What's the Deal with amp Classes?

[dantonel]

Ok so if you can't tell i'm a huge n00b and I was wondering... What's the difference between Class A, A/B, D...?

Where does a Mesa Mark IV fall? Or A Dumble?

HELP!

[Tim Armstrong]

For the purposes of guitar amp users, really irrelevant. It's a description of how the power amp operates, and not a grading system, and in the case of amps descibed as Class A, almost always wrong, with the exception of amps with just one power tube.

However, it's a term used a lot in advertising hype!

Tim

[celeste]

Originally Posted by Tim Armstrong

For the purposes of guitar amp users, really irrelevant. It's a description of how the power amp operates, and not a grading system, and in the case of amps descibed as Class A, almost always wrong, with the exception of amps with just one power tube.

However, it's a term used a lot in advertising hype!

Tim

Yep, to the end user, it is all marketing hype. In fact, I strongly most designer working today do not understand class. It is a huge can-O-worms that some people treat like religon.

It is tone, your tone, that matters. If you get it with tin cans and string, it is no better or worse then if get with an original Trainwreck, your tone is all that matters.

[Wally]

Go here for soem good info on the subject...
http://www.aikenamps.com/

AS Tim notes, with the exception of small, one tube amps; we guitarist rarely come across a true Class A amp. Gibson did build some parallel power tube amps that are therefore single-ended Class A amps. Lany builds one modern Class A amp in the same manner. Alessandro builds one or two true Class A amps which are beyond the reach of most of us in terms of cost. .
Everything else is hype...push/pull cathode-biased El-84 amps in the tradition of the Vox AC30 amps, and they are all Class A/B amps....despite the advertising hype.

[BigDaddyLH]

That still leaves class D, for example Acoustic Image: http://www.acousticimg.com/index2.html

SS Class D amps can rack up big watts. Acoustic Image's standard amp head weighs less than 5lbs and is 800W@4ohms. But are those numbers real? I mean, can you really tap all that?

This amp head, by the way, is one of the most popular with jazz guitarists these days.

[sjhusting]

Originally Posted by BigDaddyLH

That still leaves class D, for example Acoustic Image: http://www.acousticimg.com/index2.html

SS Class D amps can rack up big watts. Acoustic Image's standard amp head weighs less than 5lbs and is 800W@4ohms. But are those numbers real? I mean, can you really tap all that?

This amp head, by the way, is one of the most popular with jazz guitarists these days.

Somebody posted a link to the Peavey docs in the standby thread. There's also a pretty good explanation of amp classes, including class 'D', there:

http://www.peavey.com/support/techno...s/classact.cfm

steven

[celeste]

Originally Posted by BigDaddyLH

That still leaves class D, for example Acoustic Image: http://www.acousticimg.com/index2.html

SS Class D amps can rack up big watts. Acoustic Image's standard amp head weighs less than 5lbs and is 800W@4ohms. But are those numbers real? I mean, can you really tap all that?

This amp head, by the way, is one of the most popular with jazz guitarists these days.

Class D amps are very effiecent and do not need a lot of heatsinking relative to the output power. If you use a switching power supply, you can loose a lot of weight again. They do a really good job with "slam" which is mostly about leading edge time rise. When you get above the lower mid range they get a bit more dicy.

[celeste]

Originally Posted by Wally

Go here for soem good info on the subject...
http://www.aikenamps.com/

AS Tim notes, with the exception of small, one tube amps; we guitarist rarely come across a true Class A amp. Gibson did build some parallel power tube amps that are therefore single-ended Class A amps. Lany builds one modern Class A amp in the same manner. Alessandro builds one or two true Class A amps which are beyond the reach of most of us in terms of cost. .
Everything else is hype...push/pull cathode-biased El-84 amps in the tradition of the Vox AC30 amps, and they are all Class A/B amps....despite the advertising hype.

Even Randell Aiken contradicts RDH4 and a number of older texts, but mostly he has it right.

