For the uneducated amongst us - Class A, B, A/B means what?

hubobulous · January 2017

I have no real understanding of noise, electricity, circuits, how tubes work, etc. You get the idea.

So can someone explain in really simple terms what Class A, A/B etc means? What are the differences in sound, functionality etc?

jpfamps · January 2017

Firstly there is little practical consequence to the end user of the class of your amp, although a class A amp will burn through valves quicker than a class A/B.

An excellent explanation can be found here:

http://www.aikenamps.com/index.php/the-last-word-on-class-a

For what it's worth, there are VERY few class A guitar amps; the vast majority of amps that claim to be class A are not. It's simply marketing BS, or a lack of understanding of how the amp is really working.

I would never purchase am amp based on it's class of operation.

hubobulous · January 2017

@jpfamps - many thanks for the reply.

I guess the problem I have is that the article you posted had me completely baffled within the first paragraph and if you think that's an excellent explanation, I'm probably destined to never understand.

So perhaps my question shouldn't be about what the differences are, but why anyone buying one cares. If using the term 'Class A' can sometimes be marketing BS, that would indicate to me that there's something desirable about Class A. What would that be?

crunchman · January 2017

Adding to @jpfamps comment about very few class A guitar amps, it's worth noting that any single ended amp has to be class A. For example, something like a Fender Champ or all the modern 5W variations on that with a single 6V6 or single EL84 power valve.

I think that some of the tone that people associated with amps described as "class A" is more to do with the fact most of them tend to be cathode biased, rather than the fixed bias used by the classic Marshall and blackface/silverface Fender circuits.

ecc83 · January 2017

As our esteemed JPFA has said there are very, very few guitar amplifiers that are truly class A and even those that technically are would cease to be so when overdriven, a very likely scenario for a guitar amplifier! What they really mean is Cathode Biased which means the output valves draw around their maximum allowed anode current all the time, in fact when driven hard such stages actually draw slightly LESS current!

Class A (push pull, at least 2 OP valves) was/is used for high fidelity because it produces virtually no "Crossover Distortion" (mind you, I am old enough to know that peeps did not care a ***t about that until transistor amps arrived!) But even so called CA valve amps (Google Mullard 5-20) were in fact class AB which means a fair amount of current always flowed in the valves. By definition of course, hi fi amps are not run into distortion.

The bigger, past 30W say gitamps tend to be class AB but Fixed Biased. This allows the standing current to be even lower than an AB cathode biased amp giving less heat and greater efficiency. There will be some crossover distortion but players with 50W+ amps are generally not interested in the niceties of their amps at 100 milliwatts!

But, biasing, loading, "classes" is a huge, interrelated subject and without SOME knowledge, Ohm's Law as a very basic starter, you will soon get lost.

Dave.

jpfamps · January 2017

crunchman said:

Adding to @jpfamps comment about very few class A guitar amps, it's worth noting that any single ended amp has to be class A. For example, something like a Fender Champ or all the modern 5W variations on that with a single 6V6 or single EL84 power valve.

I think that some of the tone that people associated with amps described as "class A" is more to do with the fact most of them tend to be cathode biased, rather than the fixed bias used by the classic Marshall and blackface/silverface Fender circuits.

Actually just because it's single ended doesn't mean it's class A, and often isn't.

hubobulous · January 2017

There's a part of me that hopes that this thread is useful to some out there, promotes healthy discussion, avoids needless sniping and is a generally happy place.

But in terms of me understanding the finer workings of an amp beyond plugging in, switching on and turning some knobs, I think I will be at a loss!! But, I'm going to keep reading and see what comes about.

jpfamps · January 2017

ecc83 said:

The bigger, past 30W say gitamps tend to be class AB but Fixed Biased. This allows the standing current to be even lower than an AB cathode biased amp giving less heat and greater efficiency. There will be some crossover distortion but players with 50W+ amps are generally not interested in the niceties of their amps at 100 milliwatts!
But, biasing, loading, "classes" is a huge, interrelated subject and without SOME knowledge, Ohm's Law as a very basic starter, you will soon get lost.

Dave.

Standing current doesn't have to be higher in cathode bias compared with fixed bias, although it often is.

Cathode bias is less efficient, nit be cause of the higher standing current, but because of the voltage drop across the cathode resistor.

Moe_Zambeek · January 2017

@hubobulous

The key point is that Class A operation makes an amp more expensive and allows for more glossy advertising verbiage.

John_A · January 2017

Once had someone come over and play through a Marshall 18W clone I was selling, they loved it and were about to hand over the money but asked what class it was, I said AB and they put their money away and said they wanted a class A amp. When I asked why they said 'because the are better' Numpties

jpfamps · January 2017

hubobulous said:

@jpfamps - many thanks for the reply.
I guess the problem I have is that the article you posted had me completely baffled within the first paragraph and if you think that's an excellent explanation, I'm probably destined to never understand.
So perhaps my question shouldn't be about what the differences are, but why anyone buying one cares. If using the term 'Class A' can sometimes be marketing BS, that would indicate to me that there's something desirable about Class A. What would that be?

Unfortunately to understand this fully does require some studying.

I think the marketing angle is from hi-fi, where the lowest possible distortion is achieved by class A amps, and hence class A is seen as better than class B (or indeed class A/B), despite the fact that a properly designed class B solid state hi-fi will almost certainly not generate audible distortion below clipping.

For valve "hi-fi" then class A is almost always used.

For guitar amps, where you are actually looking to distort the signal, the "benefits" of class A are not so much an issue (assuming of course they are benefits for guitar, which they probably aren't) and the greater power output of class AB for a given valve complement is exploited.

