Class A SE vs Class A PP?

[tbryanh]

What are the sonic differences between Class A PAs with either single ended tubes or with push pull tubes?

[kldguitar]

What are the sonic differences between Class A PAs with either single ended tubes or with push pull tubes?
push pull is Class AB

[Enzo]

No, that is not true. SIngle ended versus push pull is a separate issue from class of operation. Most p-p amps you see are class AB, but that is by choice, not by necessity. There are class A p-p amp out there.

A class AB single ended amp would sound awful, but you could make one. P-p amps are either class, both are on the market. SOme p-p amps are even more or less class B amps.

[Paul P]

push pull is Class AB

I don't believe this is correct. Push-pull class A is when both tubes are
conducting all the time but doing so in opposition. When one tube is
pushing the other is pulling. I'd say that push-pull in class A is more
"push-pull" than in class AB because in the latter the tubes (at sufficient
volume) take turns in shutting off for part of the cycle whereas in class A
the tubes are pushing and pulling all the time. I find push-pull class A
pretty elegant in its symmetry.

I can't say what the tonal differences are but one thing is for sure, the
push-pull will be louder :-) I do intend to experiment with this in the
not too distant future as I'm also interested in hearing what the differences
might be. The even-order harmonics are supposed to be reduced by
cancelling out in the output transformer which may not be a good thing in
a guitar amp since these are the "good" harmonics. It may be better to
go with two output tubes in parallel instead of push-pull, another thing I'd
like to try out.

Paul P

[d95err]

The significance of the class of operation in guitar amps is generally very exagerrated. The big confusion is because some classic like the Vox AC30 or Fender "Tweed" Deluxe are often mislabeled as Class A, when they are actually Class AB. Two factors that contribute infinately more to the sound of these amps are:

1. No negative feeback loop around the poweramp
2. Cathode bias

Since the terms cathode bias and negative feedback are too technical for most people, manufacturers (in particular "boutique" builders) have found Class A a much more convenient term to use in marketing.

So, back to the original question - what are the sonic differences between SE PP? Well, not much unless we go into hifi territory. However, the PP amp needs a phase inverter. This will affect the sound (in particularly when pushed).

So essentially you can never compare two amps just by looking at the poweramp; you need to see the whole picture.

[Satamax]

Well, from what i've read, push pull class A cancels 2nd order harmonics, while class A SE doesn't.

[MWJB]

What d95err & Satamax say.

Usually SE guitar amps are built to be chaep (compared to PP counterparts) rather than for any tonal reason...if 4-5W is enough go SE, if you need significantly more go PP.

[tbryanh]

Well, from what i've read, push pull class A cancels 2nd order harmonics, while class A SE doesn't.
This answer sounds simple and clear.

That answer is making me wonder what are the tonal differences between Class A PP and Class AB PP?

[Enzo]

A class AB amp stage will be operating in class A until the signal level gets large enough to turn one tube then the other off for portions of the waveform. Class A conducts all the time, class B half the wave, the remainder in cutoff. Class AB is in between, so whatever the portion of the wave in excess of 50% it takes to turn off one side is hte portion that is running in class A. SO whatever tonal shift might occur at that threshold is the answer.

[kldguitar]

A class AB amp stage will be operating in class A until the signal level gets large enough to turn one tube then the other off for portions of the waveform. Class A conducts all the time, class B half the wave, the remainder in cutoff. Class AB is in between, so whatever the portion of the wave in excess of 50% it takes to turn off one side is hte portion that is running in class A. SO whatever tonal shift might occur at that threshold is the answer.

Who have drawing of Class A pp and SE, Class se is easy understand.but PP is difficult to understand only by words. Are the two tube parallel, use two tube amp current?But it not pull and push

[Satamax]

Who have drawing of Class A pp and SE, Class se is easy understand.but PP is difficult to understand only by words. Are the two tube parallel, use two tube amp current?But it not pull and push

http://www.aikenamps.com/ClassA.htm

[kldguitar]

http://www.aikenamps.com/ClassA.htm

which amp use class A pp?

[kldguitar]

I don't believe this is correct. Push-pull class A is when both tubes are
conducting all the time but doing so in opposition. When one tube is
pushing the other is pulling. I'd say that push-pull in class A is more
"push-pull" than in class AB because in the latter the tubes (at sufficient
volume) take turns in shutting off for part of the cycle whereas in class A
the tubes are pushing and pulling all the time. I find push-pull class A
pretty elegant in its symmetry.

