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Topic: Tube Gurus: Class A v. Class A/B |
Michael Pierce
From:
Madison, CT
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Posted 20 May 2009 12:12 pm |
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| I have done a little bit of research (enough to be dangerous) about Class A v. Class A/B tubes amps and my question for the tubular cognoscenti is, assuming having lots of clean headroom is not an issue, how differently does a Class A amp sound from a Class A/B amp? I've seen adjectives like "cleaner," "less distortion," "piano like cleans." I'm thinking Bogner Metropolis (A), Ecstasy (A or A/B) or Genz Benz Black Pearl (A). I'd be using one for pedal steel, primarily in lower volume settings. Thanks. |
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Eric Philippsen
From:
Central Florida USA
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Posted 20 May 2009 12:31 pm
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| I have several vintage class A amps and a well-known newer model. There is a definite difference between the sound of those and their class A/B counterparts. In my opinion, the tone of class A has much more of a "jangle". Very bell-like and chimey. Think Vox Ac-30. Would I use them for steel? No, although I know of some who do. They certainly develop heat, which is fine as long as it doesn't self-destruct due to that. Raw, clean power and headroom might be a problem in a live situation. Great for 6-string, not so sure for steel. |
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Eric Jaeger
From:
Oakland, California, USA
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Posted 20 May 2009 1:21 pm
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Uh, the problem with Class A vs. A/B is often a matter of definition. There are a lot of amps that claim to be class A that don't fit the technical definition.
In my opinion, the distinction is overblown, which is to say that while there may be differences, the differences between individual amps *WITHIN* each class are greater than the differences *BETWEEN* the classes.
-eric |
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Brick Spieth
From:
San Jose, California, USA
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Posted 20 May 2009 5:30 pm
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Class A is the most misused term in guitar marketing. I would venture to guess 90+ percent of all amps calling themselves class A are not.
Want a sure thing class A amp? get a Fender Champ or any other amp with only one power tube.
I've got a Gibson amp (Trace Elliot Vellocette repackaged) that calls itself class A when it is most certainly not, except when one power tube blows and it turns into a marvelously gnarly blues machine. I'm afraid to play it like that for long.
Hype and marketing run amok. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 20 May 2009 6:16 pm
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In class A, the bias is set so that each tube amplifies the full cycle of waveforms. In class B push-pull operation, the bias is set so that each tube in the push-pull pair amplifies positive and negative portions of the signal separately using a phase inverter, and reconstructs them using a center-tapped output transformer. In class AB, small signals are amplified in class A, but as the signal gets larger, it moves to a more class B operation.
To me, when pushed, the amplification characteristics of true class A and class AB or Class B are quite different. The push-pull pair cancels certain harmonics and accentuates others - to me, it creates a more crystalline tone if everything is done right. The class A approach is more raw, to my ears. They can both sound good.
Class AB is the classic sound of the blackface/silverface Fender amps - I think the Twin Reverb is its greatest exemplar, and most of the "clean" guitar/steel amps are of this design. A really nice true class A amp with more juice than, let's say, a Fender Champ, is the THD Univalve or Bivalve. Dan Tyack has posted many great-sounding sacred steel or blues tracks using these amps. I don't see any way to get exactly that sound with a class AB amp.
I've owned a bunch of true class A amps over the years - mostly for guitar - including various Vox amps, Champs, Gibsons, and so on. They're cool for certain things, but I look at them as fairly limited. If you want what they do, they're very cool. I guess I just generally prefer class AB. Just my tastes, I guess. It's not that "one is distorted" and "the other is not distorted". They can both be either distorted in varying degrees or not, but the character is different. The Twin Reverb and varying eras of Marshalls are both class AB, but when pushed, sound very different from each other, and very different than a true class A amp to me. Eric uses the terms "jangle", "chimey", and "bell-like", with which I agree for certain guitars - put a Ricky electric 12-string and they definitely do jingle-jangle. But a Gibson P-90 or humbucker into a Champ is not "chimey" to me - it's rough and raw.
