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

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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?)
 
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.
 
Ray,
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.

Cheers,
Ian
 
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.
 
hey-Hey!!!,
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.
cheers,
Douglas
 
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.....;)
 
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.
 
In class A the power stage conducts all the time, under all power levels.

This is one of the most common textbook definitions of class A. I have seen similar definitions in other books where the "under all power levels" part is left off.

My 300Beast push pull amp is biased at 70 mA per tube, a current that allows continuous current flow through both tubes over most of its normal operating range. The OPT is 6.6K ohms. This amp will make 18 to 20 watts by the strict class A definition determined by a scope probe across a 10 ohm resistor in the filament return.

The amp will make about 26 watts in class AB before clipping starts. Now do I have a 18 watt class A amp with 8 watts of headroom, or do I have a 26 watt class AB amp, or do I have a 26 watt amp that remains in class A up to 18 watts.

All of those statements are technically correct, and I really don't care what it is called. I tend to use the power at 5% distortion criteria for all of my amps, just so they are all measured the same. That makes it a classless 28 watt amp. I have seen plenty of BS advertising copy claiming rediculous Class A power levels especially in guitar amps, helping to fuel this mess.

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.

I don't belong to AA, and don't even read it. I still get emails from some of my friends when they find a a post claiming that I am an idiot, or something like that. I don't need the frustration. Fortunately only a very few of the "inmates" have escaped and found their way here.
 
TL, AA used to be a very informative place.  It has become in stages, starting with Outside, then Water Cooler, and percolating down (or up) through the actual audio fora, a place of very low signal and very high noise amplitude.  I am sorry to see this, just because I value every avenue of discourse, but there it is.  I scan those lists, but the threads of interest have become so rare that I'm starting to consider it more of a duty than an opportunity.

It's just the same ol' tired webgeek stuff.

Aloha,

Poinz
 
ray_moth said:
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?)

Here's the actual design PP loadline I did for a project. The DC loadline sets a Q-Point where Ip= 25mA; Vgk= -30Vdc. As you can plainly see, this VT will be well into cutoff when Vgk= -60V. Not Class A.

The Class B loadline is used here to determine both the power out and the h3 estimated distortion. It's the "Class B" loadline since it would be the actual loadline if the Q-Point current were set to Ip= 0. The justification for using it is that the primary of the OPT responds to differential current. If the same current is pulled by both finals at no-signal, there is no differential current (and no DC core magnetizing either) since the currents are equal. For this design, the blue loadline represents Rl= 1100R / phase.

The Class A loadline is drawn with an Rl= 2200R since both VTs will be conducting the primary current, and so appear in series. The point where the Class B and Class A lines cross is the transition point from Class A to AB. The design nominal Q-Point puts the finals deep into Class AB, though it conforms to the Pd= ~0.8Pd(max) convention. Being that the 6BQ6GTB is a TV horizontal deflection type, in actual practice it was possible to push the Q-Point current up quite a bit: to ~53mA / plate. That's ~18.5W of Pd. Though rated for Pd= 12W, that's for the more demanding horizontal deflection duty. The finals don't red plate at the higher Pd, and the sonics are improved by going more towards Class A operation.

So loadlines tell all. It does operate in Class A for low signal levels.
 
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Joined 2004
Thanks for the opinons - just what I was hoping for. I tend to believe that to call an amp Class A, it has to operate in that region at all power levels to clipping.

That of itself does not preclude the operation of an AB amp in Class A at low levels. Just whether the quality of the sound at that low level would be as good as if the amp had been designed for Class A at all levels is another issue. I would expect the Class A amp to sound (and measure) a bit better in most such comparisons.

An aspect to all this that especially interests me concerns class of operation and tube type, in PP. There have been many and varied writings on this, but the most common view appears to be that triodes are at their best in Class A, while pentodes and beam tetrodes perform better in AB, both in native 'pentode' mode and in ultralinear. This is, of course, a generalization but it seems to be the gist of what I've read.

I can't speak from experience on this, because I have neither the means nor the time to carry out any meaningful experiments of my own, but I daresay others may have done so.
 
a place of very low signal and very high noise amplitude.

In the amateur (or other) two way radio world as the signal to noise (S/N) ratio degrades more and more processing power (human or DSP) is required to dig the signal out of the noise. At some point it becomes impossible. I guess that for my feeble brain that point was reached long ago with the "my kung foo is stronger than your kung foo" attitude that began on RAT.

