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

[tubelab.com]

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.

[Poindexter]

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

[Miles Prower]

Originally posted by ray_moth
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.

[ray_moth]

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.

[Tweeker]

If maximum plate efficiency exceeds 50%, I tend to view it as not class A, whatever the power level.

[tubelab.com]

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".

[AndrewT]

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.

[Jeb-D.]

I have seen plenty of BS advertising copy claiming rediculous Class A power levels especially in guitar amps, helping to fuel this mess.

OTL manufactures are very guilty of this as well. Either they are lying about the class of operation, or they are rating using peak-to-peak power

[Miles Prower]

Originally posted by ray_moth
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.

[Miles Prower]

Originally posted by Jeb-D.
OTL manufactures are very guilty of this as well. Either they are lying about the class of operation, or they are rating using peak-to-peak power

Most commercial amps are rated in "whats", not watts. The marketing dept. likes to play fast and loose with specs in order to make bragging points. God knows what they come up with.