What is the difference between A and AB class tube circuitry?

Answer 1

Same difference for tube or solid state circuitry! The classes refer to the biasing of the transistor(s) or tube(s) during quiescent (no signal) conditions. In class A, there is one output device. It is always on, or conducting. During quiescent conditions, it is biased on so that the output terminal (plate or collector) is at about 50% of the supply voltage. This offset voltage must be removed with a transformer or coupling capacitor so the output of the stage is 0V with no signal applied. As the input signal varies, the device conducts harder, toward saturation, or less, toward cutoff. As long as the stage is not driven into clipping, class A amplifiers can have very low total distortion, and NO crossover distortion, since they never turn off. Many audiophiles consider class A to be the best possible arrangement for audio reproduction. The biggest drawback is since the device is about half on even with no signal applied, the power dissipation is huge! You can always spot a class A amp by its mammoth heat sinks or its size. I've seen class A monoblocks about a cubic foot in size that only put out 15 watts! Class B amps, commonly called push-pull, have two devices, and bipolar power supplies (where you have a positive, a negative, and a common, like 45-0-45). During quiescent conditions, both devices are off, and the output is 0V. There is no need for coupling caps or Transformers, since there is no big voltage offset at the output. As the input swings positive, one of the devices turns on, and 'pushes' the output up towards the positive rail. As the input swings negative, the first device turns off, then the second device turns on, 'pulling' the output towards the negative rail. The good news is that quiescent current is almost zero, so dissipation is much less, especially with no signal. The bad news - there is a spot, where the input signal passes through zero, where both devices are off. This creates a little flat spot in the output signal, the dreaded 'crossover distortion' that audio purists hate. What if you create a hybrid that looks like a class B stage, but where both devices are biased on just a little during quiescent conditions. Like a class A device, just not as much. What would happen? Watch the signal as it starts down from the positive peak: The positive device is almost saturated (fully on). The negative device is in cutoff (fully off). As the input starts to go down, the positive device conducts less and less. As it APPROACHES zero volts out, the negative device starts to conduct. When the output EQUALS zero volts out, both devices are conducting a little, so they cancel each other out. As the output LEAVES zero, the positive device goes into cutoff, and the negative device conducts more and more, pulling the output negative. See what happened? There was never a time when both devices were off. Instead, there was a smooth 'handoff' between the two as the signal passed zero. This is a class AB amp. Properly designed class AB stages can come close to the performance of a pure class A amp while still maintaining fairly low quiescent current, and have much better efficiency too. The audio guys will, of course remind us that they will NEVER be as good as a class A!