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What is truly class A?

Discussion in 'Glowing Bottle Tube Amp Forum' started by Ben Bishop, Oct 5, 2018.

  1. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    That just bothers me the more I think about it. Let's take an EL84, since that seems to be the favorite "Class A" tube.

    We need some assumptions to make this walkthrough easier. Let's say the bias voltage is -11V for 90% idle dissipation, and -15V will drive the tube into cutoff. This means that a grid signal of +/-4V will cause the tube to reach cutoff and make your push-pull amp cross into Class AB.

    Now, making the bias voltage more negative will cool the tube bias, so let's say Mesa achieves 50% idle by setting the bias supply to -13V. Cutoff will still be reached at -15V, so now it only takes a signal of +/-2V to reach that point. If anything, that makes this amp even closer to Class AB, not Class A as it's even more likely that it can reach this signal level.
     

  2. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Heck with Class A. Class A2 all the way baby!
     
    clintj likes this.

  3. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Just for arguements sake, or to consider the other side.


    If there are four EL84 tubes, it’s only 7.5 W per tube. The 5BQ6 is rated at 5.7 W per tube class A, at a low plate voltage, so 30 W class A out of four tubes is not that far off.


    The AC 30 schematic shows the AC 30 is not class A, it shows the cathode voltage, the bias voltage changing at full out put.


    But the AC 30 schematic shows a low cathode resistor, a low bias voltage (10 - 12.5 V) and a high cathode current, like 50-60 mA. That’s close to class A.






    I think the reason they say the AC30 is class A is because at idle it appears to be biased at about 125-130% of max plate dissipation. it may not be class A, or it isn't, but that's the reason for the claim.
     
    Last edited: Oct 10, 2018

  4. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    All tube P-P amps are pretty much biased so that half power or more is Class A so if you do not overdrive the output section you are most likely in Class A. Not that I have a bias for Class A (sorry, had to say that), the amps don'y have an appreciable distortion increase in the change so who cares?
     
    Snfoilhat likes this.

  5. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I don't care if they call it a Purple People Eater (I might name my next amp that. Hmm....) What people, myself included, do take offense to is that makers are using a term with a well defined meaning and misusing it as a marketing tool, therefore leading consumers to believe they are getting a somehow superior product and paying a premium for it. It'd be equivalent to saying a HR Deluxe has tube reverb because the preamp section is tube driven, but the actual reverb driver circuit is op amp powered. Mesa, for example, advertises the Lonestar Special as Pure Class A numerous times on their website and waxes rhapsodic about the glorious second harmonic distortion it produces as a result.
     
    JD0x0 likes this.

  6. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    There is not much difference between class A and almost class A.

    As Printer2 points out there is not much difference between the sound of class A and class AB.


    And, when a class AB amp transitions between the two, there is not much difference to listen to.





    If I had an amp in front of me I could determine if it was class A or class AB. making that determination based on descriptions, not even a schematic, like in the case of the lone star special is more difficult to see for me.
     

  7. David Barnett

    David Barnett Poster Extraordinaire

    That's a single-ended amp allowed to swing into grid current on peaks? You see that a lot on the big triode hifi amps, using 211 or 845 outputs.
     

  8. Art VanDelay

    Art VanDelay Tele-Holic

    525
    May 20, 2018
    Arcata
    Here's a great resource I've found on the interwebs. I remember getting a free printed version of this years ago in a local Portland music store but a roomie decided to mail this to his "audiophile" friend. Anyhow, hope this helps!
     

    Attached Files:


  9. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    28
    Feb 22, 2009
    New York
    I remember a few years ago Laney had some silly advertising description for the L50H, which is a 50 watt Class A amp, actually class A.. It's 5 EL34's in parallel...

    Now onto the description.. For some reason they were corelating the 5 output tubes with enhanced '5th order harmonics' and then went on to say how they're the most desirable harmonic to have... Pretty laughable, I'm glad they changed it, because that horsepucky made me not want to get one when I was researching them...
     

  10. cpk313

    cpk313 TDPRI Member

    36
    Apr 2, 2018
    Allentown PA
    Yes the long defunct Forte 4a and the 6, Threshold's junior division. The 4a - 50w amp was a really nice sounding, the 150w did not get the same amount of love
     

  11. David Barnett

    David Barnett Poster Extraordinaire

    Well, I was referring to the Threshold SA/3 and S/300... :)
     

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