#1
What is the difference in tone? I know that an A/B circuit is supposed to be more efficient and that it has crossover distortion, but is there a qualitative way to describe the tonal differences?
#2
I'm pretty sure that you could describe A/B as a tighter sound. A/B is a better voicing for thrash, shred, higher gain type stuff. Class A gives that loose, creamy, bluesy type overdrive.
I believe 5150/6505's are A/B(need verification), so you can think of that tighter, higher gain sound.

Tighter is probably the best adjective.

Edit: Most likely not always the case.

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Last edited by Checker at Jun 17, 2008,
#3
Many advertisement that proclaim class A and class AB is more of a gimmick. Typically, when they say class A in anything that have wattage higher than 15 watts, what they actually means is that they use cathode biasing. When you push into disortion, especially at 11, class A sounds the same as class AB when it's push-pull. When you are at crunch level, there may exist some sag, but only evidental when you play slowly; if you shred, even at crunch level it does not matter.

Also, class A does not automatically means it's more blusey. Bassman is a very typical class AB PP, and is your typical Blues amp.

The only real benefit of class A is that you truly do not need any biasing, as it is "auto" biasing.

What is more important is whether it is push pull or SE. Unfortunately, due to the cost of transformer, Single Ended amplifiers will be no higher than 10 watts.
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#4
Quote by imgooley
What is the difference in tone?
Pretty much nil.
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#5
Quote by Jestersage
Many advertisement that proclaim class A and class AB is more of a gimmick. Typically, when they say class A in anything that have wattage higher than 15 watts, what they actually means is that they use cathode biasing. When you push into disortion, especially at 11, class A sounds the same as class AB when it's push-pull. When you are at crunch level, there may exist some sag, but only evidental when you play slowly; if you shred, even at crunch level it does not matter.

Also, class A does not automatically means it's more blusey. Bassman is a very typical class AB PP, and is your typical Blues amp.

The only real benefit of class A is that you truly do not need any biasing, as it is "auto" biasing.

What is more important is whether it is push pull or SE. Unfortunately, due to the cost of transformer, Single Ended amplifiers will be no higher than 10 watts.

Ok, so the amps that I'm looking at are an Epiphone Valve Standard (15 watts push pull) and the Blues Custom. The Blues Custom has a 15/30 watt switch that says it switches between class A and class A/B. I wasn't able to switch modes when I was playing it last, but I know it was on the 30 watt setting.

So the only difference is the need to bias the tubes? And does a Tube Rectifier make an amp class A/B?
#6
Quote by imgooley
Ok, so the amps that I'm looking at are an Epiphone Valve Standard (15 watts push pull) and the Blues Custom. The Blues Custom has a 15/30 watt switch that says it switches between class A and class A/B. I wasn't able to switch modes when I was playing it last, but I know it was on the 30 watt setting.

So the only difference is the need to bias the tubes? And does a Tube Rectifier make an amp class A/B?


More or less. Cathode biasing (also known as class A biasing) also give out less power in general; Two EL84 can only push around 15watts RMS with class A, but can push to 25watts RMS in Class AB

Using a tube rectifier does not make it class A or class AB, but it will give it more classical compression. That would probably be the more important decision in your amp purchase.
Ibanez SA-120 (ed.2006)
BluesJr 1996-B + cathode follower + texas Heat
Crate CPB150
Homemade 4 x 10 cab Bass closeback
Metal Muff
#7
Quote by imgooley
Ok, so the amps that I'm looking at are an Epiphone Valve Standard (15 watts push pull) and the Blues Custom. The Blues Custom has a 15/30 watt switch that says it switches between class A and class A/B. I wasn't able to switch modes when I was playing it last, but I know it was on the 30 watt setting.

So the only difference is the need to bias the tubes? And does a Tube Rectifier make an amp class A/B?


Class A and A/B is really more of an efficiency thing. Class A means that the tube/s are "working" for the complete cycle of the wave form, Class AB means that they "work" for more than half the cycle but appreciably less than the complete cycle.
Class A has the advantages of not requireing bias adjustment but is quite inefficient. Class AB have leaves the possibility of cross over distortion (as one side is amplifying one portion of the waveform and the other side is amplifying the other) and so you have to adjust the bias to eliminate this. It is, however, a far more efficient way of amplifying a signal.
Single ended vs Push Pull is where the real tonal differences come in. Single ended produces more even order harmonics which are generally considered more musical. Push Pull tends to cancel these odd order harmonics and produce more odd order harmonics (as it clips the signal symetrically, like a square wave) which are considered more "harsh" to the ear.
Tube rectifier does not effect the Class of operation of an amp. You can have a tube rectified, single ended amp or a solid state rectified push pull and vise versa.
Fender Champ and the Epi Valve Junior (5watt) are both single ended if you wanna try them out vs a push pull amp to hear the tonal difference.
#8
Quote by Jestersage
More or less. Cathode biasing (also known as class A biasing) also give out less power in general; Two EL84 can only push around 15watts RMS with class A, but can push to 25watts RMS in Class AB

Using a tube rectifier does not make it class A or class AB, but it will give it more classical compression. That would probably be the more important decision in your amp purchase.

By classical compression, do you mean the amount of dynamic control? Does that mean that an amp with a rectifier will have more dynamic control, or less?
#9
Quote by imgooley
By classical compression, do you mean the amount of dynamic control? Does that mean that an amp with a rectifier will have more dynamic control, or less?

If you mean more compression, then yes. This is because tube recitifier is not as efficent as diode recitifier in following the notes (tightness), thus when you pick hard, a sag will occur. And I mean classical as in the 60s. By 70s, All manufacteurer use diode recitifier

HOWEVER, SS recitifier can mimic (though not completely) by placing a resistor to limit the voltage and current available in the power section, while tube recitifier can be tight too with heavy filtering and a choke (ala Dr. Z Z-28). However, neither recitifier types can fully mimic the benefits of the other recitifer.
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BluesJr 1996-B + cathode follower + texas Heat
Crate CPB150
Homemade 4 x 10 cab Bass closeback
Metal Muff
#10
Man, I really need to relearn all my electro-physics...

So sag is a voltage drop. In Ohm's law, is the resistance given a negative value? IDK, it's been 4 years since I studied this stuff.
#11
Quote by Yngwi3
Pretty much nil.


Exactly. The most difference in tone between a Class A/B amp and Class A amp will have nothing to do with class really...
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