# How can you tell if your amp is operating class A or class AB?

Discussion in 'Amp Tech Center' started by peteb, Jul 24, 2018.

1. ### petebFriend of Leo's

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Basically,

If the plate voltage sags, its AB.
If it doesn’t, it’s in class A.

It really is that simple.

Amps have only one class, and it is determined at full output.

Class AB amps start in class A operation at low volume and then switch to class AB with more signal power.

Where exactly is this dividing line between Class A and class AB operation?

Exactly when the signal rises high enough to cause power supply sag on the plate, is exactly when the class of operation switches over from class A to class B.

Why is that? I will tell you.

2. ### Old Tele manFriend of Leo's

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• "Class AB amps start in class A operation at low volume and then switch to class AB with more signal power."

Some people will argue(*) with this (read Aiken).

"Where exactly is this dividing line between Class A and class AB operation?"

There is none specifically, as IDLE is one END and FULL power is the other END. Statements about "in between" operation are not specifically defined except under "swept-sine" inputs.

• "Exactly when the signal rises high enough to cause power supply sag on the plate, is exactly when the class of operation switches over from class A to class B."

Power supply sag begins immediately upon input drive signal voltage activation, unlike with Class-A operation. Class-A is more like a "tetter-totter" where center (idle) doesn't move(*) and the two seats alternatingly move up and down. Class-AB is more like Old Faithful geyser where things are stable (idle) until water & steam suddenly erupt up (conduction beyond idle), but from a different location (left tube, right tube), but it's always an upward (increasing current flow) motion from stable (idle) and never a downward motion from stable...like "wack-a-mole."

Typically, but not always (tube dependent): Class-AB conduction angle (195º-205º) is far closer to Class-B (180º) than it is to Class-A (360º); the 15-25º idle overlap (both tubes conducting) serves to mitigate Class-B 'cross-over distortion' as conduction transitions from one tube to the other. Class-AB was developed to overcome Class-B distortion,

Last edited: Jul 24, 2018
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3. ### BendyhaFriend of Leo'sSilver Supporter

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Then would you say;
If the plate current rises, its Class AB.
If it doesn’t, it’s in Class A.

And if you take that to be so, could you expand upon it and state wether you think that both voltage and current remain constant in Class A, or if any shift in opposition to the given Class AB shifts are happening ?

Then what is transforming the Class AB current to voltage, and why?

Would you say Class AB is more efficient than Class A, and if so, this voltage sag which you deem to be the defining difference is then linked to this rise in efficiency......how?

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4. ### petebFriend of Leo's

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Bendyha, I know you are going to like the proof.

5. ### petebFriend of Leo's

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Aiken says it remains a class AB amp, but it exhibits class A behavior, or “acts like it is class A”

There is a crossover point when the conduction angle becomes less than 360 degrees, and we are not going to measure the conduction angle.

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6. ### petebFriend of Leo's

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This is describing what’s going on well, but it needs to go to the next step, the sag. What causes it in class AB and not in class A.

This is going to be a point of contention, I can tell, but I think you will see what I mean.

I will get to the theory and the proof, but my experience with measuring power supply sag on class AB amps is that the sag is not that easy to bring on. There is a whole range before it sags, like maybe halfway up half pick attack or maybe earlier on a bassman.

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7. ### petebFriend of Leo's

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Well, yes.

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8. ### dsutton24Doctor of TeleocityAd Free Member

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No. Plenty of class A amps sag under load.

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9. ### petebFriend of Leo's

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The class AB amp playing in class A, no sag, is a bit of an anomaly, as is the class A amp that sags. I think I can explain the class A amp that sags under extreme conditions.

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10. ### petebFriend of Leo's

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Let’s get to the theory. It involves true class A.

11. ### David BarnettDoctor of Teleocity

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Kind of a moot point since there are few if any class A push pull guitar amps.

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12. ### petebFriend of Leo's

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The cathode DC voltage stays the same. That is my best evidence that DC voltage, and DC current, and DC power applied to the plate remain constant. That’s why they say plate dissipation is the same at idle and full power in class A.

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13. ### dsutton24Doctor of TeleocityAd Free Member

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Really? Have you ever seen a Champ that didn't sag?

14. ### petebFriend of Leo's

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We are talking about class A amps. And class AB amps that play in class A at low volume, and class AB at higher volumes. (Most if not all class AB amps do this)

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15. ### petebFriend of Leo's

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Power supply sag.

Hold on, almost there.

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16. ### petebFriend of Leo's

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Please tell us more about champs that sag. Do you have numbers? I do.

My two champs never have sag, unlike my two class AB amps.

Also, my class AB amps draw more wall power with signal, and not just any signal, it needs to be significant, and my champs never draw any extra wall power with signal.

This is the difference between class A and class AB

Theory and proof next

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17. ### dsutton24Doctor of TeleocityAd Free Member

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Petb, over the last few weeks you've gone from trying to understand, sometimes in excruciating detail, how things work, to lecturing experienced amplifier techs on badly formulated 'theory'. At some point the willingness of people to help is going to diminish. I respect your thirst for knowledge, but you should really resist turning your posts into a platform for you to educate us yokels.

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18. ### dsutton24Doctor of TeleocityAd Free Member

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Ukay, I'm out.

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19. ### petebFriend of Leo's

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The definition of class A is that the tube conducts for a full 360 degrees of the cycle, class B is 180 degrees and class AB is between them. The conduction angle of class B is debatable, but Class A is only 360 degrees and class B is only 180 degrees, there is no question about that.

Class A is able to conduct the full 360 degrees because the entire waveform fits between saturation and cut off, unlike class AB which by definition reaches cutoff during part of the cycle, every cycle.

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20. ### petebFriend of Leo's

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You can’t provide numbers? Numbers quantifying sag in a class A tube amp.

Nobody can for good reason. It wouldn’t be class A.

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