[Groovey Records]

From Dave Hunter - Author of "The Guitar Player Electric Amplifier's Hand Book"

First, take this info-byte onboard: the tweed Fender Bassman, blackface Fender Twin Reverb, Deluxe Reverb and Super Reverb, Marshall JTM45, Mesa/Boogie MkI, Soldano SLO and many other classics are not class A amps, not by any twisted, spun, or convoluted assessment of the term. Phenomenal amps? Yep. Monsteriffic tone machines? Damn straight. So as we proceed with this investigation, hold on to the simple fact that many of the best sounding and most influential tube amps of all time are not class A amps, they are class AB amps. On the flip side, the Fender Champ (tweed, blackface, silverface), Vox AC4, Gibson GA-8, Kalamazoo Model 1, Wards Airline GDR-9012A, Silvertone 1481, and plenty of other similar practice, student, or just plain cheapo amps are most definitely class A amps, although they were not promoted as such in the trade ads of their day. And up on the “for further examination” bench, a vintage delectable such as the tweed Fender Deluxe—never touted as class A design but an all-time classic nevertheless—easily falls into the more colloquial definition of class A, as bandied about by many amp makers today. Buckle up . . .

Definitions of operating classes relevant to tube guitar amplifiers are really pretty simple. We’ll look briefly at these definitions as they apply to push-pull (PP) amps, which is the majority of amps other than the small, single-ended amps listed above. In a push-pull amp, which has two output tubes or four tubes working in two pairs, one tube (or pair of tubes) works to amplify the peaks of the audio signal (the waveform, which most of you will know looks like a rolling pattern of hills and valleys) while the other tube or tube pair amplifies the valleys. In other words, one tube pushes the hills while the other pulls the valleys, then they swap. Since AC (alternating current)—which is what passes your guitar signal from stage to stage within the amp—carries constantly alternating hills and valleys, each tube of the PP setup receives a steady stream of each. In many guitar amps, the majority in fact, each side of the PP setup actually shuts down briefly during some part of the cycle, when the other side is at its peak of current flow. What? One tube shuts down while the other amplifies? That’s right, and then they swap. Looks crazy on paper, but of course it all happens so fast that there’s no audible gap in the performance. Because the 360-degree waveforms that are being amplified by each side of the PP setup are reverse phase to each other, there’s no volume loss as one side dips and the other rises. The majority of guitar amps function in this way, including classics like all the big Marshalls, Fender Bassmans and Twins, Dumbles, Mesa/Boogies and so on.

Indeed, as we have already discovered, amps that perform as I have just described are class AB amplifiers. By definition, one side of the PP tube pair of a class AB amplifier rests for at least some portion of the cycle (when measured at maximum volume before clipping). In simple terms, that’s really all there is to the definition of class AB, or all that you need to worry about at least. It’s worth knowing too, though, that sharing the load makes output tubes configured in class AB design a little more efficient power-wise, and tends to make amps sound a little tighter, firmer, and punchier.

An output section operating in true class A, on the other hand, has the tubes working the entire cycle of the waveform (when measured at maximum volume before distortion). This is true even of push-pull amplifiers, where both tubes are sending the signal along to the output transformer together at all times, not alternately resting as with class AB. As such, class A output stages are somewhat less efficient than class AB stages, which can be driven to higher output levels. Players and amp makers often talk of sweeter distortion in class A amps, but true class A operation actually has less distortion content at a given output level. Class A distortion is smoother at the onset and is usually heard as being harmonically richer, too. But the fact that definitions of operating class are measured at maximum output before distortion should tell you something: a lot of voodoo is talked about class A—particularly by amp makers’ marketing departments eager to sell you a particular new model. The sound of true class A, operating within the realm of its definition, is actually something different than the advertising slogan “real class A tube amp” means to imply. Relatively few amps fit the definition for class A absolutely and beyond debate, which is not something to worry about at this juncture. The characteristic sound of different classic tube guitar amps is determined by far more than just their class definition.

