Class A Tube Amps? What does this mean?

Class A is a mode of operation where current is always flowing in the plate circuit of an amplifier tube. All "preamp" tubes run this way. A Class A tube can operate by itself, called a "single ended" amp. If you see an amp that only has one power tube, you know it's Class A.

A Class B amp (not used in guitar amps) is biased so that current flows in the plate circuit of the tube for half of the cycle of the alternating wave being amplified. It's got nearly zero current flowing at idle, and current flows during the positive half of the signal. That won't work with a single tube, it'd only pass half the signal and it'd be very distorted. So they're always installed in pairs, one tube amplifying the positive half, the other the negative half of the signal.

A Class AB amp is between a Class A and Class B. Current flows in each tube more than half the time, but less than full time. Part of the time it's "cut off." The other tube fills in. The closer it operates to Class B conditions, the more efficiently it operates and the more power it can put out within the tube's maximum power dissipation limits.

It's B.S. when people start talking about "Class A watts" as louder than "Class AB watts." Not true. Some Class A amp salesman must've come up with that one because a competitor's Class AB amp was rated at more power. Class A amps are the least efficient and waste the most power as heat. Also, within Class A operating limits, a Class A amp uses exactly as much power when it's quietly idling as when it's putting out full volume. All of that power is wasted as heat when it's idling. Paradoxically, when it's driven hard the output power is drawn from that wasted power, and the amp actually runs cooler than when it's idling. A Class AB amp runs cooler when it's idling and gets hotter as it's pushed harder. That's hard to understand without drawing a set of graphs of voltage and current (which can be found in texts like "The Radiotron Designer's Handbook"), but it's true. With a given set of tubes, you can get a lot more output power out of Class AB operation than Class A, which is why it became popular. Class A allows simpler and cheaper amp construction for low-powered amps.

Class A single ended amps do go into distortion in a different way than double ended amps, which are usually Class AB.

A single ended amp ideally would be biased exactly in the middle between the grid voltage that "cuts off" the plate current and the voltage (a little less than -1V) where the grid starts to draw some current from the electron stream. The more positive it gets, the more current it draws. Usually the grid is driven by the plate of the previous stage through a coupling capacitor, with a large resistor going from the grid to ground. When the grid draws current on the positive peak of the cycle, it limits the positive swing of the cycle. It won't easily go more positive, because the negative electrons flowing into that side of the capacitor neutralize the positive charge the signal's putting on it. It charges up the capacitor so that it goes more negative than the signal would've driven it when it goes back negative. It's the same as turnng up the bias on the grid to a more negative value. The accumulated charge can't leak away quickly, because it has to flow through that resistor. So what happens as the amp is overdriven is that you first get a soft "clipping" on the positive peak of the signal, but a harsher clipping on the negative peak as the tube's driven into cutoff. That's an asymmetrical waveform, so it contains a lot of second harmonic and other even harmonics. Push it into crazy overdrive, and you get more or less sharp clipping on both peaks, which gets to be more symmetrical. Symmetrical waveforms have predominantly odd harmonics. A single ended power amp stage will rarely be pushed to the point of symmetrical clipping, though preamp stages often are.

A double ended Class AB amp will behave differently. As the positive peaks draw current to the grids and charge up the coupling capacitors, the negative bias voltage on the grids rises. There's only very soft clipping on the peaks, but as the bias level starts getting higher, each tube begins to go into "cutoff" before the other one's come out of it fully. Instead of each half of the signal blending seamlessly into the other, there begins to appear first a slight notch or "glitch" at the crossover point, then a flat line segment between the positive and negative signal halves where no current's flowing. Nice and symmetrical. This is called "crossover distortion." It's all odd harmonics, primarily third harmonic. That's the "sweet power tube distortion" that fans of cranked amps with paired power tubes love.

