All about...Valve Amp Class

Updated: Dec 2, 2020

You've seen it a thousand times, This amp is Class A or Class B, and you automatically think that Class A must be better because A grade anything is better than B, or is it?


Peter from ToneChasers actually knows what this means...over to You...


Class A, Class A/B what does it all mean?


What are the advantages and disadvantages to each?


To give you and idea of this, some technical details need addressing…. Here comes the science!


The three major parts that make an amplifier valve are: the Cathode, the Grid and the Plate. The Cathode heats up when voltage hits the valve, causing a cloud of electrons to form. The Plate has a positive charge, which causes the negatively charged electrons to flow toward it. The Grid controls this flow of electrons. It is also the audio signal input for the valve.


An audio signal entering the valve causes a change in voltage at the Grid. This change in voltage changes the flow of electrons and causes amplification.

The behaviour of electrons described above is an example of a Class A amplifier. These amps apply a positive voltage to the Grid. Class AB amplifiers apply a negative “bias” voltage to the grid. This bias causes the electrons in the cloud to avoid the Plate. This is the standby mode of the valve.

The voltage of the audio signal entering the Grid causes the voltage on the Plate to change from negative to positive. This attracts the electrons in the cloud and causes them to flow to the plate. The valve in the Class AB amp then behaves like a Class A described above.

The need to change the charge of the plate from positive to negative causes the Class AB amplifier to feel less responsive than a Class A amp. But it also means the valve components aren’t in full use even when a signal is not passing through. This means the valves generally last longer.





Class A Amplifiers

Advantages:

The valve always warmed up and ready to amplify the signal.

Valve does not have to “wake up” from a “ready” state.

Class “A” amps have greater output than Class “AB” amps. If two EL84 output valves in a Class A design may produce only 10 watts of output power, then the same two valves in a Class AB design might produce 50 watts.

Smooth compression.

Less headroom. (NOTE: This is a disadvantage to some musicians who are looking for good, clean tone at higher volumes).

More responsive to the touch – i.e.: feels good to play.

Class A amps with EL84 valves in push-pull design emphasize high order harmonics. This emphasis makes the amp “sing”.


Disadvantages:

Keeping the valves warmed ready requires more current at all times. This constant current leads to shorter valve life.

Lower power rating than a Class “AB” amp with the same valve configuration.


Class “AB” Amplifiers

Advantages:

Longer valve life.

More headroom – cleaner cleans at higher volume.

Tighter bass response – less prone to “muddy” tone.

More efficient use of power.


Disadvantages:

Not as responsive as a Class “A” amp.

Tone won’t “break up” as easy.


Conclusion

The above details are based on the design of the amplifiers being similar as there are design changes that can be made to the amplifier circuits to combat some of the tonal disadvantages between these two amplifier classes. However, there are no design changes to combat the intrinsic disadvantages of the valves being under load continually as in a class A amplifier the other side of this disadvantage being the advantage of the immediacy of the valve to react to a signal.


In case you require a more in-depth introduction of how vacuum tubes (valves) work: -

https://vacuumtubes.net/How_Vacuum_Tubes_Work.htm


Thanks Peter, that explains a lot, my Palmer Fat 50 is an Class AB and I've wanted it to break up far more than it does, and it certainly isn't muddy, it has a gorgeous clean tone...

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