Designers seeking true class A performance in PP amps achieve it by manipulating two factors: the DC voltage delivered to the output tubes and their bias setting. They force the tubes into this state of constant operation by carefully setting their bias point, which is a very complex matter (roughly speaking, we can equate a tube’s bias with a racecar engine’s idle). In short, class A amps are usually biased very “hot,” which—when done correctly, and for the right reasons—can make them very rich sounding but also quite inefficient in terms of output level. Such biasing is far and away most often achieved in class A amps (as well as amps purporting to be class A) by a method known as cathode biasing. This will be most familiar to guitarists in the Vox AC30 and AC15, or other amps that follow those templates. Now here’s the rub: Cathode biased amps, whether they are class A or class AB in the purest, technical sense, have a discernable sound, which itself has come to be associated with some of the supposed characteristics of an amp being class A. This sound is harmonically lush, shimmering, sometimes just a hair grainy and loose, and fairly smooth when cranked up into distortion. Often it is not particularly tight, punchy, or bold—or at least less so compared to a more efficient fixed-bias output stage. And remember, neither is better or worse than the other, it all depends on what you’re seeking to achieve with your tone.

On top of this, and to compound matters, most amps billed as class A also lack something called a negative feedback loop. Applying a little bit of an amp’s output signal in reverse phase back to the front of the output stage (the input of the phase inverter) via a network known as a negative feedback loop can help to improve the overall tightness and definition of its sound. As you can see, therefore, eliminating the negative feedback loop further contributes to some of the tonal qualities that are already being emphasized in the cathode-biased amp. You can bet your bottom dollar that almost any amp you encounter that’s billed as class A will lack such a loop, and whether or not anyone is ever going to heft it up on the workbench, attach the meter and scope, and determine whether it is truly operating in class A, such an amp will still produce what we commonly consider the class A sound, mainly because it is cathode biased and carries no negative feedback loop.

Care to guess what you’d find inside a Vox AC30? Cathode biasing and no negative feedback loop. And the same goes for amps like the Matchless DC-30, Bad Cat Black Cat 30, TopHat King Royale, Mojave Sidewinder, Dr. Z Stang Ray and others that follow the AC30 template (in addition to all the smaller amps that emulate the AC15). Other amps that fall into the cathode biased/ no negative feedback category include the ’50s-era Fender Deluxe and Gibson GA-20, GA-30 and GA-40 Les Paul; early-’60s Selmer Selectortone; late-’60s Traynor YBA-2 Bass Mate and WEM Dominator; and plenty of Valco-made amps.

Toward the start of this article I listed a number of smaller practice amps that do qualify as genuine class A designs. Many of you will already have spotted that these are all what we call single-ended amps, which is to say they have just one output tube. When just one tube is working in an output stage, it is categorically operating in class A because, of course, it cannot shut down during any portion of the waveform. As employed in guitar amps, single-ended tube output stages only stray into medium-sized models (and the lower end of the category as well) in the form of dual-single- ended amps (also called parallel single ended). Dual-single-ended amps—such as the Gibson GA-8—use a pair of output tubes working in parallel to increase the potential output. Rather than working in turns like the PP pair, however, they are really acting like a single tube for operational purposes, both pushing the same signal the entire time. Modern production examples of these are extremely rare, and include the THD BiValve and Victoria Regal II, which are about the largest single-ended amps I can think of.

[Groovey Records]

PART 2

Even with two output tubes, such amps are still very inefficient compared to PP amps using the same tube complement. A design such as the GA-8 probably only puts out about eight to ten watts, compared to the 15-watt rating of an amp like the Gibson GA-20 or Fender Deluxe, and the Victoria Regal II’s maximum output is around 30 watts from a pair of 6L6s, which could produce 50 watts or more in an efficient class AB push-pull design. Still, it’s one way of getting a little more power out of a true class A design than the measly four watts a Champ can drum up.

In short, an amp’s class definition can provide some clues about its likely tonal palette, but the potential for misinformation and misdescription—coupled with the fact that it’s just good to get out there and sample as much gear as you can—means you should play as many amps as possible, whether they’re labeled class A or AB, before making any big leaps. Hone in on the heart of your tone according to what amplifier feels and sounds right to you and works best for your style of music, and let categories and class descriptions take a back seat to your ears and your fingers.

