How can you tell if your amp is operating class A or class AB?

Discussion in 'Amp Tech Center' started by peteb, Jul 24, 2018.

  1. Old Tele man

    Old Tele man Friend of Leo's

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    Yes, but it's gonna feel OH SO MUCH BETTER when you stop.
     
    Last edited: Jul 25, 2018
  2. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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  3. elpico

    elpico Tele-Holic

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    To calculate plate dissipation on the back of that envelope you sure as heck are NOT going to do that, but yes as a useless demonstration of your desire to make this harder than it needs to be for the guy, you could do that. You can imagine what you like, but I spent more than a decade writing signal analysis software at an RF engineering firm. You're not telling me anything I don't know about good old mister fourier, what you are doing is helping to confuse a guy who doesn't need to hear this right now. Ever heard of walk before you run?

    I'm trying to show him that the plate has a voltage, and it has a current, and this is as simple as multiplying that voltage by that current to get the plate dissipation in watts. It really doesn't take anything more complicated than that to understand this. There aren't magical different types of watts that heat or don't, this is really simple. You always can make it more complicated in your mind, sure, but do you see what you've got him asking now? Good job recognizing how much a person is ready to take in... :rolleyes:
     
    Last edited: Jul 25, 2018
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  4. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    If a PSYOP had actually been run here, the operation would have been considered quite successful.

    Distract, distort, divide, exhaust...
     
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  5. Old Tele man

    Old Tele man Friend of Leo's

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    My simplistic analogy:

    When people argue over CONCEPTS, it's like arguing over INTEGER values = basic understanding.

    When people argue over EXACTNESS, it's like arguing over DECIMAL values = theory & physics.
     
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  6. clintj

    clintj Friend of Leo's

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    I've added him as the sole member on my "ignore" list. The fact that he actually tried to say it's basic knowledge that class A doesn't change dissipation, then condescendingly deigned to try to tell me that knowing this basic fact is essential to understanding amplifiers, and on top of that misquoted ME to say something I did not, was the last straw. He can rot under his pathetic little troll bridge until the end of days as far as I'm concerned. This forum would be immeasurably improved if both he and his threads were forever removed.

    And now it looks like he's starting to get the inkling that what we're saying about dissipation and signal might actually be true, and I'd be seriously surprised if he issued an apology for telling us we're wrong. He'll probably just invent another dead wrong "theory" to explain this new finding with more wrong "data", and launch another half dozen threads to challenge people to disprove it.
     
  7. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I motion we all now stand on a rubber mat with a scope probe in each hand, look into the mirror, be fully cognizant of potential PSYOPS, and recite the forum oath:

    "Treat everyone here with respect, no matter how difficult!"



    I do agree though that expert-verified-as-wrong posts should be deleted or somehow clearly flagged as not-reliable from a safety standpoint at minimum. Like yo here's a picture of my (completely unsafe) power switch and fuse wiring!, and then a bunch of newbie guys copy that on a mirror site after TDPRI goes EOL someday in the future, but the assumption is it is cannonical knowledge because it came from TDPRI originally...bad.
     
    Last edited: Jul 25, 2018
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  8. TeleTucson

    TeleTucson Tele-Afflicted Ad Free Member

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    Well, despite the obvious peculiarities of peteb's posts and interactions with other posters, I do feel that --- at some level --- the replies to his posts as a whole were for a very long time evading some of his most basic questions and that the height of the condemnation is a little over the top, despite all his frenetic replies and misquotes. And ... the apparent notion that what is right in the physical world is determined by how many people seem to say it. Regardless of how a lot of you here like to think of signals, it is actually not unconventional to break any waveform into AC and DC, and even to speak separately about the DC power and AC power of signals. It is done all the time. All the time. What do you read on a spectrum analyzer? Why is noise current in units of A/Sqrt(Hz) even for your "time varying DC signals"? Of course it's not a "different kind of power", and of course average power is still < i(t)*V(t)>, but you can break them out and measure them separately, and in many circumstances understand what's going on better with this breakdown.

    What we didn't state, in quite a simple enough way to peteb. was that, to a pretty high level of fidelity, the AC power in the circuit is not sunk in the tube, it is sunk in the load. And to get him thinking from the get-go about the circuit, not just the tube.

    And to others, conservation of energy is not a replacement for understanding in detail exactly why something is happening, which is what it seems that peteb fundamentally wants to know. I don't blame him for that. The detailed understanding of what's happening of course better be consistent with conservation of energy, but IMO it's a cop out to just state that conservation of energy requires an outcome without understanding in detail why it is happening.
     
    Last edited: Jul 26, 2018
  9. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Good post in isolation.

    Zooming in: The "misquotes" that you mentioned are imo THE key point of contention, outside of the science or math here. If somebody misquotes or bends quotes repeatedly, you will piss people off because ***you are claiming somebody is saying something which they did not say***.
     
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  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    TeleTucson, this is one of many peteb threads. Go back and review some of his older threads and you'll understand our actions.

    We all understand AC riding on a DC offset and all that but the way pete was dividing them up was just insane.

    Yea, I love the, "Then we agree. . ." posts."
     
