The Class A vs. Class A/B Slugfest!

Which kick more ass? Class A or Class AB?

The recent experience making a DIY Class A Champ  reignites for me the eternal debate between the merits of Class A and Class A/B amps. (I link once more to Dave Hunter’s excellent short article on the topic, since he explains the distinction so much better than I can.)

Like most American players of a certain age, I grew up without access to great Class A amps. (I’m going to take the lazy way out here, and lump Voxes and their kin into the Class A category, despite the distinctions that Dave H. explains so well.) My initial exposure to various ’90s Vox reissues and other “Voxoids” of the era was enough to spark my interest. (And is “spark” the perfect verb for great Class A amps, or what?) But it wasn’t till I encountered Matchless amps and real vintage Voxes that I totally got it.

But still, I go back and forth constantly. And whenever I switch between categories, I think, “Ah! There’s what I was missing,” though I tend not to miss it when I’m playing an amp of the opposite type. So at least I’m usually happy.

Overall I’ve probably favored Vox-type amps over Fender- and Marshall-flavored ones, though I’m the sort of player who’s rarely interested in macho rock and metal distortion. (If that weren’t so, I’d be an A/B purist!) But having said that, my fave amp these days is the decidedly A/B brownface Tremolux I use in the lion’s share of my videos. So put me down as perennially undecided.

How about you? Do you pledge your allegiance to the explosive harmonic energy of Class A or the focused impact of Class A/B? And has anyone explored switchable schemes, as found on some Mesas and modded Marshalls?

16 comments to The Class A vs. Class A/B Slugfest!

  • Oinkus

    Switchable schemes is the thing that makes the Fender Prosonic the greatest sounding amp I have ever owned.Wanted to buy a Matchless abut 20 years ago or so was very pricey just for the head. They had an article about the Prosonic and how it compared very favorably with the Matchless. Price was much cheaper plugged ,into a head and 4×12 cab was instant WOW!Bought the combo because it had reverb and got rid of all my other amps 4×12 cabs and etc.Not sure if the uh Grid Bias ? Selection knob is exactly the thing you are talking about but it lists Rectifier/Class – Tube /Class A, Tube /Class AB2, S.S./AB2. Back to the original point it IS NOT a Matchless but it is a great sounding amp with loads of depth and character.

    • joe

      I had a similar experience: wanting a Matchless, but balking at the price. For years I used a Mesa Blue Angel, a real simple, stripped-down Class A head that’s way closer to the Vox universe than what you typically think of as a Mesa sound.

  • Schrodingersgoldfish

    I spend the most of my music-making time riding the place between cleans and breakup. The smoothness that my little class-A amp gives me when it is just breaking up would be hard to work without. It makes the difference between tickling the nickel and wacking the bugger a beautiful thing.

  • mwseniff

    Live I use Class A/B exclusively the bass is more solid and chunky. But for recording I like a mix of Class A and Class A/B as overdubs this gives a nice full tone with the best of both. I built a pair of amps for a duo that played “accessible jazz” many years ago each using 3 – WE 300B tubes in a single ended output amp circuit. The parts all came from a hifi cork sniffer pal of the guitar players (parts would have cost an arm and a leg otherwise). It put out 70 watts and used an extra clean preamp with 6SL7 tubes. The amps had a big warm open soundstage that was very detailed. The audiences supposedly really loved the tone tho’ it had a lot of chime for jazz. The output and power xfrmrs were very heavy as was the welded steel chassis (copper flashed) the amp heads weighed over 50 lbs each. The also generated a lot of heat, Class A amps use their maximum power at idle which really cooks everything in circuit.

    • joe

      I agree totally about the “solid bass” stuff — A/B amps are way stronger in that regard. Though I’m not sure how much of it is a matter of A/B amps actually moving more bass, as opposed to their superior definition between strings, or the fact that the relatively chaotic harmonic distortion of Class A designs is to complex to provide clarity in the lowest register.

      • mwseniff

        One big difference in a Class A amp and a Class A/B1 is that in Class A the amp is drawing maximum plate current at idle. This means that you don’t get the power supply sag that you do with a Class A/B1 design when pushed hard. In fact a signal will actually reduce plate current during portions of the waveform. So you don’t get the natural compression that you do with a Class A/B1. IMHO this one of the really big differences in the two classes of amps it is also why hifi cork sniffers like them as well.

  • Mil

    This thread reminds me of the mid-90s Internet. mwseniff: 70W out of three 300B tubes in Class A? FYI: 300Bs are rated at 8W. And I am not even going to debate the whole idea of using the cleanest possible audio tube for jazz but have to say that jazz is not played by sterile surgeons in white coats.
    Anyhow, neither AC30 nor any of the copies (Matchless, Bad Cat etc.) are Class A. They are biased hotter and up to a certain point they do work in Class A but then ultimately transfer to AB. Read excellent analysis from Aiken at
    http://www.aikenamps.com/VoxAC30classA_2.html
    Aiken even concluded that they start the transition at ~12W output. AC30 are not known for a good distorted sound but that’s not a proof that Class A is superior to AB. It just happen that in that particular design it works better. Besides hot biasing, an AB amp can be designed to produce some characteristics of the “A” sound by intentionally making one tube to work harder in the PP configuration, by eliminating NF and by using cathode resistors for bias. Kevin O’Connor has beaten this to death in many of his books on tube amps (check out London Power amps). Recommended reads, especially for Dave Hunters out there.