I think the only way to tell anything about this is to plug some guitars in and have at it. All the words in the world don't quite do it justice, IMO. |
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Jim Sliff
From:
Lawndale California, USA
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Posted 20 May 2009 7:52 pm
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Without getting too technical, you also would generally find that TRUE class-A amps with enough power and headroom for clean pedal steel to be extremely expensive. Class A amp, because they run the tunes "on" all the time, as opposed to a push-pull class A/B amp run VERY hot...and to get a decent amount of clean power you would need a huge number of power tubes, a monstrous power transformer, and a cooling system
If you look at the amps named, the true class-A amps are low-powered. That primarily because it's simply not cost effective or practical to build a BIG class-A amp (although it's done in things like radio broadcasting - but you'd be looking at a $150,000 guitar amp to get something comparable.
The usual misconception is that ALL EL84 amps are class A. They are not. Or cathode biased amp are class A. Wrong again.
My opinion - don't worry about it unless you are designing an amp or marketing one. As a player it makes not a bit of difference except in tone, and there are lousy A/B and lousy A amps that throw a curveball to you anyway.
If you are on a quest, get a Champ. Class A,superb tone, and MUCH louder than you'd think.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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Ben Jones
From:
Seattle, Washington, USA
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Posted 21 May 2009 5:54 am
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Man, this stuff is as bad as theory for me..it just doesnt sink in when people start talking about how tubes work, grids, plates, etc. I am trying to read dave Hunters amp book, in preparation for building my first amp but I am just not understanding or retaining the info. Maybe when i get my hands on the parts it will be clearer?
epi valve jr....class A? |
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Michael Pierce
From:
Madison, CT
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Posted 21 May 2009 6:32 am
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Don't feel bad, Ben. Everything I know about this stuff (what little of that there is) I learned from Wikipedia! Thank goodness for the Forum and my thanks to everyone who responded.  |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 21 May 2009 7:24 am
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| Quote: |
| epi valve jr....class A? |
A single output tube intending to amplify the entire waveform must be biased class A, so
single output tube => class A
But it is not true that class A => single output tube. Multiple output tubes can be run class A.
You don't need to understand all the electronic intricacies to basically understand the distinction between class A, B, and AB. If you can think about what each tube does to an AC waveform in graphical terms, that's the point. In class A, each tube amplifies the entire waveform - both the positive and negative portions of it. In class B push-pull operation, each tube in the push-pull pair amplifies the positive and negative voltage parts of it separately. In class AB, it moves from class A to class B as the magnitude of the waveform increases to the limit of the amp's capability to amplify.
The advantages of class B are partial cancellation of noise and harmonic distortion, plus greater efficiency in clean amplification. The main advantage of class A is - if you prefer it - the character of the tone and harmonic distortion.
I still say that the proof is in how it feels and sounds to a player, not the technical aspects. It is possible to simply experience the difference between these types of amps without worrying a bit about how it's done electronically. |
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Earnest Bovine
From:
Los Angeles CA USA
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Posted 21 May 2009 8:30 am
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| Dave Mudgett wrote: |
The main advantage of class A is - if you prefer it - the character of the tone and harmonic distortion.
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Are you referring to the (small) distortion that occurs when the signal is at a moderate level? Or to the extreme distortion that guitarists like to use at the highest levels? |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 21 May 2009 9:53 am
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| Quote: |
| Are you referring to the (small) distortion that occurs when the signal is at a moderate level? Or to the extreme distortion that guitarists like to use at the highest levels? |
Both. In a push-pull circuit, there is sometimes some harmonic distortion cancellation at moderate (in the context of the power curve of the amp) levels that doesn't occur in class A. To my ears, this difference gets even more pronounced as the amp is pushed harder to the limit. At low volumes, I don't hear much difference.
