Posted by: worshipguitarguy | December 20, 2006

The 411 on Guitar Amp Models: Your Questions

Hey, Brian and Matt asked some really good questions after yesterday’s article so I thought I’d throw on an extension to it that answers their questions. 

Brian’s Question:
How about a quick explanation of the differences in classes. I hear that Class A is better than Class AB, but why?

[If I get time, I’ll try to record some audio samples later demonstrating the differences..]

Technical Info:
Ok, there are some great technical explanations of Class A and Class AB amps out there, and I’ll try to link to a few of them at the end of this.  I’m definitely not an expert at this but I’ll give it my best shot… 

The terms Class A and Class AB refer to the circuit layout design in tube amplifiers.  Remember there are two major sections in a tube amp: the preamp and the power amp.  Running power through the preamp creates what most of us would call overdrive or distortion.  The power amp section gives us volume.  A cranked power amp increases the volume (obviously), but also warms up your tone.   

Preamps:
All preamps are Class A, so the difference between the two classes is the way the power amp works.     

Power amps:
Class A and a Class B power amps differ in how they handle an input signal and how efficient they are with handling power. 

Class A amps are very inefficient.  That’s not an issue at low volumes, but at higher volumes, a Class A power amp requires more and more power to get incrementally small increases in volume.  Also, Class A power amp tubes run with constant power through them and duplicate the input signal exactly, (without clipping) 

Class AB amps have two components that each duplicate half of the input signal, then reassemble it at output to create the final signal.  An AB power amp is much more efficient.  Also the power going through the amp is variable, depending on the demands from the input signal. 

An example of efficiency, A Class AB amp running at 100W with El34 tubes needs four tubes.  The same 100W amp in a Class A configuration would require 10 EL34’s to generate the same output!  (That’s why you don’t see many Class A amps with outputs greater than 30W.  Also as a note, efficiency isn’t an issue with modeling and solid state amps, since by design they are extremely efficient with power when compared to tube amps) 

So which is better?  Outside of the efficiency question, it’s a matter of preference.  You won’t hear punchy and driving distortion tones from most Class A amps (unless you’re using pedals.)  Take a cranked Vox AC30 (Class A), and you’ll notice that the sound is very saturated, and bell like, but the individual notes do not lose their clarity as much as they would through an overdriven Class AB amp, (For A think the Edge.)  Class AB will give you more “crunch” tones like those from late 60’s, 70’s, and 80’s rock.  (For AB think Hendrix, Led Zeppelin, Ted Nugent, ACDC, etc…)

As an example, the Vox AC30 is Class A, and Fenders and Marshalls are AB. Also, common tube configurations in Class A amps include EL84’s, while AB’s often use EL34’s, 6L6’s in the power amp stage 

Reference Links:
http://en.wikipedia.org/wiki/Electronic_amplifier
http://www.duncanamps.com/technical/ampclasses.html
Matt’s Question:
Can you tell more about dual and triple rectifiers? I want to know how they’re different and how they work and stuff?

A little history:  Mesa amps were born out of the mind of a guy named Randall Smith who modded Fender amps with something called cascading gain stages in the preamp.  This was a huge achievement because it let guitarists get insane amounts of distortion, which were previously unavailable in mass production amps.  Eventually Randalls designs, became famous throughout rock, metal, and punk circles under the Mesa and Mesa Boogie names. The Recto amps came on the scene in the late 80’s early 90’s, (??? I think.)  The three types, a single rectified, dual retified, and triple rectified just refer to the number of rectifier tubes in the amp.  A single has one tube (and an output of 50W), a dual has 2 (100W), and a triple has 3 (150W.)  If I remember correctly, Rectos also have a switch, letting you switch between the tube rectifiers and a solid state rectifier giving you two different tonal variations.

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Responses

  1. I read an article at a custom amp shop’s website that proved that Vox amps aren’t exactly class A:
    http://www.aikenamps.com/VoxAC30classA_2.html
    basically it says they’re a little better than an AB amp but not quite class A. Just thought I’d slap that up, sorry if you may be disappointed, haha.

  2. You’re absolutely right Josh. Vox amps aren’t true Class A amps, I think they’re categorically thrown in as Class A because they aren’t true AB and tonally they have a ton of similarities to true Class A amps. ( I remember reading that article somewhere back in the day. 😉 )

  3. Thank WGG. That cleared it up for me.

  4. A little continutation on the rectifier. Rectifiers generally don’t have a big effect on tone until they are cranked, then they can give some compression and “sag” to the tone (think of a cranked fender bassman). Dual and triple rectifiers maintain a tighter bottom end when they are cranked, that’s why you see so many metal bands playing them.

  5. This blog entry comes at a perfect time for me, as you (Gerry) and I discussed earlier tonight. I’m currently in the market for a new amp/amp rig, and for my money, tube amps/heads are the way to go. The cream of the crop, of course, being Vox. To me, Class A versus Class AB was always a matter of quality over quantity. To make amps more affordable, a lot of companies are now offering a Class A/AB switch on their amps or heads. The way it usually works is that you can choose from your amp being 15watts of Class A, or 30watts of Class AB. What you sacrifice in tone you make up for in volume. But if you know anything about tube amps, you know that 15watts of Class A power is more than enough. Right now I’m looking into getting a Vox AC30 2×12 combo, or the AC30 2×12 quarter stack. But at $1,000 and 75lbs of girth, the smaller, AC15 might be a better choice. Both are (essentially) Class A, as pointed out in the previous comments, but the AC15 sounds just as good with a little less volume. But then again, you can never go wrong with a Marshall ;-).

