Adjustment of quiescent cathode current in a thermionic valve audio amplifier

摘要

A digital controller 101 adjusts the grid bias voltage of a valve 102 until the cathode current measured by the sense resistor 106 becomes equal to a set value. Adjustment is performed only when the controller detects that there is no audio signal present on the sensing node 107. As quiet intervals often exist between audio tracks, the amplifier can thus be adjusted regularly when in use. The controller maintains amplifier performance despite rapid valve ageing caused by the overdrive which is often used to achieve desirable distortion. Valve gain is measured when the amplifier is turned on, and an alert is raised if this measured gain is too low, so that the valve can be replaced. The amplifier may be of push-pull type. The controller may be retro-fitted to an existing amplifier.