(From The Pet Peeve Dept.)
Class A tube guitar amps. Everyone’s heard the term. It’s generally associated with higher-end amps in support of an amp maker’s claim that their product sounds “better”.
I’ll leave the debate as to which is better to others. What I want to discuss is what Class A really means and, from the pet peeve perspective, to debunk many amp manufacturers’ claims that their products are Class A when clearly they’re not! You may be surprised at how many amp makers falsely claim Class A operation. So, let’s review, in practical terms, what Class A really is and learn a simple rule of thumb you can use to spot operating class BS!
There’s no denying that Class A amps have different tonal characteristics when compared to the more common ClassAB amps. However, as with anything related to tone, “better” is in the ear of the beholder. 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 bias and it’s in ClassAB.
Of course, there are always technical details that complicate things. In this case, the complicating detail is that tubes 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 about that. I mentioned it to avoid a misperception that you can simply take your amp to 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 the voltage and otherwise set it up for Class A operation. Back to regular programming!
Fixed vs. cathode biasing is another “Class A” related misconception. Many believe that if an amp is cathode biased it 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 it is biased.
Another factor in the myth is that if an amp has a “single-ended” power tube configuration it is Class A. Conversely, so the myth goes, if an amp has a push-pull power tube configuration it is Class A/B. Once again, the operating class of the amp is not defined by the power amp configuration. It is true that many (most?) single-ended amps are, in fact, Class A. But, on its own “single-ended” does not define an amp’s operating class. An amp can be Class A push-pull or Class A/B single-ended or vice versa.
The technical difference between single-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 Class A vs. Class A/B confusion.
I mentioned above that Class A amps sound different from Class A/B amps. To review, Class A means the tubes are operating 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 A produce more even harmonic content. Second, they produce less power.
Tubes biased to operate in Class A/B produce more odd order harmonics. Generally, even order harmonics sound more pleasing 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 get unnecessarily biased toward Class A amps (amp builder’s humor – HAHA!).
The fact that a Class A amp with the same power tube configuration as a Class A/B amp produces less power leads us to the simple rule of thumb you can use to check whether an amp is really Class A: just compare the power tube configuration to the claimed output power rating of the amp.
Here are the guidelines I use to tell if an amp’s manufacturer doesn’t have their facts straight:
15 watts A/B
20* watts A/B
40 watts A/B
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 15 watts – later examples are in the more typical 20+ watt range.
If the marketing hype shows output power near or above these ratings and it says the amp is Class A – there’s something wrong! If the output power is a fair chunk lower, the amp is most probably Class A. Simple!
Next time you read a guitar mag have fun by checking the tube configuration, power rating and operating class claims of your favorite amps. You may be surprised at what you find!