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What is truly class A?

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

  1. Ben Bishop

    Ben Bishop Tele-Meister

    292
    Mar 4, 2010
    Victoria, BC
    I see amps listed as cathode bias and amps listed as class A and some are advertised as both. Is Every class A amp cathode bias? Is every cathode bias amp a class A amp?
     
  2. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    Could be, but only the actual BIAS voltage will tell you for sure.

    For instance, there actually is Class-A push-pull where both tubes are conducting ALL the time...the hallmark of "true" Class-A operation.
     
  3. David Barnett

    David Barnett Poster Extraordinaire

    And the plate current at idle is the same as at full rated power.

    This condition can be achieved using either cathode bias or fixed bias, single ended or push pull.

    The chances that any production push-pull guitar amplifier has ever met true Class-A spec are slim to none.
     
  4. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    True for guitar amps where POWER is paramount, but definitely true for HiFi where 5W is considered "enough" (by some people) who use earphones.
     
  5. David Barnett

    David Barnett Poster Extraordinaire

    That's why I specified guitar amps. :) There are true Class-A hifi amps. There are also plenty of fake Class-A hifi amps, where they call it a "75W Class A" amp but it has the voltage rails of a 150W amp. "It sounds so much more powerful than its rating, must be because it's Class A."

    One hifi company made a 50W Class A amplifier and a 150W Class AB amp, they were actually the exact same amplifier except the Class A one used different taps on the power transformer secondary (+/- 34V for Class A, +/- 62V for AB), and a different setting on the bias trimpot. I don't recall if there was a price difference.
     
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  6. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    Sounds about right...what tubes did they use? The "rule-of-thumb" is about 3x (≈PI) power from same tubes in Class-AB1 due to its pi/4 relationship (0.785398163).
     
  7. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    The majority of Class A guitar amps I'm familiar with are cathode biased single ended. Champ, AC4, Marshall Class 5. There are a couple of odd designs out there that do run class A push-pull, but they seem more common as blues harp amps optimized for low power and harmonically rich distortion. Those will run in Class A self split, joined by an unbypassed cathode resistor.

    Manufacturers love to advertise cathode biased push-pull EL84 amps as Class A, as though that somehow makes them better. The reality is that they are usually Class AB at full power, with each tube entering cutoff at some point of the duty cycle. There's also no way they're getting 18+ watts from that power amp configuration in Class A.
     
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  8. David Barnett

    David Barnett Poster Extraordinaire

    Well, that example didn't use tubes...
     
  9. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    Although the device parameters themselves are different, the mathematic equations are same for SS and vacuum tubes.
     
  10. David Barnett

    David Barnett Poster Extraordinaire

    I meant my example, not yours. In hifi, class A p-p seems to be predominantly solid-state.
     
  11. Ben Bishop

    Ben Bishop Tele-Meister

    292
    Mar 4, 2010
    Victoria, BC
    As I understand it, a true class B amp will have a sharp cutoff creating problems of distortion at hand-over point. Class A has one, two or more tubes operating full-power, full-cycle. And that's the biggest reason that class A's run hot (and use up tubes faster). Single-ended amps, like the Vox AC4-TV are necessarily Class A but there are amps using more than one tube that are truly class A. I'm beginning to suspect that without reading the schematic I couldn't be sure. (And class AB has a soft hand-off, some overlap, as I understand it.)

    I was looking at the Traynor Guitarmate 15 which uses two EL-84's and gets 15 watts, which sounds about right. Thanks for your comments. I'll keep on reading.
     
    Last edited: Oct 6, 2018
  12. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    Class-AB1 "cross-over" is also sometimes referred to as "projected cut-off," the linear projection down from the linear portion of the conduction curve...goal is to mitigate "cross-over" distortion that push-pull has (specifically Class-B).
     