I read an interesting (which has been kept quiet) study that showed that as we went from analog to digital cell phones, then lowered the bit rate on the digital phones to accomodate more users, the amount of subconsious processing power required by the human brain to understand the conversation has increased. This has led to more distracted users which can be dangerous if the user is doing something important while using the phone, say driving. Some digital formats are much worse than others in that regard.

So loadlines tell all. It does operate in Class A for low signal levels......Just whether the quality of the sound at that low level would be as good as if the amp had been designed for Class A at all levels is another issue.

I guess the load line is the best explanation. As each amp is designed (or copied) differently, with different design goals, the quality of the "class A" sound and the transition from A to AB will depend on the design, and in most cases how the amp is loaded.

A typical P-P class AB design places a lower load impedance on the tubes to extract more power. This would tend to compromize the "class A" operation at lower levels since the output tubes are working harder leading to higher distortion. Then if a ton of GNFB was heaped on to reduce the distortion.....

When the amp is operating "class A" both output tubes are conducting. The output impedance of the amp (damping factor seen by the speaker) is determined by the two tubes and the OPT. When the amp goes into Class AB only one tube is conducting on signal peaks. Therefore the output impedance can nearly double as the signal reaches a peak. Some golden eared people claim to be able to hear the transition from A to AB, but I believe this is just AA speak. What I have seen is the transition from A to AB is gradual. The Rp of the tube being driven to cutoff increases as the tube approaches cutoff. The Rp of the driven tube is reduced as it is driven farther towards saturation. This effect is more pronounced with triodes. This can cause distortion on strong signals when combined with low load impedances. Again the usual cure is NFB.

My 300 Beast was originally concieved as a class A design with zero feedback, planned for operation near 100 mA per tube with 360 volts on the plates and a 6.6K OPT. During the development I found that it sounded better to me with more voltage (400 volts) and less current (70 - 75 mA). In normal operation the amp probably averages 2 or 3 watts with peaks reaching 20+ watts. That is pretty dang loud and ear + speaker compression will be dominant over the "transition from A to AB".
 
Hi,
Gingertube and Miles are right.

But, I find it much easier to refer to the ClassA current limit.

eg. a 20W into 8r0 ClassAB push pull amplifier with 100mA of bias current (SS or it's transformed equivalent for a valve/tube amp) will stay in ClassA for all output currents upto 200mApk.
Above 200mA one half of the output devices no longer control the output current.

That 20W amp is capable of 17.9Vpk into 8r0 and 2.2Apk into 8r0.
The ClassA limit is 200mApk.
A little bit of Ohm's law shows that this 20W amp remains in ClassA for all (unclipped) outputs when driving loads greater than or equal to 90r0, not just at low signal levels.
 
ray_moth said:
That of itself does not preclude the operation of an AB amp in Class A at low levels. Just whether the quality of the sound at that low level would be as good as if the amp had been designed for Class A at all levels is another issue. I would expect the Class A amp to sound (and measure) a bit better in most such comparisons.

Generally speaking, this would tend to be the case. A Class A loadline won't be as steep as a Class AB loadline. Working into a higher plate impedance tends to reduce the harmonic distortion. However, there are certain types that can't operate in Class A, and the 6BQ6GTB is one such type. If you look at the plate characteristics, you can see that the most linear part gives a working Pd= ~40W -- well into red plate territory. Forch, this type also tends to produce mainly h3 with a trace of h5, and so sounds quite good even before applying the gNFB. For the most part, 6.0db(v) of gNFB clears up whatever pentode and OPT nastiness you have running open loop, and 13db(v) of gNFB is definitely tending to a "solid statey" sound.

An aspect to all this that especially interests me concerns class of operation and tube type, in PP. There have been many and varied writings on this, but the most common view appears to be that triodes are at their best in Class A, while pentodes and beam tetrodes perform better in AB, both in native 'pentode' mode and in ultralinear. This is, of course, a generalization but it seems to be the gist of what I've read.

Triodes tend to like very high plate loads, moreso than do pentodes. Triodes are really at their best when working into an active load that makes the plate load much higher than you could accomplish with passive loads and sane Vpp rail voltages.
 
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