Why Class AB is bad

[andrei]

Class AB is often touted as having a higher efficiency than Class A (which is true), but it often overlooked that Class AB distortion characteristics are essentially the same as Class B (i.e. really bad). Here is an example:

You decide to build a super-duper 300B amplifier, and settle on a push-pull output stage. You know a single 300B is happy to drive 3k load, so for push-pull you get 6k:8 output transformer. You decide to save a little bit on power, and run the stage at 30mA idle current per tube. It's Class AB (Class A would idle at 70-80mA), so you should get 15W power output no problem, right?

Wrong. Here is what happens instead:

[andrei]

Distortion at 7W output is about 10%, and the waveform looks really bad. In comparison, if I was running the tubes in Class A at 60mA idle current per tube, I would get my 15W at 0.5% THD.

The moral is that for a given tube, Class AB operation is not going to give me more output power than Class A at a given distortion level (say, 5%). Of course, if I use feedback, I can make Class AB stage respectably linear, but I can do the same to Class A, so there is no real advantage to using AB...

[SHiFTY]

That is why negative feedback is necessary in a class AB amp. I would imagine a small amount of negative feedback would fully clear up this crossover distortion in the schematic pictured.

If anyone was making a PP amp with no feedback they would be well advised to make it operate fully in class A, even if it shortens the life of the output tubes.

[Geek]

Another often overlooked item is the P-P load difference in class-ABx. Two steps to fixing the problem, 1) Add NFB and 2) increase the P-P load

[SY]

That's odd, I've had plenty of p-p amps around here with no loop feedback, and the distortion is no higher than SE, in fact, often much lower. For example, in an amp in front of me at the moment, a pair of 7027A in AB1 triode, 8K plate-to-plate give me 10 watts at under 1% THD before feedback. 1W distortion is under 0.05%, totally dominated by third.

So, if what you're saying is that one can come up with a hack circuit that doesn't work well, OK, that's true. But if you're trying to claim any universality to that observation, I'm sorry, it's just not correct.

edit: to do your comparison properly, you need to recognize that keeping the load constant requires the plate to plate load to be 4x higher than a single tube.

[Tubes4e4]

Hi Gregg,

Originally posted by Geek
Another often overlooked item is the P-P load difference in class-ABx.

Yeah, and because of this the spice simulation done by Andrei proves nothing at all. Or since when does spice adjust the load conditions from Raa/2 to Raa/4 (as seen by a single tube in a PP pair) automagically and smoothly, when doing the slow transition from class A1 to B1?

Damn simulations

Tom

[tubetvr]

Hi,

I am assuming that the distortion figures comes from a simulator? If so then the result would be very dependant on how good the model is.

Even so the result is not that surprising for a tube like 300B and is well known. In this paper http://www.clarisonus.com/Archives/T.../BeamPower.pdf
on pages 38 and forward this phenomena is described for theoretical triodes and also make a comparison with beam power tubes showing that these can give lower distortion in AB than triodes.

I think it is important to note that distortion level in AB with real triodes depend on the triode curve itself, and the chosen operating point.
Please also read what is written in the paper about the 0 distortion class B1 operation point that gives lower distortion than AB.

In some other books like Terman it is described about using projected cutoff operating point for tubes operating in close to B, the theory here is that there is a operating point for some triodes which give a minimum of distortion, this is usually at very low idle current.

So in contradiction to what many believe at least for triodes lower idle current and operating close to class B can give better results than operation closer to A.

Some regulator triodes like 6AS7 and 6C33C have more remote cutoff than e.g 300B, and they operate with a clear distortion minima at a quite low idle current in class AB.

Regards Hans

[Geek]

Hi Tom,

I have found simulations only useful for finding DC operating points and quite useless for analog AC parameters

[Tubes4e4]

Originally posted by tubetvr
on pages 38 and forward this phenomena is described for theoretical triodes and also make a comparison with beam power tubes showing that these can give lower distortion in AB than triodes.

Yes, theoretically they can, but the ideal load range is very small. For many well known tetrodes/pentodes the manufacturers do supply distortion nomograms in the spec sheets. You can see there, that the least distortion point is very distinguished and distortion rises steep sideways from that ideal load point.

In real world circuit with real world loads (like speakers changing their impedance over frequency - thus giving a reflected load to the tubes with an error multiplied by OPT impedance ratio!) that means, that triodes usually are much more forgiving to load mismatching by real world loads like speakers.

Tom

[tubelab.com]

The impedance ratio in an OPT is the square of the turns ratio. To present each tube with a 3K ohm load you need a 12K P-P transformer.

At least that is what the theory says. My amplifier (P-P 300B) runs a 6.6 K transformer that was made for 6L6's. I set the bias at about 45 mA per tube which allows class A operation up to about 10 Watts, at that point the amp crosses into AB1. I get 26 to 28 watts per channel from this amp. It has no local or global feedback. There is no visible crossover distortion, and the measured performence is quite good also.