A majority of this data was originally derived from extensive bench measurements on real vacuum tube amplifier circuits under varying operating conditions by engineers Craig Maier and Rick Carlson in the early 1990s.
However, the output level of a VVA generally remains constant independent of drive due to internal gain compensation algorithms.
As such, the Drive of a VVA describes the degree of modulation applied to a given vacuum tube amplifier circuit centered about the set operating point.
The higher the drive level setting, the greater will be the production of predominantly even order harmonics due to the circuit's asymmetrical non-linearity.
[1][2] This is a high-mu dual triode that is generally incorporated into an RC coupled class A audio pre-amplifier configuration and its design is optimized to minimize harmonic distortion.
Thus, circuits based around the 12AT7 exhibit a larger degree of non-linearity throughout the entire dynamic operating range, including the middle.
The 12AU7 is a medium-mu dual triode often found in the driver / phase inverter stage of a push-pull power amplifier and also results in significant non-linearity in the middle of its dynamic operating curve.
Its high-gain characteristics and family of operating curves make for useful harmonic distortion and signal compression properties[clarification needed].
These devices generally consisted of a 12AU7 medium-mu triode driving a single 6L6GC beam power pentode audio output valve or similar.
Its effects are distinctive due to convolution of the non-linearity of the triode interacting with those of the pentode, with both devices operating in class-A mode.