Class A amps

Discussion in 'Amps and Cabs' started by Lerxst2112, Nov 1, 2005.

  1. Lerxst2112

    Lerxst2112 Member

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    (To avoid the inevitable, incoming argument, I'm not saying Class A are better, I'm not saying I wouldn't buy an amp that isn't class A, or anything like that... so check the "CLASS A DOESN'T MEAN IT'S BETTER! ALL THAT MATTERS IS IF IT SOUNDS GOOD!" thing at the door.)

    I know we had a thread long ago about Class A and what TRULY was Class A. Just out of sheer curiosity, what amps claim to be Class A, and what amps are TRULY Class A?

    For instance, are Orange's smaller amps really Class A? Mesa's amps (I know what Randall Smith says, but is he right)? Bad Cats? Matchless?

    I think a lot of the bias towards class A and against PCB comes from the Matchless website:
     
  2. kannibul

    kannibul Member

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    The only true Class A Orange amp that I know of isn't in production any more.

    I think it was the AD5 - basically a EL84 SE amp.

    and, from what I know, Class A is the LEAST efficient operation class - lol.

    I guess anything for marketing...
     
  3. drbob1

    drbob1 Silver Supporting Member

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    I can't speak to all of them, but true class A implies that all power tubes conduct the full cycle of the input voltage-so they don't reach 0 voltage output ever. That means that a single ended amp (or an amp that uses a pair of tubes in single ended design like the Scumbag) is true class A. Almost everything else that's listed as class A is AB/Cathode bias (including Matchless, all Mesas that I'm aware of, tweed Fenders and their copies). The exception is amps that seem to be running very low power for their output tubes (for example, Jackson Ampworks apparently is running his amps in true class A, push-pull and getting around 19w from a pair of KT66s). Does that help?
     
  4. sus4

    sus4 Member

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    Isn't the Champ true Class A?
     
  5. HeeHaw

    HeeHaw Member

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    This is probably not related, but I Recently played an amp called Bitar that was all class a design. It totally rocked and wasn't overpowering loud. I remeber it was built by a nice gentleman named George, but that's it. I shouldn't have drank so many barley snacks.:eek:
     
  6. VacuumVoodoo

    VacuumVoodoo Member

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    Anything single-ended in audio is class A by definition. There are other single-ended classes of operation i.e. B and C but we don't have to bother with these.

    You can have class A in PP with fixed or cathode bias or even a combination of both, it's a question of proper design, choice of operating point for the tubes and transformer.

    Class AB means by definition that the amp operates in class A up to a certain power level and in class B above that. Most often PP guitar amps are class A up to about 40-60% of max output power, high power amps like bass amps rated in hundreds of watts will operate in class A up to about only 10-15% of output power because running class A at higher power comes with a penalty of increased idle power consumption.
    Class A is the least efficient of classes from the point of view of power consumed to power delivered ratio.

    What Matchless say about efficiency of class A is pure gibberish and i'm wondering if they know what they are talking about - unless this was written by MBS*-guys, but then, why did they let this pass?

    Champ being single-ended is class A by definition.

    *MBS = Marketing Bull...t
     
  7. dgassie

    dgassie Member

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    THD Univalve. I really can't argue with the sound or build quality. This is a quote fro their page...

    "About the THD UniValve® Amplifier!
    The THD UniValve is a Single-Ended Class A amplifier head with a single output tube that can be switched at will among many octal-based power tubes, including 6L6, EL34, 6550, KT90, KT88, KT77 and KT66, for different tones without re-biasing the amp. Likewise, the two preamp tubes can be any combination of 12AX7, 12AT7, 12AU7, 12AY7 or 12AZ7. The UniValve delivers tones from smooth and clear to very aggressive overdrive. It is easily capable of driving a 4 x 12" cabinet, yet quite small and light. It has a built-in Hot Plate® Power Attenuator that allows for full output distortion at almost any volume. And it doesn’t cost as much as you might think.

    What is a Class A?
    Class A is a term given to an amp that runs its tubes at full current all the time, unlike most tube amps that alternate between running one set of tubes and the other set, each for one half of the wave. The set not in use is turned off by a positive swing of the grid voltage. Single-ended out-put stages always operate in Class A. Most push-pull amplifiers, including the venerated Vox AC-30 operate in Class AB when overdriven, even if they are in Class A while clean. The upshot is that Class A operation has its own unique tone characteristics that set it apart from other tube amp classes. Class A amps sound great at low volumes, and even better as you turn them up. Thus, with the relatively low wattage of the UniValve you can turn up the amplifier to take full advantage of its stunning output distortion tone without deafening anyone. "
     
  8. MVrider

    MVrider Member

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    All you need to know about the term can be found right here.

    the last word on Class A

    You can also find some hilarious mumbo-jumbo about Class A on the net, some of it from supposedly reputable sources.

    One site claims Class A is inefficient but insists their "Class A" amps gets 30 watts out of 4 EL-84's. Wow. Another site claims their design gets "maximum efficiency" from their 4 EL-84 amp which makes... 30 watts.
     
