Idiots guide to amps

[jmh151]

I'm researching amps, and I keep seeing all this talk about amps being Class A, Class B, Class AB, some being switchable from Class A to Class AB, pure Class A, particularly on the Mesa Boogie web site.

I've googled it, and still don't understand the terminology and what it means to the sound of an amp.

Can someone explain to me in guitarist (laymans) terms what this means and the effect it has on tone? I'm an idiot on this one and can't comprehend the significance.

[Koreth]

Re: Idiots guide to amps

Class A means that current flows in the tube for the full cycle of a wave. Voltage and current swing up, they swing down, but average current through the tube remains constant. It is the least efficient mode of operation, as you only get half of the tube's total possible voltage swing for each half of the wave. Class A amps are usually Single Ended (meaning output tubes sits on only one side of the transformer, with voltage fed from the other), as a single ended amplifier is useless for audio unless it is class A

Class B means the current flows through the tube for half the cycle. The input wave swings up from center, the tube turns on and current flows through it. The current swings back down to center and once it swings below center the tube shuts off. Much more efficient than Class A, as you can make use of the entire voltage swing of the tube. For a Class B amp to be useful for audio, you have to have them in a push-pull arrangement, where the transformer has tubes on both sides. One tube takes care of the positive voltage swing while the other is shut off, and when the first one shuts off, the second takes care of the negative voltage swing.

Unfortunately, pure class B will get you some nasty distortion that doesn't sound all that nice for audio and music work. Thus Class AB - current flows through the tube for more than half, but less than the full wave cycle. Thus, the first tube doesn't shut off until the second tube has already started its half of the voltage swing - there's a small space in the midle of the wave where both tubes are running. Not as efficient as Class B, but the reduction in distortion is kinda necessary for audio work. Again, a class AB amp will be a push-pull arrangement.

Class AB push-pull amps have another effect beyond their efficiency. They're good at canceling noise and harmonic distortion. Unfortunately this means a Class AB amp will be canceling out some of that 2nd order harmonic distortion that tends to sound pretty. Thus single-ended Class A amps get the mystical status they do - all the tube's harmonic distortion comes though.

FWIW, the Vox AC30 is NOT Class A. You will not get 30W of output out of a quad of EL84s operating class A. In order to get enough voltage swing to get 30W out of a quad of EL84s operating class A, you'd have to have their idle current set higher than the tubes can handle and you'd burn em out.

[GuitarDoc]

Re: Idiots guide to amps

Isn't the AC30 class AB? That's what I always thought.

[jmh151]

Re: Idiots guide to amps

Thanks a lot for the thorough answer!

[AniML]

Re: Idiots guide to amps

I actually understand this explanation. Thank you

[DankStar]

Re: Idiots guide to amps

Koreth - can you add the crate powerblock's in those terms - isn't it class D or sumthin'? or is this only for tube amps?

[Koreth]

Re: Idiots guide to amps

Class D is another beast entirely and is likely to only be found on solid state setups. At least, I've never heard of a tube Class D amplifier. However, that doesn't mean they don't exist. Class A and AB amplifiers are what're called linear amplifiers - the input signal applied to them is directly proportional to the output signal. In less words, the output signal is the same shape as the input, just bigger. Class D amplifiers are switching amplifiers - They make use of square waves altered by the input signal, a pair of active devices that switch between full on and full off to amplify the square wave, and then some filtering to turn the square wave back into the original signal. Class D amplifiers can approach near perfect efficiency because they operate the transistors as switches. Why this is more efficient gets into math and Ohms law - but by operating the transistors as high speed switches, they produce much less waste heat than they would operating Class A or Class AB.

What happened to Class C? Class C is a linear amplifier mode, where the tube passes current for less than half the cycle of the input wave. The tube doesn't turn on and pass current until the tube is well into the positive (or negative) voltage swing. They're great for high power applications in radios that operate in a narrow frequency range, but are useless for audio, and thus you won't hear about them when it comes to solid state or tube guitar amps.

[DankStar]

Re: Idiots guide to amps

wow - thanks! it's getting vault worthy in here.