping) further linearizes the power amp. So the transfer functions depicted are only accurate when Damping is zero.
In conclusion, Power Tube Bias is a powerful parameter that can allow you to fine-tune the power amp distortion characteristics to your particular style.