BIASED VIEW

The two main forms of biasing used in tube guitar amplifiers today are cathode biasing and fixed biasing. “Bias” is a term used for the way in which a tube’s operating level is set— usually with a resistor or simple network that determines how efficiently the tube uses the voltages applied to it to amplify a guitar signal. The terms for these two techniques can be misleading: fixed bias amps very often carry bias-adjustment pots, and it’s necessary to rebias them in most cases when you replace the output tubes. This is the case with all the larger Fender and Marshall amps, and others made to those templates—although early fixed-bias tweed Fenders, some Mesa/Boogies, and others have a preset bias network that is not adjustable. On the other hand, cathode biased amps most often have their bias permanently set with a resistor tied between the cathodes of the output tubes and ground. Class AB amps are usually fixed bias, and while almost all class A tube guitar amps you encounter are cathode biased, not all cathode biased amps are class A.

[Wally]

That article by DAve Hunter is informative, but an incomplete read of it MIGHT lead to a scenario whereby now folks will start calling the TWeed Deluxe a Class A circuit/amp....in addition to the amps that are already mislabelled....Vox AC30 and its clones for example.
Woe and alas! LOL

[Telesavalis]

there is a good artlcle on this topic on the Mesa website....you should read it.

[Groovey Records]

Originally Posted by Wally

That article by DAve Hunter is informative, but an incomplete read of it MIGHT lead to a scenario whereby now folks will start calling the TWeed Deluxe a Class A circuit/amp....in addition to the amps that are already mislabelled....Vox AC30 and its clones for example.
Woe and alas! LOL

Nay Woe.... Rejoice! and fix your eyes and mind to careful consideration of Mr. Hunter's written text

If you read carefully you will see he desn't say that at all. He uses the phrase "AMP PURPORTING TO BE CLASS A" while he explains how the Class A effect can be achieved in a PP A/B amp by Cathode Biasing

Such biasing is far and away most often achieved in class A amps (as well as amps purporting to be class A) by a method known as cathode biasing. This will be most familiar to guitarists in the Vox AC30 and AC15, or other amps that follow those templates. Now here’s the rub: Cathode biased amps, whether they are class A or class AB in the purest, technical sense, have a discernable sound, which itself has come to be associated with some of the supposed characteristics of an amp being class A. This sound is harmonically lush, shimmering, sometimes just a hair grainy and loose, and fairly smooth when cranked up into distortion. Often it is not particularly tight, punchy, or bold—or at least less so compared to a more efficient fixed-bias output stage. And remember, neither is better or worse than the other, it all depends on what you’re seeking to achieve with your tone.

On top of this, and to compound matters, most amps billed as class A also lack something called a negative feedback loop. Applying a little bit of an amp’s output signal in reverse phase back to the front of the output stage (the input of the phase inverter) via a network known as a negative feedback loop can help to improve the overall tightness and definition of its sound. As you can see, therefore, eliminating the negative feedback loop further contributes to some of the tonal qualities that are already being emphasized in the cathode-biased amp. You can bet your bottom dollar that almost any amp you encounter that’s billed as class A will lack such a loop, and whether or not anyone is ever going to heft it up on the workbench, attach the meter and scope, and determine whether it is truly operating in class A, such an amp will still produce what we commonly consider the class A sound, mainly because it is cathode biased and carries no negative feedback loop.

Care to guess what you’d find inside a Vox AC30? Cathode biasing and no negative feedback loop. And the same goes for amps like the Matchless DC-30, Bad Cat Black Cat 30, TopHat King Royale, Mojave Sidewinder, Dr. Z Stang Ray and others that follow the AC30 template (in addition to all the smaller amps that emulate the AC15). Other amps that fall into the cathode biased/ no negative feedback category include the ’50s-era Fender Deluxe and Gibson GA-20, GA-30 and GA-40 Les Paul; early-’60s Selmer Selectortone; late-’60s Traynor YBA-2 Bass Mate and WEM Dominator; and plenty of Valco-made amps.

