Class A amps.

Badcat comes to mind. Rivera and Diezel make one too. I'm sure there are more obscure boutique brands out there as well.
 
Matchless and bad cat are not class A despite what their marketers might say. they are nothing more than cathode biased class AB.
 
russoloco":3k59jr97 said:
Matchless and bad cat are not class A despite what their marketers might say. they are nothing more than cathode biased class AB.
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:D Correct :) cathode bias does not make class A
 
Epiphone Valve Junior 5W Head?

Jet City PicoValve?

Channel 1A of the Hughes & Kettner TriAmp MK II

Hughes & Kettner Puretone
 
Vox AC15
Bogner XTC in Class A mode

I'm not going to get into the very essence of pure Class A amps versus the 'grey' area of Class A/AB amps, as there are certainly more than fall into the latter than the former. But if you want amps that sound good, that claim to be Class A, than the above 2 take the cake to my ears.

Avagoodone,
Mojo
 
Most Class A amps today in the boutique market follow a Vox AC30 type circuit - cathode biased and the lack of a negative feedback loop. Two power tubes put it in a push/pull mode, but not pure Class A audio and very inefficient. But, they call them Class A (for marketing no doubt) as they certainly are not designed as a Class AB amp. An example of a pure Class A amp would be an original Fender Champ.
 
Mesa Electradyne. 45W ClassA or 90W SimulClass. Awesome punch and still the bloom of the ClassA power stage.
 
VC4Ever":1srekygt said:
Please educate us because your statements provide no proof.
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Now apply this to the above mentioned amps! LOL :) :D But the other poster were on the money when they said marketing plays a big roll. But in all seriousness the mentioned amps and others would have to meet the below guidelines.

http://www.aikenamps.com/ClassA.htm



As stated from Randall Aiken (The Final Word on Class A)


First of all, what is a class A amplifier?

A class A amplifier is defined as one which is biased to a point where plate current in all the output devices flows for the entire 360 degrees of an input cycle, at the full, unclipped output of the amplifier. Or, as stated in the RCA receiving tube manuals: "The classification depends primarily upon the fraction of input cycle during which plate current is expected to flow under rated full-load conditions". The key phrase being "under rated full-load condtions", which is a requirement for amplifier classifications to be meaningful.. This is typically done by biasing the output stage halfway between cutoff and saturation, with the plate load impedance to an appropriate value that gives maximum undistorted output power. This is the least efficient method of amplification, because the output devices are dissipating maximum power with no input signal.

For audio amplification, a class A amplifier can be either single-ended or push-pull. Now, you might be thinking, how can a push-pull amplifier be class A? Doesn't one side amplify half the waveform and the other side amplify the other half? Isn't this why we use a phase splitter? These are common misconceptions. You can, indeed have a true class A amplifier that operates in push-pull mode. Amplifier class has absolutely nothing to do with output stage topology. If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions. As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount. The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding. As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier. Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance.

Another misconception is that of cathode biasing. The method of biasing has nothing to do with the class of operation. You can have a fixed-bias class A amplifier or a cathode-biased class AB amplifier, or vice-versa. The presence of a cathode bias resistor and bypass capacitor is not an indication of class A operation.

There are several advantages to push-pull class A amplification. First, the bias current for each side is flowing in opposite directions in the primary of the output transformer, so they effectively cancel each other out. This lack of static, DC offset current in the output transformer means that the core can be made smaller, because it requires no air gap to prevent core saturation from the static DC offset current. A single-ended class A amplifier output transformer is huge compared to a push-pull class A amplifier of the same power level. The air gap required to prevent core saturation drastically reduces the primary inductance, so the transformer must have a larger core and more windings to achieve the same primary inductance and the same -3dB lower frequency cutoff point. Second, a push-pull class A amplifier output stage will have inherent rejection of power supply ripple and noise. This is because the power supply signal is "common-mode", i.e., it is amplified by each side equally, but since each side is out of phase, it cancels in the output.

The main disadvantage of push-pull class A amplification over single-ended class A, is the necessity for a phase splitter stage to generate the oppositely-phased drive signals. Another "disadvantage", in terms of guitar amplification, is that even-order harmonics generated in the output stage are canceled out in a push-pull output stage (hi-fi guys consider this a great advantage, by the way!). This does not mean that the push-pull amplifier generates no even order harmonics, however, because even-order harmonics generated in the preamp stages are amplified by the output stage and will pass right through to the output. Only those even-order harmonics generated in the output stage itself are canceled.
 
Thanks I will read up. I guess I am an easily lead automaton :LOL: :LOL: and trust what is told to me. You learn something everyday..hopefully.
 
Baron already pointed to the Aiken article. It's a really good read if you want to know how amps and marketers work. To get a real class A for an el84 based push-pull amp you'd have to have a plate voltage in the 200-250s or so. I haven't done load lines on el84's in awhile but I'd hazard to guess you'd have to have an OT with a primary impedance of 10-12k along with that low B+ to get you in the ball park.
 
so, can someone just accurately answer the question for guys who like guitars and amps tone, from the "guys who like guitars and amps and tone" standpoint? holy derail!
 
Vox amps and their copies like a Crate V30 are push pull amps, but are cathode biased at 100% dissipation which is in the Class A range. Only Single Ended amps are pure Class A. The Mark V is pure Class A in 10 watt mode.
 
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