The myth behind Class A

[EETStudent]

Why are some "class A" amps not considered true class A???

[TheTinMan]

Read Technical Info on Randall Aiken's site.

Chip

[Robert M. Martinelli]

Why are some "class A" amps not considered true class A???

Because they' re not - period.

I think there' s been a misundertanding going on for decades among the uninitiated, who probably thought classes were like a sort of "quality" rank - thus believing class A the best, class AB as a second choice and so on.....and the marketing guys, who are very clever in giving the folks what they want, started boasting about "class A" amps being "superior"....Vox still claims their AC30 to be a class A amp ( while it' s a hot-biased AB1 ), and even in books like "The Vox story" - By Peterson-Denney there' s a foreword by Brian May describing it as a class A amp. It amazes me that Dick Denney, the legendary AC15-AC30 designer, allowed this statement to be included in his book....

It' s enough to take a look at EL84s data sheets - 4 of them in class A can only give 20 W output - so, how come the AC30 is rated at 33 W ? Because that' s the power 4 EL84s give when working in AB1 class - period.

A properly biased class AB1 PP amp can please the ear just as much, and with lower distortion and higher efficiency, and though class AB1 designs would tend to eliminate even harmonics, this does not happen in the real world, because it' s impossible to find two perfectly identical tubes. The ringing sound qualities of an AC30 and its harmonic content should suffice to prove this assessment true.

This raises another funny point, the "matched tubes" blurb - AB1 PP amps are almost universally recognized better sounding with unmatched tubes, because their even harmonic content is not endangered by the tubes being "equal" - so there it goes the "matched vs. unmatched" tubes "querelle" ( in French it sounds better ). Seemingly, tube testing, matching and re-branding is a good source for business ( and explains why I' ll end my days as an automation engineer instead of being a successful businessman, but that' s another story ).

If output tubes are within a, say, 5-10 % tolerance they' re OK with me - period.

TinMan gave you a very good advice, Randall Aiken' s Technical Pages are a very good resource, ( BTW I' m honoured to say he is a member of this forum too... ) and can unveil you all there is to know about this matter.

My "two cents" is to always use your brains and judgement, and never believe ads, if they don' t lie, in the best case they' re hiding the truth ( but, being an automation engineer, I always start from the worst case, so maybe I' m a little paranoid ).

Hope this helps

Best regards

Bob

[Enzo]

Also, a lot of smaller amps, in particular single ended amps, that do run class A, use cathode bias instead of fixed bias. People see an amp called class A and they see cathode bias. A LOT of people think that class A means cathode bias and vice versa. Even people in the business who should know better. But that is not correct. However, I think that is one of the reasons we see "class A" amps that really are not - because someone mis-labelled the thing because it has cathode bias.

[Robert M. Martinelli]

Also, a lot of smaller amps, in particular single ended amps, that do run class A, use cathode bias instead of fixed bias. People see an amp called class A and they see cathode bias. A LOT of people think that class A means cathode bias and vice versa. Even people in the business who should know better. But that is not correct. However, I think that is one of the reasons we see "class A" amps that really are not - because someone mis-labelled the thing because it has cathode bias.

Yeah, Enzo, and even if cathode bias is deemed superior sound-wise, in fact it was introduced as a "shortcut" to cut down costs, because it allows to substitute the whole fixed bias section in the PS with a single resistor.

[jrfrond]

While I prefer and build cathode-biased amps, I never refer to them as Class A, because it is a misnomer. A cathode-biased amp COULD run Class A, but most of them are really AB1. In fact, almost ANY guitar amp could be modded for Class A, provided the transformers could handle the constant current demands. Most garden-variety guitar amp transformers would cook even with an AB1 mod.

[Robert M. Martinelli]

Another thing I noticed is some manufacturers tending to state the class of operation by referring to the tubes' biasing, but, even with very high quiescent currents ( close or above the tubes' plate max dissipation rating ) this doesn't mean the amp is operating in true class "A". "Quiescent conditions" means no signal at all is flowing. Are we interested in how the amp works/sounds when NOT PLAYING?

Classes of operation are ( uhmmm......should be ) ONLY defined signal-wise, by stating the devices' angle of operation over 360 degrees ( a full cycle ) of a periodic waveform, e.g. class "A" - only one output device ( not only valves/tubes, but BJTs, MOSFETs and the like ) can suffice because it has to be on 360 degrees ( over the whole period ) ; in class "B" - ( at least ) two devices are needed since they' re on half of the period, or 180 degrees ( with inevitable cross-over distortion because of turn-on delays that create it ); in class "AB1" - ( at least ) two devices are on for more than half of the period but appreciably less than the full period ( keeping them "on" during zero crossing thus eliminating cross-over distortion by eliminating turn-on delays ); class AB2 is a "variation on the theme" of the latter, the "2" indicating that some grid current is flowing because the grid is operating above zero. I won' t go further because the matter has already been analyzed and "dissected" by others better than I could ever do.

I'll only add that it always amazes me how far some ads can go in trying to mislead/deceive potential customers : time ago I've read an ad saying "class A preamp" - ( which to me is much like saying "four wheeled car", or "bladed knife". ).

Regards

Bob

[Enzo]

Lest we get TOO pedantic on the issue, yes class of operation depends upon what the signal and power tubes do, but one must set up the circuit and the bias for this. If that manufacturer makes the mistake of confusing cathode bias and class A, then I am with you. But if they are trying to say the amp is set up so that any reasonable expected signal level will result in class A performance, then I wouldn't yell yoo loud about the quiescent thing.

Likewise, if an amp circuit operates in class A most of the time but could possibly be driven into AB on extreme peaks or something, I wouldn't quibble with the ClassA label.

[Robert M. Martinelli]

Lest we get TOO pedantic on the issue, yes class of operation depends upon what the signal and power tubes do, but one must set up the circuit and the bias for this. If that manufacturer makes the mistake of confusing cathode bias and class A, then I am with you. But if they are trying to say the amp is set up so that any reasonable expected signal level will result in class A performance, then I wouldn't yell yoo loud about the quiescent thing.

Likewise, if an amp circuit operates in class A most of the time but could possibly be driven into AB on extreme peaks or something, I wouldn't quibble with the ClassA label.

Sorry if I sounded pedantic, Enzo, I just wanted to make things clear to the uninitiated too, maybe that' s probably why my explanation WAS pedantic, ....maybe I' m getting old before time......Oh no! I just looked in the mirror and found some gray hair!!! I AM getting old!!!!

Apart from joking, as usual, I thank you for your advice, and I beg you to continue correcting me if I say something wrong, or express myself out of the boundaries of politeness....or common sense!

Thanks and best regards

Bob

[jrfrond]

Class A preamp.....HAH! I always love when I see this load o'sh*t in ad copy.

What kills me about the music industry is how they rope the consumers into buzz-words and minutia that only designers and techs need worry about. The end result, no matter WHAT the amp class or design is should be SOUND, and that's ALL it should be, save for a quality build. People read this stuff and get all huffy and snooty over it, especially when it comes to solid-state. For instance, it's tough to explain to the uninitiated that there are excellent-sounding SS amps, and crappy-sounding tube amps.

[Old Tele man]

...something to remember: the "CLASS" of tube operation is determined at the tube/circuits' "...maximum rated output power..." and not at idle or any other operating point...strictly at "...MAXIMUM rated output"!