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ray_moth 22nd January 2009 09:20 PM

Can a Class AB PP amp be said to be operating in Class A at low signal levels?
In a debate on AA, this age-old issue is is being hotly contested.

Some say that as long as both halves of the PP are conducting 100% of the time, then it's operating Class A, regardless of whether it's designed to be Class A or Class AB at higher signal levels.

Others say that this is ignoring the full definition of Class A, which requires 100% of conduction at all signal levels up to clipping.

Who is right? (And does it matter?)

adason 22nd January 2009 09:25 PM

I am no expert here, but just for fun I take one tube out from the PP amp. So it has only one output tube. Sounds actually good at low levels. Must be class A.

Poindexter 22nd January 2009 09:33 PM

Ray, I got in this same argument over there, and got roundly scolded for describing my amp as 'Operating in class A under normal signal levels, only into class B on peaks'.  As I recall, it was a doubleE that was doing the scolding, so perhaps that is in fact the formal definition of class A, AB. As you know, in technical and scientific fields definitions don't have to be logical or sensical, as long as everybody knows what they are.

If this is correct, I'm a bit disappointed, as it deprives us of some descriptions for PP operation at various signal levels that are very useful.  On the other hand, we could keep describing in these terms and let 'em scold away.

Poinz, who, with his pair of tiny AS, ain't got no tech cred anyway.

dsavitsk 22nd January 2009 10:14 PM

Re: Can a Class AB PP amp be said to be operating in Class A at low signal levels?

Originally posted by ray_moth
Who is right?


Originally posted by ray_moth
(And does it matter?)

It's a matter of semantics -- define Class A how you will, and your amp will either fit into the definition, or not. Poinz' amps conduct 100% of the time at low signal levels, and not at high. Call it Class P.

gingertube 22nd January 2009 10:28 PM

I saw the thread on AA but did not bother posting a reply there.

Many make this argument much more complex than it is.

If the tubes in a push pull amplifier were biased at zero idle current with one tube handling the positive signal swings and the other handling the negative signal swings then that would be pure Class B. Each tube handles 50% of the cycle.

If (for example) the output tubes were instead biased to idle at 45mA and the signal level is such that you are swinging +/- 35mA (that is up to 45 + 35 = 80mA and down to 45 - 35 = 10mA) both tubes are conducting for 100% of the signal swing and that by definition is Class A.

If on the other hand the signal level is such that you try to swing +/- 70mA then the tube current will swing up to 45 + 70 = 115mA and will swing down to 0mA for (it can't swing to a negative current) for a part of the cycle - that is the output tubes are conducting for more than 50% BUT less than 100% of the signal swing then it is neither Class A nor Class B but somewhere in between and that by definition is Class AB.

With RF Amplifiers (But NOT Audio Amplifiers) operating into a tuned "Tank" Circuit were you ONLY need to give the "tank" a "Kick" to keep the oscillation going, it is possible for the tube(s) to conduct for less than 50% of the time and that is by definition Class C.

That is, a Class AB Amplifier is always Class A for low signal levels and at high signal levels it transitions from Class A into Class AB for some part of the input cycle.

This leads to expressions we see used such as "First Watt". In tube Class AB amplifiers the first watt is often pure Class A.

Confusion arises when the terms are miss used. The classic miss use most often sited is the VOX AC30 Guitar Amp which always made the claim that it was 30 Watts Class A. Actually because of the high bias currents used it can put out about 14 to 15 watts while staying in Class A BUT at 30 watts out its is definitely Class AB.

Other confusion arises when we see people talk about Class AB Amplifiers and saying something like"above a certain signal threshold the amplifier transistions from Class A to Class B". To be strictly correct what should be said is that above a certain signal threshold the amplifier transistions from Class A to Class AB.

My 90 Watt Class AB Solid State Amp can put out a few milliwatts in pure Class A since it idles at just enough current to eliminate (well reduce anyway) cross over distortion.

My 10 Watt Class AB Baby Huey Tube Amps can probably (educated guess) put out about 3 watts of pure Class A.

I did not post to AA as I often view many of the threads there as a waste of time. Since it is un-moderated there are sometimes almost as many posts/threads which are intended to provoke as those intended to inform. I ignore those threads and there are several inmates that when I see their name I don't even bother to read.
Counter to that is the fact that there are several "Cluey" guys who seem to like the "free wheeling" attitude over there and so I continue to at least look at it and ONLY ocassionally post.

I prefer the polite ambience here.


jjman 22nd January 2009 10:46 PM

I'm far from an expert but my understanding is that the class is a "design" and not a condition at any given operating point. I believe that something like "at full rated output" or something like that is part of the definition?

"My Class AB amp operates in Class A until it gets up to........."

It would probably be more appropriate to say "My class AB amp doesn't reach cutoff until........"

But I think most people understand what a person is saying when they loosely (or wrongly) use the term.

Many advertisements for guitar amps claim they are Class A when they are not. This for marketing purposes since "Class A" sounds like it must be "Class Best." And there are no Class Police stopping them from doing it.

Bandersnatch 23rd January 2009 12:25 AM

In class A the power stage conducts all the time, under all power levels. If it is SE the definition is quite straightforward and easy to see( IMO ). In PP, it is still simple, the finals conduct all the time, under all power levels. If you violate that, then it is something else.

AuroraB 23rd January 2009 12:57 AM

By all definitions, most class B amps do have a "class A region" where the output devices does not switch off. This is the very definition of class A. One cannot, of course, call a 50W amp with a mere 1W "class A region" a class A amp. A class A amp must of course operate all output devices at all power levels. The rest is just semantics.

OTOH- while I am slowly moving towards some tube projects myself, I cannot nothingbut wonder why so many tube fundamentalists seemingly claim tube amps to operate under other laws of physics than SS.....;)

Eva 23rd January 2009 01:30 AM

By definition, class AB involves class A operation at very low output current levels.

The problem is the discontinuity in the transfer function that arises when output current demand increases and each half of the output stage becomes unbiased. With ideal output devices, open loop "gain" is theoretically doubled when both halves are working at the same time (because current is rising in one side and falling in the other). This results in distortion unless the bias point is carefully chosen to compensate for the reduced gain that real output devices exhibit at very low currents.

I have no experience with tubes, but after looking at a couple of arbitrarily chosen datasheets (300B, EL34), it seems that the reduced gain at low currents phenomena is far more drastic in tubes than in bipolar transistors, so class AB makes even more sense.

Poindexter 23rd January 2009 01:46 AM


You see?



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