VOLT
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Post by VOLT on Sept 23, 2010 8:13:48 GMT -5
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Post by newey on Sept 23, 2010 11:08:54 GMT -5
Well, from the description, it seems that the control varies the amount of the signal amplified in a "class A" single-ended fashion versus the amount that is amplified in a "class AB" push-pull fashion. Or at least that's the way I understood it. Wikipedia describes, in a very technical manner, the differences between the A and AB classes of amplifiers: Amplifier Classesbut I don't understand half of that. From a guitarist's standpoint, just understand that "Class A" is what a Fender Champ sounds like, and "Class AB" is more like a Vox AC, and like some other well-known "British Sound" amps. The variable control apparently allows one to blend a bit of Champ with a bit of Vox, so to speak. Many amps have a switch to switch between "British" and "American" sounds, which are probably doing the same thing as this pot, but without the ability to blend the 2 sounds. IIRC, some other amps also use a variable control like this, although they may not call it a "texture" knob- Mesa Boogie has something similar, I think. |
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Post by ashcatlt on Sept 23, 2010 12:19:26 GMT -5
For as long as I've been paying attention, the Vox AC30 has always been described as the quintessential Class A amplifier. On my V-Amps, for example, the model labelled "Brit Class A" is meant to emulate a Vox. This guy makes a pretty convincing argument that it isn't exactly true Class A, but hasn't yet changed the popular conception. I can't speak to the Champ, but most Fender amps a AB. I think most Marshalls are as well. So maybe it will go from an "American" sound to a "Liverpool" sound, but I think you've got yourself confused about which side of the pond is which. I wouldn't really call the Class A "British" because a lot of folks think Marshall when they hear that term. The big difference is crossover distortion. In a class AB amp the two sides of the waveform (positive and negative) are amplified but different tubes. Since these are most likely not going to agree on exactly where 0 is, there's often a little bit of "juggling the handoff" that happens. That causes a form of distortion distinctly different from other types. |
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Post by sumgai on Sept 23, 2010 21:37:07 GMT -5
To set the record straight:
Class A is defined as one tube amplifying the entire signal - all 360° of it. Class B is defined as one tube amplifying exactly one half of a signal, or 180° of said signal. Obviously it would take two such tubes in order to amplify the entire signal, so that we hear all of what went in, only louder as it came out. Doing so is classically called a "push-pull" circuit, and usually involves a transformer (for tubes, at any rate).
Class AB, OTOH, means just what ash said above - more than 180° (and less than 360°) of the signal is amplified by one of the two tubes in a push-pull configuration. Since each tube is set to do this, we get a bit extra at the cross-over point. At one point in time, and you'll still see this on many specification datasheets, there was Class AB1 and AB2. These were meant to denote how much beyond the cross-over point the tubes were set to operate. This isn't so important in audio work as it is in radio, but it does explain why you might see these figures on a datasheet, and why they're slightly different.
So what we have here is the fact that the Engineers who originally sat down and wrote the spec never guessed that Nutz would appear on the scene. IOW, there's no law against running two tubes in push-pull, and setting them both to operate in Class A!!! That's right, both tubes are conducting at all times on the entire waveform. And since there's still a transformer involved, the interaction between the two is..... ummmmm........ interesting. Yeah, that's it, it's an interesting phenomenon. Some folks like it, some don't, it's as simple as that. (Well, besides the technical aspects of it, such as it's a dreadful waste of energy, it gets hotter than sin, etc. etc.)
Next:
Describing the difference in sound, such as "American", "Black-face", "British", etc. are all exercises in mental masturbation. The whole of an amp's sound depends on:
The tone stack;
The tube types (both pre- and power-amp);
The tube biasing circuitry;
The quality of the output transformer;
The speakers;
The cabinet (open vs. closed back, etc.);
The power supply;
The input signal itself!!
And that's just the major stuff - the list goes on and on, trust me on this.
To my way of thinking (and for all you youngsters out there, you can easily discount my old-fart thinking as fast as you wish), the reason some folks call a particular sound "British" is that it sounds crappy - it has no bottom end, it has a muddy mid-range, it has screechy highs, and it's distorted worse than a pair of tin cans with a waxed string. But in the 1950's and 1960's, that was all that a British lad was able to procure - no American tonality was imported for mass consumption, it had to be "brought back" by someone who toured/visited the USA. That didn't happen any too often, you can be sure of that..... sad to say.
And for those of you about to jump my bones about being a Fender-aholic, the single word is "No". If I could, I rather have a Gibson GA79-RVT, or perhaps an original Epiphone Reverberocket. Both of them could, can, did, and still do "out-tone" any Fender, each and every day of the week, way back when. But neither of them had the advertising budget or the marketing savvy to make any headway against "that California upstart". The fact that they are scarcer than hen's teeth, and yet command no more than half of what a FTRBF can fetch on eBay, says to me that someone's not letting the cat of of the bag.
Hmmm, perhaps I should be going "oooops" about now........ ;D
HTH
sumgai
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VOLT
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Post by VOLT on Sept 27, 2010 20:04:46 GMT -5
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Post by DarKnight on Sept 28, 2010 2:05:38 GMT -5
My experience with that variable class -control is that on A it makes amp somewhat "mushy" in the midrange and thicker sounding. AB on the other hand is more "tight". Somewhat what you would expect. Its effect isn't as pronounced as one would think. It's a small flavor added.  Considering ValveKing, VOLT: If I traced schematic correctly the clean channel is "fenderish" without master volume (the volume lies between two halves of preamp tube just after clean tone-stack). Lead channel on the other hand has a master volume just before effects loop and phase inverter and when lead channel is active the clean channels tone stack and volume are bypassed with lead channels gain-pot. So you get "both worlds" in theory - Marshallish and Fenderish (insert other brands here) topologies. My friend has a VK212 and I'm not really sure I like it. Never really spend good time setting it up. It is very loud amp (as is VK112) and really has nice crunch and distortion sounds. Clean channel is great too, but they both lack some bottom end I think. Don't know if a speaker/tube change would help. Anyway I can recommend it, but I think you should try it before buying. I say again: It is LOUD. I hope you don't have neighbors if you try to soak the power tubes. Dark |
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Post by Yew on Oct 1, 2010 14:06:24 GMT -5
In the UK, Class a seems to generally mean it has valves, and class b is solid state, with AB as a valve hybrid....
Pretty screwed up
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Post by sumgai on Oct 2, 2010 11:16:57 GMT -5
In the UK, Class a seems to generally mean it has valves, and class b is solid state, with AB as a valve hybrid....
Pretty screwed up I don't think that holds true for the entire UK, only those, ummmm..... individuals who persist in thinking that series wiring is the only way to obtain humbucking capability. Don't think I'm disparaging these people, I'm only wishing that they didn't get so confused by the facts after they've made up their minds.  sumgai |
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