I read that the class of an amplifier is determined by how it is biased. So if it is Class A the cutoff voltage is below the bias voltage, and with AB the cutoff voltage is halfway between the Class A cutoff voltage and the bias voltage. Is this true? If so, can you change what class an amplifier is by adjusting the bias? I've heard that you often have to adjust the bias of a new amp when you put new power tubes in, but I didn't know what that meant. Are they implying that some power tubes run in Class A and some in Class AB? Thanks.
The Class of the amplifier is determined by how much bias current is present. If there is a lot of bias voltage, the grids are held 'way negative, then only the tube which is driven by the positive going half wave of the signal at any moment is conducting. As the bias voltage is made less negative and allows both tubes to conduct a little, the crossover notch diminishes swiftly, and you are in class AB2; a little less negative, and they both conduct more, and you have class AB1. If you go further, you get to the point where both tubes always conduct, making the amp work in class A.
There are amps designed that can be run both ways. Most Guitar amplifiers that call themselves "Class A" are really just Cathode-Bias Class AB Push-Pull circuits.
If its a new amp you shouldn't have to put new tube in it, unless you change to a tube that is a non-factory installed value or brand, something has happened to the tubes that are in it or something is wrong inside the amp. Generally if you get a set of matched tubes (duet or quad) and install them. However checking the bias when installing tubes is really the way to go and then after a month when the tubes get broke in check and adjust again. You should re-bias the amp whenever you change power tubes or modify the power amp circuits.
Hope this Helps
Joe
Here's an interesting article that explains the difference between Class A and Class AB.
There is a lot of hype about Class A amps. Many amps advertised as Class A are really AB.
If you know something better than Rock and Roll, I'd like to hear it - Jerry Lee Lewis
