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are class "a" amps louder than class "a/b" amps ???

Franktone

Gold Supporting Member
Messages
3,101
Generally one 30W tube amp should be about the same power and loudness as pretty well any other 30W tube amp. Differences can be attributed to the efficiencies of speakers or the the way the amps were tested for their rated output.

But if you are comparing cathode bias tubes amps to fixed bias tube amps, generally with roughly the same output tubes and comparable transformers (plate voltage etc), it can be seen that fixed bias amps generally put considerably more power than the cathode biased amp.

For example most big bottle amps such as the 50W Marshall running two EL34's puts out 50W fairly cleanly and has considerable more peak power available when cranked up full. Cathode Bias amps with the same tubes and comparable transformers (plate voltage) typically out around 30W clean and considerable more power when cranked up full.

So why would anybody buy a cathode biased amp. Well they usually have a different sound and personality that some people want.
 
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Seegs

Member
Messages
10,170
My Matchless DC30 30 watt combo easily competed volume wise with my other guitar players 100 watt Marshall half stack...used to frustrate him...

I now have an SC30 which is the same amp with only 1X 12" speakers...gets really loud as well!
 

hogy

Member
Messages
13,514
It sure is easier to swap tubes with a Class A, that's why I like them.
That has nothing to do with operating class. I think you're referring to cathode biased amps, which may or may not be Class A.


My Matchless DC30 30 watt combo easily competed volume wise with my other guitar players 100 watt Marshall half stack...used to frustrate him...
The Matchless is not a Class A amp.
 

teemuk

Member
Messages
3,225
Given the terrible inefficiency of class A it is my experience they are often not loud enough with their pathetic output power ratings. ;-)

Class AB amps come in output powers of 15W - 2500W while a reasonable output power demand for a class A design is usually about 5 watts at best.
So I would say class A/B amps are generally louder. If you want to think of it THIS way.

If you just want to compare class A vs. class-AB operation -as is- I fail to see how it effects anything meaningful wrt loudness.
 

mscaggs

Member
Messages
629
My Matchless DC30 30 watt combo easily competed volume wise with my other guitar players 100 watt Marshall half stack...used to frustrate him...

I now have an SC30 which is the same amp with only 1X 12" speakers...gets really loud as well!
Ok, so having owned Matchless since 1993 and also owning Vintage Marshall Superleads I can tell you there is no way on earth the 30 Watt C-30s can outrun a 100 watt Marshall. I love em both for different reasons but the Marshall is over the top louder.
 

DaveKS

Member
Messages
16,704
Of the same wattage output in the amp configuration, class A is more poignant, not nessesarily louder.
 
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J M Fahey

Member
Messages
2,642
As they (correctly) say: "a watt is a watt is a watt" .
30W at clipping, coming from a Class A amp (if such a beast really exists) and a regular Class B amp are the same, the speaker has no way to "know" where they come from, only so many Volts and Amperes it's getting.

Yet I understand where the idea may come from: some top class (no pun intended ) amps are usually (and incorrectly) referred to as "Class A amps" , are expensive (think Matchless, VOX AC30, etc.) and come paired with killer sounding **efficient** speakers, think a pair Vox Blue or the old Matchless standard of mixing 2 different but complementary Celestion speakers.
 

cochese

Silver Supporting Member
Messages
2,170
Class A amps are usually single ended. They have 1 output tube. THD's Univalve is a true class A amp. Vox AC30's are cathode bias and from what I understand do run in class A up to a point and then become class AB. I have a Fender Deluxe that was modded with a cathode bias switch that also removes the negative feedback loop similar to a Vox style amp. When you switch to cathode bias no negative feedback the amp becomes more aggressive and is louder at lower settings. Once you turn up the amp higher to where the output is maxing out there is no volume difference.

So basically like the previous poster said a watt is a watt. There was an article in one magazine a few years back and the cathode bias amp's were referred to as fast response. The notes seem to "catch" faster at lower volumes.

You should go to Aiken amp's website he has information regarding the Vox AC30 cathode bias vs true class A. He explains it in great detail and Aiken really knows a lot more about tube amps than most.
 

