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Class A tube guitar amps. Everyone’sheard the term. It’s generally associated with higher-end amps in support of anamp maker’s claim that their product sounds “better”. I’ll leave the debate as to which isbetter to others. What I want to discuss is what Class A really means and, fromthe pet peeve perspective, to debunk many amp manufacturers’ claims that theirproducts are Class A when clearly they’re not! You may be surprised at how manyamp makers falsely claim Class A operation. So, let’s review, in practicalterms, what Class A really is and learn a simple rule of thumb you can use tospot operating class BS! There’s no denying that Class A ampshave different tonal characteristics when compared to the more common Class ABamps. However, as with anything related to tone, “better” is in the ear of thebeholder. There are plenty of butt kickin’ amps out there of both classes. What does “Class A operation” mean?Technically, it refers to where on a tube’s operating curve, it’s biased.That’s it. Bias a tube one way and it’s operating in Class A, change the biasand it’s in ClassAB. Of course, there are always technicaldetails that complicate things. In this case, the complicating detail is thattubes biased to operate in the Class A “zone” require a lower voltage supply.Otherwise, they’ll quickly fail. Enough tech stuff, no need to worry aboutthat. I mentioned it to avoid a misperception that you can simply take your ampto a tech for a 5-minute re-bias job and you’re in Class A land. Can’t happen.Fundamental changes to your Class A/B amp would be required to lower thevoltage and otherwise set it up for Class A operation. Back to regularprogramming! Fixed vs. cathode biasing is another“Class A” related misconception. Many believe that if an amp is cathode biasedit is Class A. Not true. An amp can be A or A/B and fixed or cathode biased.Again, it depends on where the tube is biased on its operating curve not how itis biased. Another factor in the myth is that if anamp has a “single-ended” power tube configuration it is Class A. Conversely, sothe myth goes, if an amp has a push-pull power tube configuration it is ClassA/B. Once again, the operating class of the amp is not defined by the power ampconfiguration. It is true that many (most?) single-ended amps are, in fact, ClassA. But, on its own “single-ended” does not define an amp’s operating class. Anamp can be Class A push-pull or Class A/B single-ended or vice versa. The technical difference betweensingle-ended and push-pull power amps are perhaps a topic for another article;I mention it here because they are common terms and often enter into the ClassA vs. Class A/B confusion. I mentioned above that Class A ampssound different from Class A/B amps. To review, Class A means the tubes areoperating in a different part of their operating zone as compared to Class A/B.Two important things happen as a result. First, tubes operating in Class Aproduce more even harmonic content. Second, they produce less power. Tubes biased to operate in Class A/Bproduce more odd order harmonics. Generally, even order harmonics sound morepleasing than odd. That’s why Class A and Class A/B sound different. However,as I mentioned earlier there are MANY GREAT sounding Class A/B amps. Don’t getunnecessarily biased toward Class A amps (amp builder’s humor – HAHA!). The fact that a Class A amp with thesame power tube configuration as a Class A/B amp produces less power leads usto the simple rule of thumb you can use to check whether an amp is really ClassA: just compare the power tube configuration to the claimed output power ratingof the amp. Here are the guidelines I use to tell ifan amp’s manufacturer doesn’t have their facts straight: Power Tube
EL84
6V6
EL34, 6I6 | 2-Tubes
15 watts A/B
20* watts A/B
40 watts A/B | 4-Tubes
30 watts A/B
40 watts A/B
80 watts A/B |
*Some smaller Class A/B Fenders (e.g.early Princetons and Deluxes, etc.) that feature two 6V6s are rated around 15watts – later examples are in the more typical 20+ watt range. If the marketing hype shows output powernear or above these ratings and it says the amp is Class A – there’s somethingwrong! If the output power is a fair chunk lower, the amp is most probablyClass A. Simple! Next time you read a guitar mag have funby checking the tube configuration, power rating and operating class claims ofyour favorite amps. You may be surprised at what you find!
[ 本帖最后由 kldguitar 于 2012-10-24 18:36 编辑 ] | 
发表于 2012-10-24 18:30:13
