24-07-2017, 07:34 AM
(This post was last modified: 24-07-2017, 07:38 AM by Murphyv310.)
In simple terms in class A the bias should be arranged that it is near mid point of the characteristic curve, the resultant anode current will then be "x" it's mean DC figure, when a signal is applied it will be "x" + or - the signal appearing on the anode, run say a 1khz signal to the grid and the mean anode current will remain virtually the same, measure it with a scope and then you will see the AC component out of phase with input and obviously amplified. This is in simple terms.
In class B the grid bias is near to the bottom of the curve, and there should only be a small anode current, you cannot fully bias the valve off as you will land up with crossover distortion so as the valve is fed with signal it will only conduct on the positive half cycles as they make the bias more positive and turn the valve on harder. A phase splitter is needed to reverse the signal so the lost lower part of the waveform that valve 1 cannot amplify on is then amplified by valve 2 fed from the phase splitter are combined in the output transformer after amplification from valve 1 & 2 the output pair.
The standing current or quiescent current is therefore low in a class B amp, the current is directly proportional to output, the louder the more HT current is required.
Finally there are some circuits that require a positive grid voltage, these can be in valve shaping or clipper devices, anode currents though are usually low.
In class B the grid bias is near to the bottom of the curve, and there should only be a small anode current, you cannot fully bias the valve off as you will land up with crossover distortion so as the valve is fed with signal it will only conduct on the positive half cycles as they make the bias more positive and turn the valve on harder. A phase splitter is needed to reverse the signal so the lost lower part of the waveform that valve 1 cannot amplify on is then amplified by valve 2 fed from the phase splitter are combined in the output transformer after amplification from valve 1 & 2 the output pair.
The standing current or quiescent current is therefore low in a class B amp, the current is directly proportional to output, the louder the more HT current is required.
Finally there are some circuits that require a positive grid voltage, these can be in valve shaping or clipper devices, anode currents though are usually low.