There's some misinformation on the Web to the effect that "tubes sound better than transistors because they produce even harmonics, which sound good, while transistors produce odd harmonics that sound harsh and unmusical." That's totally untrue on every count. I've just shown how tubes can produce both even and odd harmonics, which predominates being dependent on the operating conditions. The same is true for solid state devices. And both even and odd harmonics are musical. The second harmonic is an octave. Kinda sounds like playing a twelve string guitar when it's the predominant harmonic. The third harmonic is a perfect fifth in the next octave up. The fourth harmonic is another octave, two octaves up from the fundamental. The fifth harmonic is a perfect major third, two octaves up. If you listen carefully as you increase the gain distortion on a single note with a single ended preamp, you'll hear it go from clean to a 12-string like sound as the second harmonic comes in, a "power chord" as the third harmonic becomes more prominent with more symmetrical clipping, and finally you can detect the sound of a major chord as the fifth harmonic becomes audible.
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Wow! That's the best explanation of class A & AB I've read, Ricochet - and I've been thinking about this since the early 70s 8)

You even begin to deal with the differences and similarities between tube & SS amps - but I'd love to hear more from you on this. I know the early SS amps were sadly deficient in overdrive. I had a Triumph 100W solid state amp (4x12 combo - weighed a ton) which was tremendously loud, but had horrible fuzz-like, inflexible overdrive, and a (class A) Vox AC30TB, which went from bell-like clean to rich creamy fruity overdrive seamlessly just by turning the volume knob. I think I used the Triumph on one solo in the studio, and that was that. Hated it for anything but live, and then cringed all too often

But I've a little 15W Fender SS amp now which is use for practice, and it has a lovely, warm, bluesy natural overdrive. Never been able to work that one out - I can understand the sparkling cleans it gives, but how does it get that overdrive without tubes (or digital models of tubes)? And the Marshall MG series is the same, beautiful flexible, responsive overdrive - boggles my mind - not that it's boggle-threshold is all that high, I fear :shock:

Ric ...that must have taken you time i bet.

I guess you can write a whole article on Amps (including this one !) and post it in the lessons sections out here.My HO, :D

Rahul

If the question was "what time is is it?" and I built a watch, I apologize.

I love this stuff. I've been interested in electronics, especially of the vacuum tube sort, since I was 4 or 5 years old. Had to figure out what happens when a resistance-capacitance coupled tube amp's overdriven on my own. The old textbooks just say "Don't overdrive them, it produces unacceptable distortion." There's another subclass of amps not normally found in guitar amps with a subscript 2 after the class letter, like "Class AB2." The 2 means that the amp's designed to operate while drawing grid current. It's got a low-resistance path to ground to "get rid of" the grid current so it doesn't cause the bias changes described above.

That's how junction transistors (the original type) operate all the time. A transistor is just like a tube always operating with a positive grid drawing current. In the case of the transistor, that's base current, rather than grid current. Since there's never a point where "grid" current starts flowing and discharges a coupling capacitor, when it's overdriven it hits a limit when it "saturates," i.e. the current in the collector circuit ( the collector's analogous to the plate of a tube) won't go any higher no matter what the base voltage. That happens pretty abruptly, whereas the R/C-coupled tube goes through the changes I mentioned above fairly slowly. The nice thing about a tube amp is playing in that transitional zone where it's clean if you play softly, very "crunchy" if you play loud, and it gives this wonderful feeling of "touch sensitivity," being able to modulate the tone with your playing fingers. Field-effect transistors work more like negative-grid tubes, and the amp makers have gone to using them extensively in modern solid state amps, which are far better than the first attempts at building SS amps. Also, early SS amps were largely designed by people with experience in engineering hi-fi amps. What sounds good in a hi-fi for reproducing music isn't what works best for producing music in a guitar amp.

I'm still not very familiar with that new-fangled solid state stuff. If anybody sees flaws in the explanation above, please correct them. Don't ask me any detailed questions about SS circuitry, I probably can't answer. Tubes are my thing.
:lol:

Thanks! That was theb est explanation ever. I LOVE GUITAR NOISE!

You're welcome. :D

Thanks! That was theb est explanation ever. I LOVE GUITAR NOISE!

Me too.. Thanks Ricochet. :) Very Much.