  9. JacksonAmpworks

    JacksonAmpworks Member

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    You're right. Pure Class A and High Power do not go together (at least not in guitar amplifiers). Class A is a combination of Plate Voltage, Primary Impedance of the Output Transformer and the Idle Current. All of these things affect whether an amp is in Class A. Check out Randall's site for a wonderful explanation. Actually just check out Randall's site period. It is required reading for anyone interested in tube amp theory.

    :D
     
  10. Lerxst2112

    Lerxst2112 Member

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    Randall has been my hero since I got my Invader last December. Awesome guy, awesome amp... It's a honor and a privelege to be one of his patrons.

    And I'm not sure I'll take any company's word for it on whether their amps are class A or not. I mean, look at Matchless? Look at Bad Cat. The very top of the Bad Cat page right under their logo is their slogan, ".:Class A amplifiers:." Look at Mesa's website. Look at Orange's website.
     
  11. JacksonAmpworks

    JacksonAmpworks Member

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    If Randall says it's Class A - trust him. The man knows more about tube amps that anyone I can think of, and I can think of a lot of guys.
     
  12. Lerxst2112

    Lerxst2112 Member

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    I don't know anything about you, and I apologize for asking this so bluntly... but are your Class A amps truly Class A?
     
  13. 1964

    1964 Member

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    > "Isn't the Champ true Class A?"

    Yes.
     
  14. G'OlPeachPhan

    G'OlPeachPhan Member

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    On the issue of Matchless and circuit boards, it appears to me that they're not only speaking out against the use of PCBs, but ANY circuit boards in favor of true point-to-point wiring... I will agree with them that this method of construction is more difficult, but I can't agree that it's necessarily better, or more "rugged."

    Class A has become a grossly overused (incorrectly more often than not) marketing buzz word.

    Yes, the single-ended Champ is Class A. All single-ended designs are Class A.
     
  15. kannibul

    kannibul Member

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    :(

    They, uh, mean...

    The amps have a great Class A (like) Tone - yeah - that's it!

    It's a typo!
     
  16. Guinness Lad

    Guinness Lad Silver Supporting Member

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    Class "A" is a single ended design and all single tube amps I know of are class"A" This being said the crap that Matchless or Badcat is spewing is pure hyperbole. The only reason they say this is because that's how they make their amps.

    Just because it's class "A" doesn't mean it will be better then a similar "AB" design. Also PTP is not better then circuit board construction, it just costs more to make because of the labor.

    Most amps claimed to be class "A" are not, it's just marketing. The same can be said for PTP construction, guys get all hard when they hear clas "A" and PTP so that's why manufacturers spew it.

    Read up on Joule's law, it's all there.
    Power= current*voltage.

    It's hard for me to believe anyone who say's they have a class "A" amp that runs on 450+ volts. Just ask the guy what the resistance and voltage values are for the circuit and then do the math. Find out what the dissapation is for the tubes and you will know if it's B.S. or not.

    I don't claim to know everything, far from it, but take the whole class "A" thing with a grain of salt.
     
  17. Scumback Speakers

    Scumback Speakers Gold Supporting Member

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    I can verify that the Scumbag Amp line is class A, single ended, whether you talk about the dual or quad 6V6 amp we make (Capt. Crunch & Major Crunch, respectively).

    The single ended design isn't as efficient for the power tubes used, but strangely enough, the volume of our dual 6V6 amp winds up competing nicely with a dual EL84 Class AB (push pull or cathode biased) style amp along the lines of an 18 Marshall watt clone. I'm not the most knowledgeable on how "Class A single ended" watts compare to Class AB watts, though. I just know the 9 watt amp we make has a volume that is comparable the 18 watt when dimed, all speakers being equal in the comparison.

    My circuit designer tells me that once you actually push the amp up into the OD range that he feels the class changes from Class A to a "hybrid Class A" circuit, due to the power tubes overdriving.

    All I know is that it sounds very rich, very detailed, and has excellent string to string definition even when overdriven to a high saturation level.
     
  18. JacksonAmpworks

    JacksonAmpworks Member

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    It is not an offensive question. Of course they are. They are Fixed Bias Class A. There is software that you can buy that I use when designing an output section. With this software, I can plug in various values such as, Plate Voltage, Primary Impedance, Load, Idle Current, OT efficiency etc. With all of those values entered, I then run the simulation and view the results. I tweak the entered values to get to the exact operating point I want. With this software, it is easy to confirm whether the output section is indeed in Class A.

    If you look at Randall's site, he has a screen shot of a similar program. These type's of programs really make designing an ouput section much easier.

    Hope that helps!
     
  19. Lerxst2112

    Lerxst2112 Member

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    Cool! Ain't technology great?
     
  20. heybulldog

    heybulldog Member

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    i started a thread a while back "class A who is correct and how do we know?

    my purpose was to examine if the vox, matchless, bad cats are not class A argument was ever documented in a third party print editorial (magazine -book NOT ON- LINE WEB article) and no one could come up with one. I pointed out that aspen pittman (groove tubes) called the AC30 circuit class A in his tube amp book. I wonder still, if it is all marketing bs why i never saw this in guitar player, vintage guitar, etc or how aspen could be wrong?
    just asking .....again
     

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