What Is A "Class A " Amplifier?
Moderator: jingle_jangle
What Is A "Class A " Amplifier?
Don: I have read a number of discussions online about what constitutes a Class A amplifier and there seems to be some debate, by some, whether a Vox AC-30 is technically a Class A Amplifier or not. My thinking has always been that it is an amplifier that is running full out all the time regardless of the gain chosen. Would you please explain what a Class A is and what makes fit this description?
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ted_williams
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A Class A amplifier amplifies the entire waveform. A Class B amplifier, which is more typical of a power amp, separates the positive and negative components of the waveform, sends each "half" to its own amplifier, and then re-combines the amplified "halves" back into a complete waveform again. These are also referred to as push-pull amplifiers.
As to whether the AC-30 is true Class A, I'll let someone who is familiar with them comment.
As to whether the AC-30 is true Class A, I'll let someone who is familiar with them comment.
A "Class A" amplifier is one that never goes into cutoff. A single ended Class A amp varies its output between cutoff and saturation, and is thus not "full on" except at one point in the waveform at full power. The bias point is typically set in the middle of the load line so that the current can increase or decrease by equal amounts before waveform clipping occurs.
An amplifier does not have to be a true Class A amp in order for you to play it in Class A. Most push-pull tube guitar amps only go into Class B operation at high power levels. These are known as Class AB amps and are the most common type in modern audio power amp designs. There are NO Class B audio amps as crossover distortion would be a very apparent problem at low power levels. The Vox AC-30 is probably what is referred to as a High Bias Class AB amp. At normal operating power levels both push-pull halves are doing some conducting on the entire waveform. At high power levels, each half is cut off during part of the waveform, and current flow from the power supply is increased through the conducting tube as the output rises above the cutoff point of the unused half. This allows "rectifier sag" to be induced and influence the tone. A true Class A push-pull amp never undergoes rectifier sag as the current flow through the power supply rectifier is ALWAYS THE SAME. The reason for that is because as current increases in one push-pull half, it is decreased by an equal amount in the other half. Both transformer halves receive their current from the single center tap supply wire. A single ended Class A design is a different matter. The power supply will see a varying demand throughout the waveform even at moderate power levels. There are not two halves available to vary their current flow by equal and opposite amounts. The varying current flow through the tube also varies the gain of the tube in a dynamic manner and gives the single ended amp its unique tone. In spite of the hoopla put forward by audiophiles who favor single ended Class A designs, a properly designed push-pull Class A amp is the highest fidelity tube power amp known. The phase splitter is usually where problems lie in the push-pull design. You need phase splitter tubes with exact equal gain in each half to make the phase splitter work as intended and allow distortions in each push-pull half to cancel each other out. A transformer with exactly matched dual output windings makes the best phase splitter (they are connected in opposite phase to each output tube section), but years ago the tube phase splitter was chosen because tubes are MUCH cheaper than precisely matched phase splitter transformers that last the life of the amp. Tubed phase splitters are simply more cheap **** that became standardized due to overwhelming popularity in a price competitive market. This is not necessarily a problem in guitar amps, as the distortion produced by unmatched phase splitter tubes may be sonically pleasing to some people. Audiophile amps are being produced that addresses the problem, and similar Class A amps would be best for acoustic guitar amplification if somebody bothered to produce them. I suspect old amp designs such as Vox and Fender that were known for clean tones at low power levels had precisely matched phase splitter tubes installed. Grabbing any old mismatched twin triode tube and slamming it in the phase splitter socket would give a different tone for sure.
I mention this because it is a critical component for the reason that Class A push-pull amps were invented in the first place: DISTORTION CANCELLATION. Those thinking otherwise would probably be better served by a Marshall amp with the most mismatched phase splitter tube they can locate.
An amplifier does not have to be a true Class A amp in order for you to play it in Class A. Most push-pull tube guitar amps only go into Class B operation at high power levels. These are known as Class AB amps and are the most common type in modern audio power amp designs. There are NO Class B audio amps as crossover distortion would be a very apparent problem at low power levels. The Vox AC-30 is probably what is referred to as a High Bias Class AB amp. At normal operating power levels both push-pull halves are doing some conducting on the entire waveform. At high power levels, each half is cut off during part of the waveform, and current flow from the power supply is increased through the conducting tube as the output rises above the cutoff point of the unused half. This allows "rectifier sag" to be induced and influence the tone. A true Class A push-pull amp never undergoes rectifier sag as the current flow through the power supply rectifier is ALWAYS THE SAME. The reason for that is because as current increases in one push-pull half, it is decreased by an equal amount in the other half. Both transformer halves receive their current from the single center tap supply wire. A single ended Class A design is a different matter. The power supply will see a varying demand throughout the waveform even at moderate power levels. There are not two halves available to vary their current flow by equal and opposite amounts. The varying current flow through the tube also varies the gain of the tube in a dynamic manner and gives the single ended amp its unique tone. In spite of the hoopla put forward by audiophiles who favor single ended Class A designs, a properly designed push-pull Class A amp is the highest fidelity tube power amp known. The phase splitter is usually where problems lie in the push-pull design. You need phase splitter tubes with exact equal gain in each half to make the phase splitter work as intended and allow distortions in each push-pull half to cancel each other out. A transformer with exactly matched dual output windings makes the best phase splitter (they are connected in opposite phase to each output tube section), but years ago the tube phase splitter was chosen because tubes are MUCH cheaper than precisely matched phase splitter transformers that last the life of the amp. Tubed phase splitters are simply more cheap **** that became standardized due to overwhelming popularity in a price competitive market. This is not necessarily a problem in guitar amps, as the distortion produced by unmatched phase splitter tubes may be sonically pleasing to some people. Audiophile amps are being produced that addresses the problem, and similar Class A amps would be best for acoustic guitar amplification if somebody bothered to produce them. I suspect old amp designs such as Vox and Fender that were known for clean tones at low power levels had precisely matched phase splitter tubes installed. Grabbing any old mismatched twin triode tube and slamming it in the phase splitter socket would give a different tone for sure.
I mention this because it is a critical component for the reason that Class A push-pull amps were invented in the first place: DISTORTION CANCELLATION. Those thinking otherwise would probably be better served by a Marshall amp with the most mismatched phase splitter tube they can locate.
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toneman
Peter; I don't think I could answer this any better than Phillip already has done.
I usually refer to AC-30's as a Class A "type" Push-Pull amp. It's not a true Class A amp. But is a sound that I prefer to hear over just about anything else.
Perosnally I prefer at least two output tubes in an output section. Single-end sounds a bit harsh to my ears. I don't care for Fender Champ's or Vox AC-4's. The only single end type amp I've heard that I actually liked (but wouldn't buy) is built by my friend Andy @ THD. I've got too many amps right now. I do have my first amp that is from 1965 that's a single end. It's an old Kalamazoo Model I made by Gibson. Bought it brand new...
I usually refer to AC-30's as a Class A "type" Push-Pull amp. It's not a true Class A amp. But is a sound that I prefer to hear over just about anything else.
Perosnally I prefer at least two output tubes in an output section. Single-end sounds a bit harsh to my ears. I don't care for Fender Champ's or Vox AC-4's. The only single end type amp I've heard that I actually liked (but wouldn't buy) is built by my friend Andy @ THD. I've got too many amps right now. I do have my first amp that is from 1965 that's a single end. It's an old Kalamazoo Model I made by Gibson. Bought it brand new...