Guinness Lad

Senior Member
Messages
15,860
My Matchless DC30 30 watt combo easily competed volume wise with my other guitar players 100 watt Marshall half stack...used to frustrate him...

I now have an SC30 which is the same amp with only 1X 12" speakers...gets really loud as well!
I owned a Soldano SLO100 and a HC30 and SC30, no way the Matchless is as loud, not even close. Also these amps are not class A, they are hot running class A/B amps. Vox and Vox type amps being labeled as class A is possibly the single biggest marketing lie I can think of.
 

Papanate

Member
Messages
19,847
I'm referring to 15-30W amps...I figured class A would be a bit louder...
Based on what? Watts are Watts -
Now if you asked if Class A amps
are rated differently than Class A/B
amps you would be correct.

The only other piece you might be
'hearing' is that Class A amps can
have peaked out Midrange Freq's - so
your perception is they are 'louder'
even though they are not.
 

Flyin' Brian

Silver Supporting Member
Messages
30,178
Not necessarily. There are many variables to be considered. Speaker efficiency, distortion levels to name two.
 

hogy

Member
Messages
13,514
To those "a Watt is a Watt" advocates, you are forgetting two things.

First, the human ear is not a linear device. It is more sensitive to certain frequencies than others, and can perceive something as "louder" that is not.

Second, an amp needs much more power to reproduce low frequencies than it does for highs. If an amp is voiced to have lots of bottom end, especially inaudible subharmonics, it will have less power available for the midrange and treble frequencies. Such an amp, while nominally producing the same wattage as an amp voiced with an emphasis in the mid and up spectrum, can appear less loud
 

teemuk

Member
Messages
3,225
Vox AC30's are cathode bias and from what I understand do run in class A up to a point and then become class AB.
No such thing.

If you look at how classes of amplifier operation are derived, a class A amplifier is a class A amplifier if it conducts through a full cycle (360 degrees) at the very limits of its linear operating area.

In class B, on the other hand, the device only conducts a half cycle (180 degrees) and therefore in order to amplify a full wave with class-B amplifiers we need two of them in a "push-pull" arrangement.

This is why "single-ended" amplifiers must be class-A whereas "push-pull" amplifiers can be either class A, B or - as usually - class AB.

Since class A bias means that half of operating current is constantly flowing through the device when its idling the efficiency of opearation is terrible. Given moderately symmetric signal amplification the average current draw is about the same since it "swings" below and above the idle rating.

Class B, on the other hand, idles in cut off where no current flows at all and current flow approximately increases in interaction with output power generated by the amp. Efficiency wise this is much more ideal.



In practical class B designs, however, there is a dead zone at the crossovering region from one half wave to another where neither device conducts. This introduces distortion. Class "AB" bias is therefore a compromise between inefficient class A and 360 degree conduction or much more efficient class B with only 180 degrees of conduction. By exteding the conductive region the crossover distortion can be decreased and made so low that it practically becomes inaudible in most situations.

So what exactly is this "AB" bias? By definition it's less than 360 degrees but more than 180. That leaves quite a lot of room for variation.

Alternatively, we could shift the bias of such push-pull amp to full class-A operation. Now both devices would conduct 360 degrees but to opposite directions.

Here's the fine line between "hotter" and "colder" biases. But whichever way you bias it, operation is either defined as class A or class AB. No "in betweens", the AB is already that. If conduction is less than 360 degrees at the limit of linear range of operation then its not class A. "Hotter" bias doesn't mean class-A unless its indeed hot enough to be class-A bias.

...And for sake of optimal performance, or even best tone in general, "hotter" bias isn't always better than a colder alternative.

So in practice the advantage of class A is that its devoid of crossovering distortion. The disadvantage is terrible inefficiency, which usually limits the output power a class-A amp can produce to "practice amp levels" along with equivalent "headroom". As they tend to run inefficient and are usually simple "single-ended" design chances are harmonic distortion levels generated by other distortion mechanisms than crossovering far outweigth those of a decent class AB amp.

Class AB's advantage is realistically decent efficiency. Drawback is higher crossover distortion, which in practice is almost completely eradicated by class AB bias or design imlementations such as negative feedback... So it's really no drawback at all.