For example 2 EL34s in push pull are good for 25W in class A operation, but can generate 100W in class B. A typical guitar amp will get 50W from 2 x EL34s in class A/B.

hubobulous · January 2017

So if I've got this right, a lot of it stems from hi-fi amps, where the lack of distortion is paramount since you want to hear the music you're playing in a pristine sense, even when loud.

But somehow, this construct has bled over to guitar amps, where of course, distortion can be a good thing.

jpfamps · January 2017

hubobulous said:

So if I've got this right, a lot of it stems from hi-fi amps, where the lack of distortion is paramount since you want to hear the music you're playing in a pristine sense, even when loud.
But somehow, this construct has bled over to guitar amps, where of course, distortion can be a good thing.

Indeed.

Although I think you mean "distortion is a good thing"!!

professorben · January 2017

Where is John Majors 'classless society' now eh?

fucking politicians.

ecc83 · January 2017

"Standing current doesn't have to be higher in cathode bias compared with fixed bias, although it often is."

I think it would be tricky to cathode bias a pair of EL34 much below 30mA using normal anode and G2 voltages?

A pair of 34s running at 30mA apiece will dissipate around 27W for Va= 450V but should deliver around 50watts. Four cathode biased EL84s would run at 48W total (and that's if the amp designer is being kind!) and push only around 30 watts.

But guitar amps aside I get REALLY pissed off with the adpuff men that pimp up "class A" microphone amplifiers! WHY TF would anyone make them otherwise?

Dave.

jpfamps · January 2017

ecc83 said:

"Standing current doesn't have to be higher in cathode bias compared with fixed bias, although it often is."
I think it would be tricky to cathode bias a pair of EL34 much below 30mA using normal anode and G2 voltages?
A pair of 34s running at 30mA apiece will dissipate around 27W for Va= 450V but should deliver around 50watts. Four cathode biased EL84s would run at 48W total (and that's if the amp designer is being kind!) and push only around 30 watts.
But guitar amps aside I get REALLY pissed off with the adpuff men that pimp up "class A" microphone amplifiers! WHY TF would anyone make them otherwise?
Dave.

You can cathode bias an amp to any current draw you like.

At lower current draws you may reduce maximum power a bit as you will be effectively reducing the HT voltage, although this will depend on other factor too.

This might explain why higher currents are habitually employed with cathode bias, although ignorance is also a possible explanation.

ThePrettyDamned · January 2017

Class a sounds like a higher "grade" than class A/B.

I've heard someone claim class a amps have a nicer transition into power stage overdrive, referencing small fender combos and vox amps - but the sound from those (and most other amps) seems tied into the preamp far more than the power section.

I think it's just marketing rubbish. Buy an amp for its sound and build, not its class or even whether it's valve, digital or solid state.

ICBM · January 2017

ThePrettyDamned said:

I've heard someone claim class a amps have a nicer transition into power stage overdrive, referencing small fender combos and vox amps - but the sound from those (and most other amps) seems tied into the preamp far more than the power section.

That's more to do with the lack of negative feedback in things like the 5E3 Deluxe and AC30. NFB sharpens the transition to distortion because it effectively breaks down when the stage it's working around clips.

NFB or lack of it also has nothing to do with the class.

One thing that's particularly ridiculous is talking about 'Class A overdrive' or 'Class A compression'. The first is an oxymoron because the class definitions *only* apply to clean signals, and the second doesn't happen in a true Class A amp because one of the key characteristics (and one of the reasons it's good, for hi-fi) is that the current draw is constant at all signal levels so there is no power supply sag.

Otherwise, everything that jpfamps said...

Gassage · January 2017

A very simple way of descibing it is thus.

If you drive a car, you could, notionally, control the power and speed one of 2 ways:

1. By adding more fuel and power as required (not class A)
2. By having the engine running flat out all the time and using the brake to control the speed and power. (Class A)

A very simple but, I am told, accurate explanation.

FelineGuitars · January 2017

I use a few single ended amps which with a EL34 or 6L6 tube can generate about 10w
these have been made for the most part by @martinw at MJW Amps (currently taking a break)
Not entirely sure if these would be class A but they seem very rich sounding and MORE THAN ENOUGH POWER FOR HOME USE...I probably never push them into an operating zone where the power section class becomes that much of an issue as while I'm relying on preamp drive for my crunch and the power section isn't clipping much it will probably not be too different to a push/pull amp design (I could be wrong but I think this is so)
I love what has been achieved and Martin and I did some nice experimentation with a couple of transformers and settled on one that gave a better low end to the amps.

jpfamps · January 2017

ICBM said:

ThePrettyDamned said:

I've heard someone claim class a amps have a nicer transition into power stage overdrive, referencing small fender combos and vox amps - but the sound from those (and most other amps) seems tied into the preamp far more than the power section.

That's more to do with the lack of negative feedback in things like the 5E3 Deluxe and AC30. NFB sharpens the transition to distortion because it effectively breaks down when the stage it's working around clips.

NFB or lack of it also has nothing to do with the class.

One thing that's particularly ridiculous is talking about 'Class A overdrive' or 'Class A compression'. The first is an oxymoron because the class definitions *only* apply to clean signals, and the second doesn't happen in a true Class A amp because one of the key characteristics (and one of the reasons it's good, for hi-fi) is that the current draw is constant at all signal levels so there is no power supply sag.

Otherwise, everything that jpfamps said...

There is some potential for sag in the power supply with class A valve amp as the screen supply may sag, depending of course on the impedance of the screen circuit.

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For the uneducated amongst us - Class A, B, A/B means what?

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