  6. I have been using one of the new VOX AC30 CC2’s for almost a year now and recently purchased a Fender Blues Deville 4×10. I’m liking it better than my VOX. I’m trying to figure out if it sounds that much better to me or if it’s just new.. ha. I thought VOX would be my last stop, but the tones I’m getting with this Deville are sweet. You should check it out:

    http://www.guitarcenter.com/shop/product/buy_fender_blues_deville_410_reissue_guitar_amp?full_sku=103298501

    Chris

  7. Blues Deville Amps are awesome! I’m wondering if the tone difference you’re noticing is because the 4×10 cab is a little beefier? (I’m curious to hear how different the Vox would sound if you could run it through your Deville speakers. 😉 )

  8. Here’s a GREAT article written by Randall Smith himself on the difference between Class A and Class AB: http://www.mesaboogie.com/US/Smith/ClassA-WebVersion.htm

    While Class A and AB differ in efficiencies, they also differ in the tone they produce. Don’t believe it when people tell you one is better than the other – they’re completely different animals, as this article will explain.

    Personally, I like an amp that employs switching between the two; better yet, an amp that can blend the two classes together is even better still. The Vox AC30 will do it, plus the Marshall JVMs. A lot of boutique amp builders also employ this. Tone is all about personal preference, so which class you use is a purely subjective matter.

  9. have you ever used any peaveys? i just bought a classic 30/112. it’s my first tube amp, and i’m liking it alot. you can’t get any real high-gain distortion on it, but it’s good for what i play, mainly classic rock and stuff.

  10. I have used a Peavey Classic 30 before. I recommended it to a friend of mine looking for a good tube amp in his price range. The Classic’s are solid amps, I think they’re related tonally to Fender Blues amps???

  11. Hey man-a good general explanation for class ab vs class a, except for one thing-the vox ac30 is not class a at all! This is such a common misconception! Cathode biased class ab amps with high plate current/dissipation are often assumed to be class a and are not. It is sometimes said that it is class a at lower volumes, which is also partially true. If you don’t believe me, ask the guys at http://www.aikenamps.com

  12. Thanks for posting the difference between class A/AB guitar amps. I’m looking for a new amp now. The custom blues 30/ and the peavey valve king are the 2 amps I have been looking at because both offer class A and class AB in one combo. One amp that offers both is just the ticket. mike

  13. I currently have a Mesa single rectifier combo (6L6), and an orange 7/15 watt tiny terror (EL34). Both amps are great, but I honestly prefer the Orange over the mesa… at least for church applications. The mesa is great for high gain sounds and LOUD clean headroom, but My style has evolved to the point where i’d rather just have a great clean sound and just enough overdrive.

    It’s all personal preference thought when it comes down to it.

  14. I have a Vox AC 15w, and am looking to step up. Can you stack a cab on a AC15, preferably Alnico speakers, or is there not enough juice to power it? Should I just look into getting a new 50-100w amp to accomplish the volume and depth of clarity?

  15. Frustration abounds. Is there an amp out there that covers clean to mellow metal (jazz, blues, rock) and handles effects well? I am looking for suggestions. Thanks,

    John

  16. John this is a very tricky question to answer cause tone is very subjective. My Metal may not be your Metal. that a side.
    There are a lot of great amps out there. I think the vox amps are great for a Worship environment. Diverse with tone and take pedals good. Ac15 is a great amp. small but plenty of power.
    I have a couple tophat amps that i really enjoy playing thru. One is a single 12 combo that is 20 watts and has a 6v6 vibe. Easy to carry, class A great tone and loves pedals. i have seen them as low as 600.00 on ebay.
    I’ve had a couple fender amps which i really liked but were heavy and a bit more difficult to get good pedal tones.
    If you NEED to be diverse modeling is the best road in my opinion. unless you can have 2 amps on stage and a roadie 🙂

    My opinions hope this helps a bit.

  17. Hi I am considering buying amp from a friend for my 10 year old son who has been playing for 2 years he currently has a early 1980s japanese fender strat, Would this amp be good for future giging and also practice. Also what would be a fair price for this amp and is it rare as he discribes it. The specs are as follows • Fender London 185 – Head 185W + Cabinet (4×12″ speakers). Has 2 channels, mid-boost/cut, reverb, and input for foot-switch. I would appreciate any advice you could over.
    Thanks.
    Richard.

  18. Hi Hi,

    I have been playing Valvetrain 427 amp for a year or so, but I decided to buy a Music Man amp after I played my friends. However, the tone in the Music Man 112 65 brings out the twang in my Tele but there is no bottom end. I can’t seem to get it to sound like my Valvetrain. Now I don’t expect it to be exactly the same tone but my guitar is not sounding like it did before. I have custom Fervor Pickups (unreal pickups) but I don’t know if I am missing a setting adjustment or something unkown to me. What would cause this and how can I correct it? I have used pedals and plugged straight into it but to no avail. What’s the problem?

    Thanks in advance,

    Jeff

  19. is one trim pot normal 4 a valve amp even when there are 2 or 4 valves

  20. iam currently using a yweed deville with blue alnico,s an jus got a tc electronics nova systom stomp board love the anolog and digital reality of this new stomp board,iam diong a new praise n worship project,please iv us some prayer input,christ is so real,jmze johnson.

  21. also prayin 4 a ac 30.i love the big bell tone thunder it produces.jmze


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