    Last edited: Jul 26, 2018
  11. mherrcat

    mherrcat Tele-Holic

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    I don’t understand any of this...and I can’t believe I read every bit of it...and now I think I’ll go shoot myself...
     
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  12. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Man up and don't shoot yourself. Your obligation as a registered member is to:

    "Treat everyone here with respect, no matter how difficult!"
     
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  13. SSL9000J

    SSL9000J Tele-Meister

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    Please forgive me for beating the proverbial dead horse here, but having read all six pages (and counting) of this, I must pose a question that I didn't see addressed anywhere. Going back to the very first statements of the very first post:
    Voltage sag at the plate has to do with the AC to DC rectifier circuit in the power supply, or so I thought? (Tube rectifier = sag / solid state rectifier = no sag*) If so, then by Peteb's reasoning the class of an amplifier would be determined by the power supply. Or, at least, by the B+ rail of the power supply, and would have nothing to do with the output tube configuration. Which seems a bit confusing and counter-intuitive. To me, at least.

    *Assuming, of course, no modifications that would emulate sag, such as those described by Robrob here, were applied.

    Apologies if this just adds more fuel to the burning dumpster of entropy.
     
  14. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Burning dumpsters of entropy are fine with me (they kinda are always THERE), as long as: they aren't blasting smoke and flame that obscures everything else, or pushing participants away.
     
    Last edited: Jul 29, 2018
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  15. robrob

    robrob Poster Extraordinaire Ad Free Member

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    What peteb was saying is that Class A amps idle at max power so when you play a loud passage more current isn't pulled through the power supply so you don't get voltage sag like you do with Class AB.
     
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  16. ocduff

    ocduff TDPRI Member

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    I think this much we can agree upon: Whenever there is a design that is not single ended Class A, there is always debate about whether it is true class A or not.

    I believe this is due to the widespread belief, perhaps due to marketing or mystique, in the general guitar amp buying public, that Class A has a certain cache to it - that it is better, more special, or more unique (perhaps due to the terminology of "CLASS A") - and then there are the tech types who work equally hard to dispel those myths or disparage the designs that perpetuate the myth.

    Vox, Williamson, Matchless...no design is safe.

    Also, I believe also that in my experience, some degree of tone - particularly the sonic difference of an AC30 - is, at least by myth, purported to come from its Class A design. And Class A is this a generic term applied to that kind of sound, when it's far more complicated than that.

    And yet I do think that Class A has a sound. I build an amp regularly that is based closely on a Williamson (Standel) design, and it sounds a bit Voxy to me, with some early tweed 5E3 thrown in. And yet that design will not clip or distort like a Fender or Vox even at high volume, which is due more to the low gain preamp topology I believe.

    I've even heard it said a tweed Deluxe 5E3 is Class A when voltage is low enough. But I doubt anyone would be able to agree on that either.

    Regardless, these are all distinctive and amazing sounding amps. They also all run hot - any of these amps have chassis that get too hot to touch after an hour.

    To me, that's not an efficient design - and I don't know what is gained by a cathode bias Class A design, unless we are getting something unique out of it sonically.

    Where I am in my experience, is not being able to find anything that sounds more perfectly designed than the 5F6 or 5F8 tweeds. I believe - for better or worse - that tube amp design could've stopped right there. That is, who ever improved upon that? What else was there's left to "say" after that? Or who didn't end up just adding to what had been already "said" after that?

    But I digress...
     
  17. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Wasn't looking for it but since I stumbled across it just now in print (in a coffee shop), I thought I'd share a quote of someone quoting someone else.

    "The definition of Class A operation is that the output tube conducts signal for 360 degrees (or the full signal swing)."

    Ken Fisher, Trainwreck Pages, p24

    as cited by

    Gerald Weber, A Desktop Reference of Hip Vintage Guitar Amps, paperback edition, copyright 1994
     
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  18. ocduff

    ocduff TDPRI Member

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    That Weber book is and was an amazing resource for the time but it has several notable and well documented errors (one being that Leo Fender did not design the 5F6 Bassman, but that it was a Western Electric circuit). Kendrick amps are amazing and Weber was at the forefront of this clone world of ours - but those "mods" in his books aren't mods at all.
     
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  19. elpico

    elpico Tele-Holic

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    Are we still doing this one? Okay I'll bite. How you actually tell if your amp is operating class A or AB:

    You'd have to insert a small current sense resistor between the power tube plate and OT, turn the amp up to whatever "max clean output" metric you've chosen (5% THD etc), then use a scope to look at the voltage across the current sense resistor. (always check your scope/probe voltage ratings carefully before putting it on a power tube plate, the voltages present can be very high)

    If the voltage across the resistor falls to zero at any point (ie you see a flat spot at the low end of the waveform) then you have a class AB amp. If not, it's class A.

    Does any of this really matter? Heck no :D
     
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  20. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    My twisted take on it;

    peteb said: "Basically, If the plate voltage sags, its AB. If it doesn’t, it’s in class A. It really is that simple."
    -
    This is based on the assumption that current draw is at maximum in Class A at quiesence, and will not rise more than this throughout the duty cycle, thus demanding additional drain on the power supply.

    A paraphrase of the statement might be "Basically, If the plate voltage isn't at maximum sag, its AB. If it is, it’s in class A. This is a simplistic stultification."
     
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