Class A and A/B what is the difference

jadams71

Member
Messages
466
I am in the market for a new amp and when I bought my first amp I bought waht sounded good to me and was very uninformed about what makes an amp work. Now that nearly 20 years have past I am interested in how amps work. Could someone explain to me what Class A and A/B mean? I hear it all of the time and am very curious.
 

GT100

Member
Messages
3,942
I am in the market for a new amp and when I bought my first amp I bought waht sounded good to me and was very uninformed about what makes an amp work. Now that nearly 20 years have past I am interested in how amps work. Could someone explain to me what Class A and A/B mean? I hear it all of the time and am very curious.
This has been discussed waaaaay too much already.
Do a search.

All I'm going to say is that one is not better sounding than the other -don't listen to anyone who says so...
 

donnyjaguar

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4,194
Class A amplifiers generally have no global negative feedback. That teamed with their nature of producing even-order distortion products can make them sound quite musical. They seldom have enough power to drive speakers to gigging/jamming volume though.
 

phsyconoodler

Member
Messages
4,305
Unless you use 4-EL34's at 250v plate supply for about 20 watts class A.
Or 4-6550's at about 300v for 30 watts class A.
What a waste of tubes and it makes a great space heater.You could toast bread over the vents or pop Orville popcorn.
 

Mike Fleming

Member
Messages
1,228
Class A amplifiers generally have no global negative feedback. That teamed with their nature of producing even-order distortion products can make them sound quite musical. They seldom have enough power to drive speakers to gigging/jamming volume though.
Do you mean single-ended class A? Don't even-order harmonics generated by a class A power section cancel out in a push-pull configuration?
 

phsyconoodler

Member
Messages
4,305
No,it's the way the tubes are biased.Single ended Class A and push-pull class A are almost the same.The advantage to push-pull is the size of the OT.It doesn't need to be 20 lbs.
 

Mike Fleming

Member
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1,228
Right, and what i mean is -- any even-order harmonics being generated by class A output tubes would be cancelled out by the transformer if they were push-pull, wouldn't they? I.e. if you had tubes running class A in a push pull configuration, you wouldn't necessarily get more even-order harmonics from them, since the transformer is cancelling the even-order harmonics; right?
 

GT100

Member
Messages
3,942
No,it's the way the tubes are biased.Single ended Class A and push-pull class A are almost the same.The advantage to push-pull is the size of the OT.It doesn't need to be 20 lbs.
Push pull (regardless of Class of operation) eliminates the even order harmonics produced (by the power tubes).
 

GT100

Member
Messages
3,942
Right, and what i mean is -- any even-order harmonics being generated by class A output tubes would be cancelled out by the transformer if they were push-pull, wouldn't they? I.e. if you had tubes running class A in a push pull configuration, you wouldn't necessarily get more even-order harmonics from them, since the transformer is cancelling the even-order harmonics; right?
Right
 

DT7

Member
Messages
2,794
Do you mean single-ended class A? Don't even-order harmonics generated by a class A power section cancel out in a push-pull configuration?

Only if the tubes are identical in performance...and how often does that happen in real life?

Otherwise, you're going to get all kinds of harmonics generated.
 

donnyjaguar

Member
Messages
4,194
I usually consider push-pull circuits operating under heavy bias to be Super-A and not a classical Class-A design, but that term comes from my solid state experience. When I think of Class A tube designs I think single-ended by default. A distinction needs to be made with respect to distortion products, which are relevant in MI designs. Sorry for any confusion.
 

killer blues

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3,307
Class A means that all devices are biased at and conducting 100% through the full cycle of operation. If a class A amp with a push pull OT goes into cutoff, it is now running class A/B.
 

GT100

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3,942
Only if the tubes are identical in performance...and how often does that happen in real life?

Otherwise, you're going to get all kinds of harmonics generated.
Yeah, but since they are closely matched most of em ARE ELIMINATED/
REDUCED SIGNIFICANTLY.

Ever have a pair that wasn't matched very well -did it sound better?
 

DT7

Member
Messages
2,794
Yeah, but since they are closely matched most of em ARE ELIMINATED/
REDUCED SIGNIFICANTLY.

Ever have a pair that wasn't matched very well -did it sound better?
Anytime somebody says "better" you have to be careful...it's subjective. But in my experience, a little mismatch in gm can be good if you're going for an overdriven sound. By nature, such a mismatch introduces second-order harmonics. I'll pay attention to matching power tubes for current-draw, but if the gm of each tube isn't too far off I don't worry about it if it's going into an amp that's going to be overdriven. This whole "matching for gm" thing is more for audiophiles than musicians...we're creating sound, not reproducing it. I'll agree that there are certainly going to be times where what you're going for can benefit from very closely matched tubes, but I think that's going to be the exception rather than the rule.

Now...the subject of "matched tubes" is a whole other animal. The only way you KNOW they're matched is when you put them in the amp they're going to be used in and run them at that voltage...which, in most cases, is probably not the voltage they were "matched" at...and then view the performance on a scope. This shows up how the tubes in question operate with the OT; something else that is also never perfectly balanced. You can't tell this by reading current-draw from a meter...at least not practically.

A little tip...I know this may initially sound stupid, but if you're using a pair of power tubes in an amp and aren't satisfied with how they sound, try swapping the power tubes with each other.
 

GT100

Member
Messages
3,942
Anytime somebody says "better" you have to be careful...it's subjective. But in my experience, a little mismatch in gm can be good if you're going for an overdriven sound. By nature, such a mismatch introduces second-order harmonics. I'll pay attention to matching power tubes for current-draw, but if the gm of each tube isn't too far off I don't worry about it if it's going into an amp that's going to be overdriven. This whole "matching for gm" thing is more for audiophiles than musicians...we're creating sound, not reproducing it. I'll agree that there are certainly going to be times where what you're going for can benefit from very closely matched tubes, but I think that's going to be the exception rather than the rule.

Now...the subject of "matched tubes" is a whole other animal. The only way you KNOW they're matched is when you put them in the amp they're going to be used in and run them at that voltage...which, in most cases, is probably not the voltage they were "matched" at...and then view the performance on a scope. This shows up how the tubes in question operate with the OT; something else that is also never perfectly balanced. You can't tell this by reading current-draw from a meter...at least not practically.

A little tip...I know this may initially sound stupid, but if you're using a pair of power tubes in an amp and aren't satisfied with how they sound, try swapping the power tubes with each other.
I agree with most of what you say here.
I can tell you that in my Guytron GT100 the transformers are balanced.
I've played musical tubes a number of times (both tranformers) and nothing changed.
Maybe on cheeper stuff this isn't allways true?
The advantage to me of closely matched tubes is being able to set the bias on all the tubes exactly were it sounds "best".
 




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