    • joe

      Yes — the Dave Hunter article I linked to as well makes the same point about which circuits are truly Class A, hence my disclaimer above. But non-engineer that I am, I tend to view the distinction in sonic terms, rather than technical ones. And both the true Class A amps (like the Champ) and the not-really Class A amps (like the Voxes) share many of the characteristics, good and perhaps no so good, that distinguish them from Class A/B amps.

    • mwseniff

      You are right I had 300Bs stuck in my mind somehow. I had to dig out my old notebooks from the 1980′s in my file cabinets. The amps used KT88s in single ended operation triode connection (the amps were built in 1983 according to my records). The reason these amps got built in the first place was that the “hifi cork sniffer” ponied up the parts and labor money to create the amps for his pals. They came to me with the idea for the hifi-style clean guitar amps, it was not something I designed but rather built to spec. I built a rough test version for them to approve made some changes they wanted and then built the final amps. Honestly at that time in the early 80′s I was interested in distorted amps at low volume not clean ones like these amps. The duo used the amps for 10+ years AFAIK. I am not a huge jazz fan especially the “accessible jazz” these guys played so I only saw them do one show with the amps. While the amps were clean they were still very musical sounding but not sterile by any stretch of the imagination. Heck they sounded far better than the solid state Vox amps they were using before. They were some happy campers after playing some gigs with them. The duo didn’t wear white coats to play music but they did wear them in their day jobs doing chemistry at the U of I.

  • Digital Larry

    I think I’ve only played electric guitar in public a few times. Most performing has been on electric bass, with solid-state amps yet; after that acoustic guitar and mandolin.

    The only tube amps I have ever owned are the ones I bought off eBay to restore them and try to learn about all this stuff! My current guitar amp arsenal consists of a Boss Micro Cube and a Tech 21 Trademark 60. Oh, the shame!

    My first tube amp experiment has been with a low wattage single ended amp. Having only one triode stage it does not get much in the way of crunchiness or sustain without turning the guitar volume all the way up and hitting the thing as hard as possible. I find the results to be “splatty” and “cluttered” especially on the low strings.

    I’ll go on record as saying I don’t exactly like this sound.

    I tried my Joe Gore boost pedal on it and actually got into some realms where some other type of distortion mechanism took over – the sound actually sort of disappears at the first attack. Could it be the first triode grid is getting forward biased? I’m not sure. There is no grid stopper resistor here.

    Dave Hunter’s article is interesting but I think he’s oversimplified the cause and effect. He does allude to lots of different things beyond the amplifier class which are probably more important.

    The reason I started these restoration projects was to see if I could learn what the different causes and effects were. I doubt I will discover anything that is not already known, but it’s one thing to read about it and quite another to experience it and measure it and try to control it and see what happens.

    I’m still on the quest but I don’t think I’m going to put a lot more effort into single ended designs.

  • I do love the all-in poweramp distortion of single-ended amps. Ever check out Willie Johnson, Howlin’ Wolf’s guitarist from the early days in Memphis? That’s what I’m talkin’ about!
    I never went the Vox route beyond fetishizing them a bit. In my first band the second guitarist used a Korg era AC30; it cost a mint and blew through tubes at record rate, so I just got over the idea.

    • joe

      LOL — in building amps that break every two weeks, Korg was merely being faithful to the ’60s originals.

      I don’t know much about cars, but I watch Mad Men — and I suspect there are definite parallels between the Vox amps and Jaguar automobiles of the ’60s, with both being very cool and very unreliable.

      • joe

        Though I should add that, despite the technical unreliability, I actually relied a great deal on a ’90s circuit-board-built AC30 as a primary studio amp. I sold to recently to another musician friend, who loves it too. Really, it’s a great-sounding amp.

  • mwseniff

    I’ve serviced so many AC30s over the last 35+ years I have lost track of even an estimate of the number. One thing they all had in common was they were all cooked to death by their own heat. I’ve seen some that the leads from the output transformer had all the insulation crumbling away. I’ve replace a lot of output tube sockets as well as they were crispy critters from the heat. For the most part the AC30s circuits were crammed together making them very time consuming to service because sometimes the parts needing replacement etc. required removing other components to reach the bad ones. In a way the later pc board models were a little easier to service than the 60′s ones. AC30s could definitely use a fan (or 2 or3 or 4) to make them last longer or even a redesign that would give them better ventillation. It’s a shame because they do sound fabulous when working properly. If you really need an AC30 for gigs you better have at least a one or two spares in your vehicle outside the venue(having a handy spare is the easiest way to never have a breakdown Murphy be damned).

  • Digital Larry

    OK here’s a dumb question – supposing you have a push-pull configuration where both of the output devices are running class A. How is this any different/better than a single ended amp with the two output tubes in parallel?

    Seems like Aiken’s extrapolations are to what would be needed to get a push pull stage to run class A are academic rather than practical (I’ve been accused of that!).

  • Mil

    @Digital Larry
    It’s the economics; PP output transformers are much smaller (read cheaper)and easier to make than parallel SE. Another argument against parallel SE configuration would be requirement for a more complex power supply. In PP configuration you can skimp a little bit on filtering as noise gets cancelled by the very nature of the design. These are important things for mass producers. Most of the boutique guys out there are more or less just copy cats; they are either not willing to experiment much or would like to but don’t have enough $$$.
    As far Aiken goes, his aim was just to break some marketing myths about class “A” and AC30. I am sure that he likes the sound of AC30 regardless. I know that I do.

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