Note that what most manufacturers pass off as "true Class A" or "pure Class A" push-pull operation is, in reality, merely cathode-biased class AB push-pull operation. If you use the class A biasing rules for these amplifiers, you may burn up the tubes. These amplifiers should be biased according to the class AB rule of thumb, but can sometimes be biased a bit hotter due to the larger voltage drop that occurs across the cathode resistor under full signal conditions. For example, if a push-pull amplifier using EL84 tubes is operated much above 250V, it is probably class AB, because you cannot get true class A operation at much higher plate voltages without exceeding the plate dissipation limits, unless you run a very high plate load impedance, which is not typically done. The operating point in a true class A amplifier is chosen to correspond to a point on the load line where the load line intersects the maximum plate dissipation curve in such a way as to provide relatively symmetrical swing at the output. This maximizes the output power before clipping and makes the amplifier relatively linear.
Αγγελίες