I can't say what the tonal differences are but one thing is for sure, the
push-pull will be louder :-) I do intend to experiment with this in the
not too distant future as I'm also interested in hearing what the differences
might be. The even-order harmonics are supposed to be reduced by
cancelling out in the output transformer which may not be a good thing in
a guitar amp since these are the "good" harmonics. It may be better to
go with two output tubes in parallel instead of push-pull, another thing I'd
like to try out.

Paul P
It seems Class a PP used in guitar amp fewer.
which amps used this way?

[Paul P]

It seems Class a PP used in guitar amp fewer.
which amps used this way?

I don't think there are any, at least not mass-produced ones. They seem to
be fairly common in the hi-fi world.

Paul P

[Enzo]

I can't recall, it seems a few are marketed as class A. It is a matter of bias.

[Bruce / Mission Amps]

Who have drawing of Class A pp and SE, Class se is easy understand.but PP is difficult to understand only by words. Are the two tube parallel, use two tube amp current?But it not pull and pushWell, in my opinion ... yes and no.

I think as far as DC goes, yes, the two power tubes are in parallel in class A PP or class AB PP, for that matter any class of push pull.
But, the AC current part, which is generated in the power tubes and output transformer, is not in parallel... because the phase inverter section ahead of the two PP power tubes drives them with AC, one at a time, 180 degrees (or close to it) but out of phase which each other.
You can make a class A SE amp with many parallel power tubes, sharing a SE output transformer with no usable center tap.
Then all power tubes are in parallel with respect to DC and AC current.

[d95err]

I can't recall, it seems a few are marketed as class A. It is a matter of bias.

In marketing, "Class A" means "cathode bias and no negative feedback". I guess "Class A until you turn the volume up really loud" wasn't quite as catchy... :)

[mooreamps]

A class AB amp stage will be operating in class A until the signal level gets large enough to turn one tube then the other off for portions of the waveform. Class A conducts all the time, class B half the wave, the remainder in cutoff. Class AB is in between, so whatever the portion of the wave in excess of 50% it takes to turn off one side is hte portion that is running in class A. SO whatever tonal shift might occur at that threshold is the answer.

Yes It Is !! A push/pull gain stage ; in an amplifier with a "varying" level input signal, like a guitar amp ; will be operating in Class A until the drive signal gets large enough to shut down the power tube on the negative swing of the input signal. Since the VOX amps work in this manner, perhaps there in lies the answer.

It is just that, push/pull output transformers are smaller and can handle more power than there SE counterparts. As long as both sides of the primary coil are pretty evenly balanced dc wise, anything else is fair game...including tossing out the PI...



-g

[pdf64]

This analysis isn't meaningful, as even regular class B semiconductor amps are biassed just into conduction, just a few mA sufficient to get rid of the crossover gap. On this basis would they be class A as long as the power was kept below 0.1 watt?
The class of amplifier operation is only relevent / meaningful when applied to the maximum sine wave that the amp can put out ie max rms power before clipping.
Aiken explains this very well - Peter

[Enzo]

OK, get pedantic. The point was to try and dispell the idea that Class A is some magic thing with night and day differences.

In your class B solid state example, we would call it a class B amplifier, of course. Personally I would differentiate between what we label the amp as, and the class of operation at any given set of operating parameters. So as long as everyone knew the context of the remark, we could say that it operates in class A for that special condition. WOUld it be useful information? No, of course not.

Something analogous - the difference between being a baseball player and playing baseball. If you pitch a ball to me and I swing a bat at it, we are playing baseball. I would not legitimately call us baseball players, but we would indeed be playing baseball.

But in the case of a tube amp and someone wanting to know what the different classes sounded like, we bent your rules to show that an amp will sound like class A until you turn it up. When you turn it up enough to cross that threshold where alternate half cycles are not comlpete, then the sound of class AB operation results. This is served up only to offer a means to tell how much diference in sound there would be and would not be. To that extent I find meaning in the discussion.

[pdf64]

'There is a very real difference in tone and operating conditions between a true class A 10W amplifier running at say, 1W, and a 10W class AB amplifier running at 1W. Same output level, same overall power level, *but* a different class of operation, different amount of distortion, different efficiency, *and* a different tone, even though neither one of them is in cutoff for any portion of the output cycle at that low level. This is due to the bias point differences and load line differences. The differences become even more apparent when the amplifiers are run at their full undistorted output power. The true class A amplifier will have no crossover distortion, while the class AB amplifier will. The average plate current for the true class A amplifier will not change, or will change very little, from idle to full output power, while the average plate current in a class AB amplifier will increase dramatically. This will lead to "sag" in the power supply that doesn't exist in the true class A amplifier, which again results in a tonal change.'

http://www.aikenamps.com/ClassA.htm