I think my problem with the typical class A setup is that there isn't any way to get the sound very clean and spanky at moderate volume the way a good class AB does. In most situations I'm in now, I guess I like it pretty clean at moderate volume and only start to get some hair at the limit.
A while back I sold an 80's Vox AC-30 - it definitely had that sound on guitar - a Gretsch, P-90, or humbucker guitar really sounded great with it. But it isn't a sound I can use very often in the context of the stuff I'm playing right now. |
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Dan Tyack
From:
Olympia, WA USA
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Posted 22 May 2009 12:56 pm
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As has been pointed out, there's more hooey out there about 'class A' amps than just about anything in the tube amp world. My main amps are true class A (THD Univalve and BiValve), and there is definitely something really unique to the sound. I believe a lot of it does have to do with 'distortion': not extreme distortion, but the kind you get when the amp is pushed a little, but sounds 'clean'. Class A amps emphasize even order harmonics, which sound really sweet. You simply can't duplicate the sound with an AB design. And when you really push the power amp stage, the character of the distortion is even more distinctive. The words I would use to describe my THD amps when pushed are sweet and clear. Even when totally pushed, you can still hear each note in a chord. Great for blues or rock and roll pedal steel, which can sound harsh and buzzy with a lot of guitar amps that are set up for rock and roll guitar.
In terms of volume, my BiValve with 2 KT88s can get fairly loud and still stay clean. Certainly louder than a Deluxe, for example. And because of the distortion characteristics of the amp, if it gets pushed a little, it still sounds good for country steel. Now if you push the amp, it will get seriously loud, easily as loud as a Super. If I need to play really loud and clean, I just take the line out from the BiValve (post power amp section) and run it into a little Crate Power Block into a seperate cabinet. This combination is as clean and loud as any twin I've ever heard.
Last edited by Dan Tyack on 22 May 2009 2:20 pm; edited 1 time in total |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 22 May 2009 2:15 pm
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| Quote: |
| My opinion - don't worry about it unless you are designing an amp or marketing one. |
Precisely, and I'm glad someone said it! |
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Dan Tyack
From:
Olympia, WA USA
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Posted 22 May 2009 2:25 pm
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| FYI: According to Andy Marshall, the head of THD amplifiers, the AC30 is not a class A amp, although it and similar EL84 amps are typically marketed that way. It's a push-pull design. However, he says that when pushed, AC30s do operate in ways that are similar to class A amps. Not sure of the specifics, but that's the view of a guy who knows. |
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Ben Jones
From:
Seattle, Washington, USA
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Posted 23 May 2009 6:52 am
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Ive had someone tell me Dave Hunters book is not a good one to learn from. Can anyone recommend something else? Is there a book that explains how amplifiers work that maybe doesnt read like a tech manual? Hows Pittmans book?
I'd like to actually understand, at least somewhat, what i am building rather than just soldering a bunch of parts to an eyelet board..on the other hand i dont want to read a tech manual. maybe there is something in the middle for me?
tweed deluxe kit arrives next week. |
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John Billings
From:
Ohio, USA
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Posted 23 May 2009 7:19 am
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Is the Williamson circuit Class A? I have an old power amp that has that design, using Genelec KT-66s.
Doctor Z told me that my Hiwatt Lead 30 is biased so that it's almost running Class A. |
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Tony Prior
From:
Charlotte NC
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Posted 23 May 2009 7:51 am
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The Vox AC15 and AC30's are class AB, also, it is also stated that all tube amps may be considered class A up to approx 50% of the rated output power.
There are several great articles to this regard all over the net,here's a great one on this exact topic.
http://www.aikenamps.com/VoxAC30classA_2.html
I still want an AC30 but I don't want it to be 70 pounds ! Yes, I am thinking head and cab...
t |
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John Billings
From:
Ohio, USA
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Posted 23 May 2009 7:53 am
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Tony,
You might want to try out a Doctor Z Prescription Amp. Available as a combo or head and cab. I've got one, and it's incredible! |
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