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#81
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Output classes A and AB
"Eeyore" wrote in message ... John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Actually, there is cancellation regardless of whether one side has ceased conducting or not, because the cancellation comes from the fact that the transfer functions of the two sides are identical but opposite. During the portion of the wave where both sides conduct, there may be cancellation of odd order distortion. In school I was taught the law of half-wave symmetry. Any wave, no matter how distorted, that has matching halves has no even-order distortion. A wave that is composed of one half of any kind and the other half is zero, has only even-order distortion. |
#82
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Output classes A and AB
"Don Pearce" wrote in message ... On Thu, 25 Oct 2007 15:40:55 +0100, Eeyore wrote: John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Graham Because if you add an even harmonic to a signal, you have to make it asymmetrical. You always get a peak coinciding with a trough on one half cycle, followed by a peak coinciding with a peak on the next. If you modify the signal to remove any asymmetry, you must by definition remove the even harmonics. Agreed. |
#84
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
(Don Pearce) wrote: On Thu, 25 Oct 2007 15:40:55 +0100, Eeyore wrote: John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Graham Because if you add an even harmonic to a signal, you have to make it asymmetrical. You always get a peak coinciding with a trough on one half cycle, followed by a peak coinciding with a peak on the next. If you modify the signal to remove any asymmetry, you must by definition remove the even harmonics. Finally a man who understands the theory! Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#85
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
"Arny Krueger" wrote: "Eeyore" wrote in message ... John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Actually, there is cancellation regardless of whether one side has ceased conducting or not, because the cancellation comes from the fact that the transfer functions of the two sides are identical but opposite. During the portion of the wave where both sides conduct, there may be cancellation of odd order distortion. Are you sure about this? I guess I will have to do some homework. In school I was taught the law of half-wave symmetry. Any wave, no matter how distorted, that has matching halves has no even-order distortion. Aren't there two symmetry laws, I can never remember the second one, or is it the first? A wave that is composed of one half of any kind and the other half is zero, has only even-order distortion. Could you say that again please? Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#86
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
"Arny Krueger" wrote: "Patrick Turner" wrote in message ... In class AB, each man pulls the saw about 1/2 way across the stroke then lets go, and the other guy grabs his saw handle and pulls the saw back the other way. Each man only mainly pulls the saw in turn, and applied force is jerky, and frankly, a difficult way to work; the Union will be down soon to have a go at the boss who told the men to saw the log that way. So speaks someone who obviously has no experience cutting wood with a cross-cut saw. You can pull on a saw, but pushing on it can easily cause it to bend and bind. When 2 men use a cross-cut saw, each man pulls far more than he pushes. Class AB verging on pure class B is the preferred mode of operation for a cross-cut saw. I was going to call him on that, maybe they don't have many trees in OZ. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#87
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
(Don Pearce) wrote: On Thu, 25 Oct 2007 13:43:56 GMT, John Byrns wrote: In article , (Don Pearce) wrote: Can you overdrive a class A amp to cutoff? Yes, it is painfully easy. In my experience what happens when you overdrive a class A amp is that one device saturates, and the other sticks with its normal bias condition. No, the coupling capacitor charges up as a result of grid current shifting the bias point so that cutoff becomes even easier. There is no circumstance in which I have ever managed to put a class A amplifier output device into cutoff. You haven't tried very hard then. The designs I have used must have been better matched in drive level between the driver and output stages. They tended to limit almost simultaneously so that the drive level to the output stage did not go on rising as the input signal increased, just squared off. As I say, I never saw an output valve go into cutoff. That sounds like a dangerous way to go, I would think that running the drivers that close to "limiting" would create a lot of distortion in the driver stage before the output stage starts clipping. I think Patrick just posted, expressing his dislike for this sort of wimpy driver. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#88
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
On Oct 25, 7:40 am, Eeyore
wrote: John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Graham Holy ****! Did I say yet that Poopie is ignorant and incompetent? Nah, nobody can be that stupid and uninformed about tube basics. Poopie must be cracking a joke. For the first time in his life. Good on yer, cobber! If you can't be smart and informed, at least you can try to be a clown, give people a giggle. Andre Jute Entertainer |
#89
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
Coupla superbly detailed posts here, Patrick, that I've copied to save
the calculations if I ever sink so low as to build a Class AB amp... Andre Jute Class A1 rules the waves! |
#90
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article .com,
Andre Jute wrote: On Oct 25, 7:40 am, Eeyore wrote: John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Graham Holy ****! Did I say yet that Poopie is ignorant and incompetent? Nah, nobody can be that stupid and uninformed about tube basics. There are a lot of "stupid and uninformed" people around, there are at least three people involved in this discussion that have expressed this same belief as Eeyore, they are Multi-grid, Patrick Turner, and Eeyore. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#91
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
On Oct 25, 4:16 am, "Arny Krueger" wrote:
