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7 years ago
I was going to try to answer this post yesterday but knew it would be a mouthful...Props to 'Guitarplayer' for a great explanation of tone vs feel. That's good stuff right there.
Thanks, that's very helpful. If the manufacturer doesn't choose to spell it out in promotional material, is there any way of knowing whether a particular amp model is A or AB?
Okay, here's a different viewpoint, in favor of Class A having a different (and better) sound, from a review of a class A amp on premierguitar.com:"What makes the Class-A amp desirable for many guitarists is the way the output tube distorts. Class-A output distortion strongly favors the even order distortion overtones, while class AB favors the odd order overtones. Even order distortion is far more pleasing to the ear in most situations and is commonly described as having a “singing” quality."Is there anything to this?
3 years ago
This is from their review of the 57 Champ re-issue, I believe. A great example of what I was bitching about in my earlier post... : )It's partly true, partly hokum. The part that's true is that the Champ is a Class A amp - the reason for this is it has only one output tube, which means it has to operate in Class A by definition, unless you like the sound of having half your waveform chopped off. The part that's hokum is the part about the Class A amp "favoring even-order harmonics". Operating class is nothing more than the way the tubes are biased. Class A means tubes conduct throughout 100% of their duty cycle. That's all.The thing that makes the Champ's output distortion different from, say, a Bassman is that the Champ is single-ended (one tube) and the Bassman is push-pull (two tubes operating in a complementary pair, wired to opposite ends of a center-tapped transformer). In the push-pull circuit the second harmonic distortion of the two tubes are mostly canceled in the output transformer. This was the main incentive behind development of the push-pull amplifier circuit that has been in use since at least 1930. So the idea that the Champ has more "2nd harmonics" in its distortion because it's Class A is misleading - if it has, which is debatable, it is because it's single-ended rather than push-pull. Coincidentally, single-ended amps are all Class A, but not all Class A amps are single-ended. You can make a Class A push-pull amp easily enough. It will behave similarly in terms of distortion to an AB amp because they're wired the same with respect to the output transformer. The reason I would argue that amplifier class is not very significant in terms of one being "better" than the other is that most amps are chock full of single ended (and thus Class A) operating tubes in their preamps. A lot of the distortion you hear when you crank an amp comes from these tubes rather than the output tubes, therefore there are plenty of 2nd harmonics being generated that are not being cancelled, more than enough to make an amp "sing". I've had plenty of push-pull AB amps that "sing" like crazy, and it is mostly because they play really loud and have high-gain preamp circuits. A quick recap of this mostly tedious topic:All single ended amps are Class A by definition. (Champ, etc. - anything with only one output tube, even (gasp!) the $100 Epiphone Valve Jr., so it's not limited to the high end.) Back in Leo Fender's day no one would have thought of using operating class as a marketing buzzword, it's just silly.Not all Class A amps are single-ended - you can make a Class A push-pull amp by having two tubes operating thru 100% of their duty cycle in Class A.Significant trade-off in power output, heat and tube life though, which is why you don't see that many. They also cost more, mostly because the output transformer has to be bigger.All Class AB amps are push-pull, it's inherent in how they work. You can bias a Class AB push-pull amp really hot, very close to Class A but it's not a good idea, lots of amps have been destroyed doing this. It's an old joke that "it never sounded better, right up to the point where the smoke came out"...Many of the most highly regarded tube amps in the world operate in Class AB2. It's their careful design, quality components, and robust construction that makes them great, not what operating class they are. SF
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