Class A/B whats the difference?

[John]

Evening all, just wondering what the diference is between a class A amp and a class B or any other classes that may exist.
Oh yeah keep any answers simple please
Cheers John

[Amanda]

Hi John,

A class "A" amplifier is an amp where the output valve(s) are conducting all of the time,
i.e. throughout the whole signal, examples of Class A amps are the AC4HTV and other amps using a single
output valve, in these circuits the output valve is biased so that the signal remains in the
straight part of the valve's operating curve, thus minimising distortion.

In a class B amplifier we set the bias point to where the valve almost stops conducting.

The input signal is a lot larger now in order to drive the valve hard enough. Also, the output current is only for half of the waveform.
To use Class B, we have to have a "push-pull" output stage which employs two valves (or two banks of valves) so that each side amplifies
each half of the waveform.

Class AB is somewhere between class A and class B. In class AB, a small amount of bias current flows through the valve.
For the output valves in a typical class AB guitar amplifier, this is around 30-40mA, with peaks of approximately 250-300mA.

The AC15 / 30 is a good example of a Class AB amplifier, although some people regard it as class A.

Regards

Amanda

[peter roest]

Hi John,

In the figure you'll see the Class A, Class AB and Class B. Signed is the signal (vertical) send into the input of the phaseshifter triode valve before the power valves (ECC83 and EL84 in Vox AC15 and AC30). In the figure you find the output signal (horizontal)of the "left half"of the doubletriode (the "right half" gives also the same output as the left only the signal is turned upside down 180 degrees)).
1. Class A: the complete shape of the input signal appears at the output of the end valves.

2. Class B: only the positive part of the signal appears at the output of the endvalve(s) the other endvalve(s) gives the negative part.

3. Class AB: Moves between Class A and B (also both endvalves gives a part of the signal also 180 degrees phase shifted).

In the output transformer the both signals out of endvalves will be putting in phase.

in the pictures you can see that going from Class A via Class AB to Class B the input higher signalstrengh is possible , so you can get more watts output.

Peter

[TWANGER 59]

Hi Amanda , is my Pro Junior class A ? it sounds amazing, and and stays clean at high volume , as you know i have changed the valves ,and speaker , plus it has the mod that you did for me to allow more treble when required , regards Ron Sutton

[Graylion]

Hey! I'm glad that John only asked for the short answer! Mostly all Greek to me! Does it aways follow that an amp with 2 output valves has them in push/pull as a Class B or AB? I'm wondering what my Bird Golden Eagle is. It has 2 x EL34s and is rated as 25 watts by the manufacturer. It uses 3 x EF86s as preamps and 4 x ECC83s as oscillators etc. for the effects and phase splitter. The original advertising leaflet says "ultra linear output" in the Technical Information part! It has a GZ34 rectifier and a 15" Goodman speaker. I also have a WEM Dominator MkIII with 2 EL84s - A or AB?

In my ignorance I always thought that they put valves in push/pull to increase the output. My Bird is an example - 2 x EL34s only giving 25 watts whereas later Marshalls, and others, gave 50 watts with the same valves (100 watts with 4 x EL34s).
Keep it simple guys! Cheers, Lionel

[RayL]

Lionel,

That 'Ultra Linear Output' is a bit of a clue. My guess is that your Bird amp has negative feedback. Negative feedback is good for hi-fi amps because it reduces distortion, but the trade-off is loss of power. This is why your 2 x EL34 output is only rated as 25 watts. Negative feedback sounds like a good idea (lower distortion) but it's not good for guitar amps because the effective dynamic range is reduced and dynamic range is important in allowing the player to go from lightest touch to a powerful chord. Does your Bird amp sound great at bedroom volume but a bit weedy in a big hall?

Neither the AC30 nor the Marshall range have negative feedback. The Watkins Dominator with it's '17 watts' of output (world famous thanks to Mo Foster) is squeezing about as much as you can get from a pair of EL84s, so Charlie probably biased it somewhere very close to class B.

Ray L

[Didier]

is my Pro Junior class A ?

As far as I know the Fender Pro Junior is class AB (two EL84 in push-pull).

Didier

[RogerCook]

There's an in depth discussion here (one for the techies )

http://www.aikenamps.com/VoxAC30classA_2.html

Roger

[Didier]

I'm wondering what my Bird Golden Eagle is. It has 2 x EL34s and is rated as 25 watts by the manufacturer.

25 watts output is very low. Hi-Fi amps used to have 30 W with a pair of EL34 and negative feedback, guitar amps up to 50 W without feedback.

Didier

[RogerCook]

There's a Bird Golden Eagle ciruit diagram here. Is this the same as yours Lionel?

http://www.vintagehofner.co.uk/britamps ... matic.html

I've not had time to look carefully but at first glance it has feedback and ultra-linear taps on the output transformer much like the classic Mullard 50-20 (think that's what it was called) of the 50s/60s

Roger