I hope this helps you understand class A/B and class A amplification a little better. In a class A/B amp, the life span of tubes and every other component is going to be much longer. With a class A amp you have something like a high performance race car that, unless you keep your foot on the brake, is gonna take off like a rocket! Much like any other piece of high performance equipment, almost all components, especially tubes, in a class A amp will have a shorter life span because they are taxed so much from being on all the time. When the light turns green and you step on the accelerator, there is a “rise time” involved before the car achieves cruising speed. The best analogy I can think of is that a class A/B amp is like a car idling at a stop light.

Class A amps also commonly display an interesting phenomenon in which a 30-watt amp will seem as loud as a 100-watt amp. The biggest musical difference between these two electronic circuits is that you get a lot more of a “bark” and a much faster attack from a tube that is always on (class A), as opposed to a tube that is opening and closing when you start and stop to playing (class A/B).

This type of bias enables the current to always flow in the tube, which enables the amp to always be “on.” That means that the output tubes on a class A amp does not have an adjustable bias. Some vintage Marshall amps use EL84 power amp tubes, like the 20-watt PA head, along with British-made Selmer amps are also class A amplifiers.Īnother difference between class A and class A/B is that class A amplifiers are cathode biased, and class A/B amps are grid biased and adjustable. The most well-known examples of class A amplifiers are the Vox AC30 and Vox AC15. N the case of class A circuit, plate current is always flowing in the tube, so the tube is always on, even when you’re not playing and no signal is being applied to the grid. Most amps that we know and love, like Marshalls, Fenders, Ampegs and Hiwatts, Kendricks, to name a few, utilize this type of circuit. In this circuit, signal is applied to the tubes when you play. In a class A/B circuit, although the tubes have voltage applied to them so they are always powered and lit up, the tubes aren’t really on until signal is applied to the tubes’ input, which is called the grid. The basic answer is this: These two classes of amplification illustrate two different types of output tube (or power amp tube, as some people like to call them) wiring. Many people are not familiar with the difference between class A and class A/B circuits, even tough it has a huge effect on an amplifier’s tone.

Adjustable bias guitar amp heads