Controlling the Performance of a Thermionic tube

Abstract

Controlling the performance of the thermionic tube ( 102 ) having a cathode ( 103 ), a plate ( 104 ) and a grid ( 105 ) is disclosed. The tube is configured to provide amplification of an audio derived signal ( 106 ) and is arranged to apply a grid bias voltage to the grid. The absence of an input audio signal is detected whereafter output current between cathode and plate is measured to identify actual output current. The actual output current is compared against a preferred output current and the grid bias voltage is adjusted so as to bring the actual output current value towards the preferred output current value.

1 - 20 . (canceled) 
     
     
         21 . A method of controlling the performance of a thermionic tube comprising the steps of:
 measuring a current in said thermionic tube;   comparing the current to a preferred current; and   adjusting a parameter in said thermionic to bring the current closer to the preferred current.   
     
     
         22 . A method according to  claim 21 , wherein said thermionic tube has a cathode, a plate and a grid. 
     
     
         23 . A method according to  claim 22 , wherein said thermionic tube provides amplification of an audio derived signal by applying a grid bias voltage to said grid. 
     
     
         24 . A method according to  claim 23 , wherein said step of measuring a current comprises measuring an output current between said cathode and said plate to identify an actual output current value. 
     
     
         25 . A method according to  claim 24 , wherein said step of comparing the current to a preferred current comprises comparing said actual output current value to a preferred output current value. 
     
     
         26 . A method according to  claim 23 , wherein said step of adjusting a parameter comprises adjusting said grid bias voltage so as to bring said actual output current value towards said preferred output current value. 
     
     
         27 . A method according to  claim 21 , wherein said step of measuring a current is only performed in the absence of an input audio signal. 
     
     
         28 . An apparatus for controlling the performance of a thermionic tube comprising:
 a measuring device for measuring a current in said thermionic tube;   a comparison device for comparing said current to a preferred current; and   an adjustment device for adjusting a parameter in said thermionic tube to bring said current closer to said preferred current.   
     
     
         29 . The apparatus of  claim 28 , wherein said thermionic tube has a cathode, a plate and a grid. 
     
     
         30 . The apparatus of  claim 29 , wherein said thermionic tube provides amplification of an audio derived signal by applying a grid bias voltage to said grid. 
     
     
         31 . The apparatus of  claim 30 , further comprising an input terminal for providing said measuring device with a representation of actual output current between a cathode and a plate of said thermionic tube. 
     
     
         32 . The apparatus of  claim 31 , wherein said comparison device compares said representation of actual output current against a preferred output current value. 
     
     
         33 . The apparatus of  claim 32 , wherein said comparison device is configured to compare a plurality of representations of actual output currents. 
     
     
         34 . The apparatus of  claim 33 , further comprising an output terminal for supplying said grid bias voltage to said thermionic tube. 
     
     
         35 . The apparatus of  claim 34 , further comprising the step of adjusting the grid bias voltage so as to bring the actual output current towards the preferred output current when no audio input signal is present 
     
     
         36 . The apparatus of  claim 35 , further comprising non-volatile storage for storing preferred currents. 
     
     
         37 . An audio signal amplifier, comprising:
 an input terminal for receiving an audio input signal;   an output terminal for supplying one an amplified output signal via an output transformer;   a thermionic tube; and   a performance control device that monitors a current in said thermionic tube, compares said current to a preferred current, and adjusts a parameter in said thermionic tube to bring said current closer to said preferred current.   
     
     
         38 . An audio signal amplifier according to  claim 37 , wherein:
 said thermionic tube has a cathode, a plate and a grid, and provides amplification of an audio derived signal by applying a grid bias voltage to said grid so as to control a flow of current between said cathode and said plate; and   in the absence of an audio input signal, said performance control device adjusts said grid bias voltage to bring said flow of current closer to said preferred current.   
     
     
         39 . An audio signal amplifier according to  claim 36 , comprising four thermionic tubes, in which:
 a first thermionic tube and a second thermionic tube amplify a first channel of a stereo pair;   a third thermionic tube and a fourth thermionic tube amplify a second channel of a stereo pair; and   said performance control device adjusts said grid bias voltages so as to minimise harmonic distortion.   
     
     
         40 . The audio signal amplifier as claimed in  claim 36 , wherein:
 a plurality of thermionic tubes are divided into a first set of thermionic tubes and a second set of thermionic tubes configured to operate in a push-pull configuration; and   said performance control device adjusts said grid bias voltage to encourage a degree of harmonic distortion.