[Wally]

GrooveyREcords, just as I noted....an incomplete read MIGHT lead one to believe that a Tweed deluxe or in fact any amp that is push/pull cathode biased wihtout a negatvie feedback loop is Class A. IN fact, that is not what Dave Hunter says at all. Please read it carefully. He says that such amps "have a discernable sound, which itself has come to be associated with some of the supposed characteristics of an amp being class A." Take note of the 'has come to be associated with' and 'supposed'. This does not replace the technical aspects. Hunter clearly states that the public has come to associate the sounds of such amps as being the sound of Class A because of the hype that is based on something other than technical reality.
HUnter writes "how the Class A effect can be achieved in a PP A/B amp by Cathode Biasing"...he doesn't say that the amp becomes a Class A amp. HE says that the results mimic what the public has come to think of as Class A sonics. Once again, perception/belief and reality are not always the same thing. Hype is strong.

IN fact, what Hunter lines out is exactly what the great confusion is all about. That is, the hype has gone beyond the reality to establish a perception. PErception may be 'real' to the perceiver, but it is not necessarliy the technical truth.
Whatever...this debate will go on. And, with this article, even more amps will be 'perceived' as being Class A amps. Many early FEnder amps will be so. BAssman
5A6?? Cathode biased, no negative feedback loop. Class A? OR perception thereof?
All of the early PRos through the 5C5 series. Class A or perception thereof?
Hunter is talking about harmonic production. IF you greatly increase the resistance in any negative feedback loop, you will get much richer harmonic content in any FEnder amp. Have we made it into a Class A amp? I don't think so. Change the biasing to cathode biasing in a BFTR and remove the feedback loop. Class A or perception?
IT is possible to take a TR into Class A, but it takes more than cathode biasing and removal of the NFLoop.
So, now we all have a better chance to own a Class A amp. IN fact, based on accepted perceptions, I very quickly went from having none to having 6 Class A amps. OR so some perceive. LOL

[dantonel]

So all those "Class A Switches" on Mesa's and Pentode/Triode Switches have absolutely nothing to do with the supposed Class.

So where does a Dumble Fall?

[Tarnisher]

What does "class A electronics" mean? L.R.Baggs uses this phrase to describe their active soundhole pickup:

The M1 Active takes the M1 pickup to a new level of performance and convenience. The built-in all class A electronics eliminate the need for any external preamps and makes the M1 Active compatible with any outboard equipment. And the new volume control gives you fingertip access in any performing situation.

[Tim Armstrong]

Originally Posted by dantonel

So all those "Class A Switches" on Mesa's and Pentode/Triode Switches have absolutely nothing to do with the supposed Class.

So where does a Dumble Fall?

Wherever you drop it!

Tim

[valiant]

So this is why low watts sound so good.....

[Tim Armstrong]

Originally Posted by Tarnisher

What does "class A electronics" mean? L.R.Baggs uses this phrase to describe their active soundhole pickup:

It doesn't really mean ANYTHING significant, but as many electronics marketing types have figured out, "Class A" has a strong subconscious appeal, no doubt because when we were all school kids, getting an "A" grade meant the best possible grade.

Tim

[Tarnisher]

Thanks Tim, I had a feeling that was the case. It's really too bad- the M1A is a great product (it's on the top of my holiday wish list). Putting all that nonsense in the description isn't necessary and kinda degrades it- like putting a slice of baloney on top of a really good steak.