This all of course begs a question, why would any of this really matter if judged from solely how waves get amplified and how much power such wave generates to drive a transducer that converts electrical energy to acoustic energy? End result is still always a complete wave, although class AB does it at much more admirable efficiency than class A. In the end that allows nice things, like having amps with more than about 5 watts of output power, that don't heat up like sauna stove, and don't weight ridiculous amouns wrt output power.

I have a Fender Deluxe that was modded with a cathode bias switch that also removes the negative feedback loop similar to a Vox style amp. When you switch to cathode bias no negative feedback the amp becomes more aggressive and is louder at lower settings.
Well, needless to say, feedback loops or bias arrangements have absolutely nothing to do with class of operation.

But since you brought it up, yes, by a good chance a fixed bias amplifier will, especially in terms of overdrive, behave quite differently than a similar but cathode biased circuit - or a mix of both topologies. But this is a difference of bias topologies not difference between classes of amplifier operation. Similarly NFB vs. non-NFB systems (and in betweens) will display great amounts of all kinds of differences, given feedback's contributions to distortion, bandwidth, output Z, etc. These characterstic again have nothing to do with class of operation of the output devices, though.

Certain amplifier architectures, however, are more vulnerant against phenomenons where sustained overdrive conditions can shift the bias. This generally happens towards "colder" direction so its not uncommon that in many "push-pull" tube amps sustained overdrive triggers crossover distortion. Single-ended amps would not and could not "crossover" like this but class-A biased push-pull amps theoretically could.

This is, of course, happening at that magical area outside that "linear range of operation" at which class of operation is defined to take place. We can't really evaluate on this basis objectively, although a lot of musical effects borderline this territory.
 
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riffmeister

Member
Messages
16,608
I skipped ahead in the alphabet and now I only play "Class E" amps. That's because I'm a "Classy" guy.
 

Franktone

Gold Supporting Member
Messages
3,101
Here's some nice easy diagrams so you can see the difference between Class A, Class B, and Class AB. I know that these diagrams have transistors but the idea and concepts are about the same for tubes. You can see in the first photo why the Class A circuit is always feeding current, even when the transistor is idling, because the Q point (operating point) requires that a large amount of current is passing through the transistor at idle. This is because you are using the full curve in order to amplify the full 360 degrees of the wave. Yes the diagram in this 1st photo shows a single ended amp.

The Class B circuit is much more efficient because the idle current is zero, but cross-over distortion is introduced as the operation is passed from the 1st transistor to the 2nd transistor because each transistor needs at least 0.7 volts to go from not running to running and they do not work below 0.7 volts. So as the signal passes through zero from +or - 0.7 volts nothing is being amplified by either transistor and this distorts the sound. This 2nd photo is a push-pull amp.

In the last photo (Class AB) you can see how the Q point (operating point) is raised slightly above 0 volts in order to avoid the considerable cross-over distortion caused by the Class B circuit. This creates some overlap of operation between the two transistors so that there is no time span where both transistors are both off at the same time to create considerable cross-over distortion. Here you get less wasted current, greater efficiency, and moved the Q point (operating point) out of the 0 volts zone to greatly reduce cross-over distortion. This 3rd photo is also a push-pull amp.



 
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voodoosound

Funk & Grooven member
Gold Supporting Member
Messages
6,224
I skipped ahead in the alphabet and now I only play "Class E" amps. That's because I'm a "Classy" guy.
I'm going to skip ahead even more and have a class z amp built. Then there will be non louder. Of course it will be all black and go to 11 x 10. It's the only amp I think I'll be able to truly hear the full range of an H chord. It will be 5 watts and sound at least 23 times louder than a Marshall 100 watt because of course a Marshall is only class a/b. And I will be running class Z.
 

sharpshooter

Senior Member
Messages
4,012
Yeah, the THD UniValve, while rated at 15watts, has the iron to support a 6550/KT88, and get up
to around 18>20watts, some have measured 22 watts.
The THD BiValve, and the Victoria Regal II, are both "parallel single ended", and will easily get up
into the 35>40watt range.
IIRC, some outfit was building a single ended amp using a Russian tube about the size of a beer bottle,
and getting about 50>60 watts,, a real high voltage tube, originally used in some military app.
 




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