Class A vs Class AB Amplifiers

MNM7

Ok. I know this has been discussed before and I'm too lazy to go and search for it. I have also heard all the technical explanations between the different valve amplifier classes. Cause everywhere I ask, everywhere I search in Google it gives me the same technical explanation with graphs and everything, and I get the technical aspect of it but it doesn't really tell me what I want to know. So...

Example: Two guitar amps. For all intensive purposes they are the same except for the amp class. One is Class A and the other is Class AB. What is the difference in Gain, tone. Does the sound differ? Does the class A break up earlier or later in comparison with the class AB?

AlanRatcliffe

As I've recently come to understand it, there isn't that big a tonal difference because of the class itself. It's more to do with how the other circuitry is configured - feedback, biasing, etc.

makepeace

Just in case you skipped these ones, have a read:

http://www.aikenamps.com/ClassA.htm

http://www.aikenamps.com/SingleEnded.htm

http://www.aikenamps.com/VoxAC30classA_2.html

+1 to what Alan's said, the topology of the rest of the amplifier far overshadows that of the class of operation.

You're example is a bit of an abstract one, but I understand what you're getting at, however - two amps operating in different classes can't be the same. My advice to you is that the hype RE Class A/Class AB is something that you shouldn't worry about. I also think that no body will have definitive answers to your questions. Tone isn't really very well described by words, and gain and distortion are not the same. It is an established fact that Class A is inefficient when compared to Class AB, therefore you would get less power for the same amount of real estate, so the overall signal gain would be less. That is not to say that the signal will be more or less distorted. In Aiken's article it is mentioned that as one of the steps to turn an AC30 into a Class A amp, you would have to lower the plate voltage about 100V to get the plate dissipation within the limits when biasing towards Class A. This is just a guess, and I couldn't say for sure without doing the experiment, but I'd say that you would lose headroom as you'd have lower plate voltages. Then again, your output power is also decreased, so your headroom, or the upper border of your clean signal - where the amp begins to break up, as a percentage of the output power might remain in more or less the same place.

Can't help you further than that sir, apart from urging you (if your curiosity regarding this topic is the bane of your existence) to learn some theory on amp design, build a few, and do these experiments yourself.

Gearhead

There is something we can say. Let's start off by asking how much volume you want. Choose between bedroom practice / church use with electronic drums / keeping up with drummer in band practice or small venues / cut through in loud band with ham fisted drummer in not so small venues / full on rock in stadium thing.
Then, tell us what type of music you play. From the two answers we should be able to figure out how much power you need. Do not assume 1W is not so much or 100W is better than 30W.
The thing is, you would be hard pressed get more than 10-12W out of a class A tube, no matter what the type. I have both types of amp and even stuff in between (called ultralinear) and for electric I hardly ever use anything but class AB. The class A clean is wamer, friendlier, more musical and sweet, whereas the AB clean is more sparkly, more pronounced not-a-solid-state. In driven tone, class A is fuzzier, more Santana whereas the AB is more in-your-face kickass, more Angus Young. I reckon jazz guitar and bedroom practice are the only applications for class A stuff.
All this goes for the power stage. Almost all preamp stages in all tube amps are class A anyway.

MNM7

Well it's not a matter of power or what I need. Luckily I know what I need and what I have is sufficient in where I use it. I have a Blackstar HT-5W valve amp. Which just perfect for where I use it(church hall). Cause I have found that anything bigger than 30W is kind of a watse as you have to play at lower volumes, so the valves don't get pushed that much. My 5W is perfect as I can play it loud enough to have the clean room and also loud enough so that the valves are getting nice and warm. it gets mic'd up so it's cool. I was just simply wondering about it. Let me say this. I was checking out Laney from their LC series. And there they have a 30W class A amp and a 50W Class AB amp in the series. Both have the same channel setup. But the 50W had more drive on the clean channel at a lower gain than the 30W had on the clean channel. That's why I was wondering. But It's not a end of the world thing for me. I was just really wondering. But thanx for the replies. ?

Airguitar

Ok I'll give this a go:-
Class A amps refer to the power stage, normally a single power tube which is set up to amplify 100% of the signal, ie both negative and positive parts of the wave.
A tube behaves differently at the top of its conductivity compared to the bottom end. This means it does not amplify in a perfectly linear fashion.
Therefore an amplifies wave in a class A amp will have differing harmonics in its negative cycle compared to its positive one, which will give it a unique sound.
Class A amps are biased right in the center of their conductance so they can swing up as well as down do amplify a full wave. This is why they run hotter than class AB, which I'll cover next.

Gearhead

MNM7 wrote: I have also heard all the technical explanations between the different valve amplifier classes.

Airguitar

Class B amplifiers are not used for audio. They only amplify for a portion of the full wave, so of no use to us guitar players. High powered radio transmitters use class B. Their advantage is that they can be boosted past their rated power levels because they get to shut off for a portion (normally 50%) of a full cycle, therefore getting some "rest".
Now comes the clever stuff:-
A class AB amp is one where two tubes stand on top of each other in a series configuration.
One tube will amplify the positive cycle while the other rests, then the negative cycle gets done in reverse fashion. A greater degree of symmetry is achieved when the tubes are matched, giving a smooth, glassy tone.
This configuration gives the greater power handling capabilities of a class B without its associated distortion problems. (Not overdrive!! That's different!)

For comparison, a Fender Champ has a single class A 6V6 output tube and runs at 5 Watts, while a Deluxe Reverb runs two 6V6's in class AB and is 22 Watts!

I hope all this made sense and helped some.

Airguitar

The technical description ought to help you understand why they sound different.