[Wally]

Tarnisher, I don't think Tim is saying that the advertisement for that particular active pickup is deceptive. IT may well have Class A preamp circuitry. Tim is saying that Class A is a buzz phrase in the tube guitar amp world....and gets a lot of attention. Most of the time the hype doesn't measure up in the guitar amp world when splitting hairs over class of operation. What really matters is sound.
There are reasons why Class A gets a lot of attention in HI-fidelity, guitar amps, and musical reproduction/recording equipment. I don't think that you can lump Baggs in with companies that hype Class A when it doesn't exist in the amp being hyped....unless you get the schematic and understand the operation of the preamp and find that it truly is not Class A.
When it all comes down to it, whether something operates in Class A or A/B or whatever doesn't really matter to 99% of us. The sonics are what we seek. IF an amp or a preamp yields what you want, play on and enjoy.
Hype? There is a lot of hype around. There are amp companies that tout certain amps as 'all-tube' when in fact the circuit processes some of the signal funcitons in SS circuitry. Clue: if a modern amp has reverb and has only 3 12AX7 tubes, ime the reverb is in the SS domain. Some people are upset when told this, and others couldn't care less. Believe me, one can hear SS reverb even if there is a spring tank. It only takes attentive ears with a little bit of experience to hear it, and yet these large companies keep spreading the hype....all-tube. Right? Class A? Right. At some point it all gets comical.
Years ago, I heard a very good tech talk to a owner of a then new JCM900 Dual Gain Reverb amp about 'all-tube'. The player was ecstatic and bragging that his new Marshall was the best tube amp he had ever had. The tech diplomatically told him that the amp didn't sound like an all-tube amp. Without ever having seen the circuit or a schematic, the tech named the IC's in the amp. I think there are at least 13 TL702's in that amp performing various things....one of which is the reverb.
Yep, that amp has 3 12AX7's to do all of that. The largest selling amp with a FEnder badge is the same story.....fewer chips than the big Marshall but SS reverb and 3
12AX7's.
Hype....Step right up, son. The biggest show on Earth! She dances, she prances.....got a ruby in her belly a big as Egypt! 4 bits a dance.....come on kid! IT's a class A act, I guarantee.
Ya'll have a good day...and....
Caveat emptor....

[Tim Armstrong]

Yeah, I wasn't saying the Baggs unit WASN'T Class A, I was more trying to indicate that it's ultimately irrelevant!

Tim

[valiant]

In England the phrase "Class A" refers to hard drugs and is a legal category....

[dantonel]

Originally Posted by Tim Armstrong

Wherever you drop it!

Tim

ok so literally yes but what about metaphorically or.... oh never mind... I gotta get out of this hole.

What class would a Dumble be Classified as? (if that is the "end all be all" amp)

[Tim Armstrong]

Originally Posted by dantonel

ok so literally yes but what about metaphorically or.... oh never mind... I gotta get out of this hole.

What class would a Dumble be Classified as? (if that is the "end all be all" amp)

Dumbles, just like practically every guitar amp made that uses more than one power tube, are Class AB.

Tim

[Steve G]

Originally Posted by valiant

In England the phrase "Class A" refers to hard drugs and is a legal category....

Especially in Bentilee....

[Groovey Records]

Wally I understood that you understand, my comment and reiteration was for those who might of misconstrued your post thus leading them to misunderstand.


Clear? ;)

[Wally]

Groovey REcords,
Your statement...."He uses the phrase "AMP PURPORTING TO BE CLASS A" while he explains how the Class A effect can be achieved in a PP A/B amp by Cathode Biasing"
is what threw me. I think we are thinking the same thing but words get in the way! LOL
What Hunter says is in effect that one can achieve an effect that fits the current public PERCEPTION of Class A...."such an amp will still produce what we commonly consider the class A sound" as Hunter states. The public's 'common' consideration of what Class A is bent by the hype.....and many players have never played through a Class A amp. They base their 'perceptions' on the result of hype about A/B push/pull amps that are cathode biased with no negative feedback loop. That is my point. The hype has distorted the 'common consideration', right?
All in all, it doesn't matter to most of us what class an amp is operating in as long as the amp does what a player wants it to do. IT does matter to the designer of the amp. IT does matter to the tech who works on them. And....I suppose that on some level it matters to the advertising department. Class A hype sells amps. Just don't tell those folks playing those cathode-biased, no negative feedback loop class A/B amps that they are acheiving those marvelous sonics with anything BUT a Class A amp. They won't enjoy the experience nearly as much. LOL

Hype? WE don't need no stinkin' hype?