    Last edited: Oct 6, 2018
  13. fmmlp

    fmmlp TDPRI Member

    Age:
    45
    70
    Jul 19, 2017
    Buenos Aires
    Cathode bias is a form of polarization, which is a way of setting the operating point of the tube. It's possible to use cathode bias to set the operating point over a wide area by this method.
    Amplifier class is a classification of amplifiers by conduction angle of the active device ( A, AB, C, D, etc.) and a subindex indicating if current flows in the grid of the tube for not, some or all of the conduction angle, so AB1 has no grid current but AB2 has grid current flowing some of the time, because the driving signal makes the grid voltage to go positive at some level.
     
  14. 3-Chord-Genius

    3-Chord-Genius Friend of Leo's

    Apr 3, 2015
    Winchester, VA
    I think the boxy-sounding low-wattage single-ended amps, which have become popular in recent years, are the only amps truly capable of Class A operation when distortion is reached.
     
  15. deadbeat son

    deadbeat son Tele-Afflicted

    Apr 5, 2010
    Evergreen, CO
    I don’t. A couple that come to mind quickly are the Laney Lionheart L20 and the Gjika amps.
     
  16. Mike Eskimo

    Mike Eskimo Doctor of Teleocity Ad Free Member

    Nov 9, 2008
    Detroit
    I’ve owned 3 Trace Elliot Velocettes - 2 of them with 10’s and one with a 12.

    Supposed to be class A.

    Hell if I know.

    Great sounding amps though.

    No sag/very immediate, with a ‘66 335 into it it sounded very, very good...:)
     
  17. brogh

    brogh Super Moderator Staff Member

    Jun 26, 2010
    italy
    Cool post !

    Mesa Boogie lonestar special is a pure class A, so it says in the manual, in all three wattage modes (5 15 30 )
    I was looking for some schematics but didn't find any would be cool to understand how that works :)

    cheers
     
  18. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    They say that like six times on their website, too. It's bovine excrement. A single EL84 in Class A will make about 5W. Two in Class A push-pull will make about 8 to 10W max, nowhere near the 15W they claim. You need 4 in Class AB to make 30W. Of course, it should properly read Pure Class A with a US Patent No. to be truly a Mesa product.
    Edit: what's really head scratching is they reference Randall Aiken's white paper on Class A operation to explain how A is better. You know, the same paper that unequivocally debunks the Vox AC30 as being class A and getting 30W from 4 x EL84.
     
    Last edited: Oct 8, 2018
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  19. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    696
    Apr 8, 2016
    Oakland, CA
    Posting this to enrich the discussion, not as an endorsement of any particular view. I have read the question posed by this thread before, and I have read this essay by Randall Smith before, and I still don't quite understand!

    On rereading it just now, I think the ongoing confusion and failure of everyone to agree (considering this is ancient technology about which there are no mysteries), is that different folks are using subtly different definitions for some of the key concepts. Fallacy of equivocation.

    Smith is making all his claims about bias with respect to current through the power tubes, operating point w/ respect to plate current, and mapping different input voltages at the control grid with respect to plate current.

    In my very limited experience and understanding of this topic, I know I am thinking about plate current some of the time, but inevitably switch to thinking about power (V x I). Like plate dissipation.

    Smith talks about biasing pairs of EL84s to exactly 50% of their total possible plate current. He implies that this is very much hotter bias than a typical Fender/Marshall class AB amp which might be biased to 60-70% maximum plate dissipation. But he describes the Fender and Marshall as biased between 10-30% total current. Well what kind of supply voltages are we talking about in a real Lonestar Special or any of his hypothetical examples? Why be cagey about it, why not just say?
     

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    Last edited: Oct 8, 2018
  20. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    The guy sure likes his patents, doesn't he? I lost count of how many times he said something was patented or patent pending in that yarn, but at least there's a helpful list in the back.

    And regarding Class A single ended, that couldn't be farther from the truth on how that mode operates regarding idle dissipation and audio power. I encourage anyone interested to look up Merlin's articles on it, or even the RCA tube handbooks if you enjoy vintage writing.
     
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