"Don Pearce" wrote in message Andre Jute wrote: Now this poor dumb slow learner Poopie Stevenson, an embarrassment to everyone who comes into contact with him, claims to know more than the RDH4! Yo, Poopie, in the RDH4 (Newnes 1997) on p 545 we find this nugget of authoritative information by F. Langford-Smith B.Sc. B.E. himself: "Class AB operation indicates overbiased conditions, and is used only in push-pull to balance out the even harmonics." No such thing in the RDH4 at hand. Do you just lie from habit, Krueger, or are you incapable of using the contents list or the index of a reference book? The reference is from RDH4, Chapter 13, Section 1 (ii) Classes of Operation. Unsigned out of contempt |
#92
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
"John Byrns" wrote in message ... In article , "Arny Krueger" wrote: Actually, there is cancellation regardless of whether one side has ceased conducting or not, because the cancellation comes from the fact that the transfer functions of the two sides are identical but opposite. During the portion of the wave where both sides conduct, there may be cancellation of odd order distortion. Are you sure about this? yes. This looks about right: http://dave.uta.edu/dillon/ee5301/lecture11.htm In school I was taught the law of half-wave symmetry. Any wave, no matter how distorted, that has matching halves has no even-order distortion. Aren't there two symmetry laws, I can never remember the second one, or is it the first? Thanks for the memory jog. Please see the reference. There are lots of symmertry laws! A wave that is composed of one half of any kind and the other half is zero, has only even-order distortion. Could you say that again please? A wave that is composed of one non-zero half wave of any kind, and the other half is nothing (zero), has only even-order distortion. |
#93
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
Poopie Stevenson aka Eeyore
wrote: Andre Jute wrote: Eeyore wrote: Andre Jute wrote: All of that follows logically from Poopie's absurd redefinition of Class A as a Class in which "the output device(s)never cease conducting *under any signal condition*," (emphasis added). It's ludicrous. It's actually the only accurate definition. I've already demonstrated several times that your words "under any signal condition" make your definition grossly inaccurate. But you're an ignorant **** and what you say is a load of ********. Even when I'm right? Tsch, tsch, Poopie. That's not even an original thought. Ron Bales had it first. "In a Class A circuit, the amplifying element is biased so the device is always conducting to some extent"http://en.wikipedia.org/wiki/Electronic_amplifier#Class_A You're blowing smoke through your fat arse, Poopie. That reference doesn't say anything about "under any signal condition." Even Wikipedia isn't as misinformed as you are. That must be a new record for you. You're confusing cause and effect but your brain is too addled to understand the difference. It still wasn't me who said "under any signal condition". It is still you who sstupidly said "under any signal condition", thereby voiding the rest of the definition of Class A operation. Graham You're a clown, Poopie. Wiki in, Poopie out. Same as GIGO. Unsigned out of contempt |
#94
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
On Oct 25, 2:12 am, Eeyore
wrote: Andre Jute wrote: Eeyore wrote: You really should constrain yourself to talkind about stuff you understand. Which would seem not to be very much going by your posting history. Me? Come on, Poopie, I'm not the one who claimed for several days that a Class A stage is one in which "the output device(s)never cease conducting *under any signal condition*." You're the one who committed that stupidity, and so many others. And * so many others* too eh ? You do rather make a habit of being wrong, Poopie. Even your mother noticed that. Ever consided we might actually be right ? Of course I have. I am a connoisseur of unlikely and bizarre events. If you are ever unequivocally right in an argument with me, I get to win a couple of thousand Euro that I've bet on the statistically probability that nobody can be wrong *all* the time. Why, even a broken clock shows the right time twice every day! You're a ****ING CRETIN Joot. Go back to the miserable hole you crawled out of. No point in abusing me, Fatso; it won't make me any less right or you any less wrong. Graham AKA Poopie for a good reason. Why don't you share it with us. The reason for your name is surely the one thing you do know. Hey, Poopie, aren't you the same Graham Stevenson clown who announced three or four years ago that he came to RAT "to be on Jute's arse". You will let me know when you start, won't you. Andre Jute Rodeo rider |
#95
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
On Oct 25, 1:17 pm, John Byrns wrote:
In article .com, Andre Jute wrote: On Oct 25, 7:40 am, Eeyore wrote: John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Graham Holy ****! Did I say yet that Poopie is ignorant and incompetent? Nah, nobody can be that stupid and uninformed about tube basics. There are a lot of "stupid and uninformed" people around, there are at least three people involved in this discussion that have expressed this same belief as Eeyore, they are Multi-grid, Patrick Turner, and Eeyore. From Eeyore (Poopie Stevenson) I expect only the worst; he says whatever comes into this head as the opposite of what is said by someone he dislikes, without any reference to the facts in electronics; I have made a separate thread to illustrate that Poopie is joined in this perversity by Pearce and Krueger. I've given up reading Cuddles Multi-grid's posts; it is too wearing to think up new ways of explaining the same simple thing to him over and over again. Patrick is entitled to a mistake now and again; he makes so few. I saw the above and knew you would call him on it, so I read on before I wasted my own time telling Patrick he'd better explain himself -- and there you were, pointing out the faux pas. Actually, I regret that Patrick is both honest and not as slippery with words as some we have seen here, or we could have had a dingdong as he tried to explain it away without admitting he was wrong. I expect he will just say he slipped up. I actually find Patrick's total ignorance of woodsawing the most amazing thing in this thread -- especially considering that we have a fellow with real expertise (and ruler braces) on board RAT: Iain Churches. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ Andre Jute Visit Jute on Amps at http://members.lycos.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review |
#96
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Don's limp driver Output classes A and AB
On Oct 25, 10:29 am, (Don Pearce) wrote:
On Thu, 25 Oct 2007 13:43:56 GMT, John Byrns wrote: In article , (Don Pearce) wrote: Can you overdrive a class A amp to cutoff? Yes, it is painfully easy. In my experience what happens when you overdrive a class A amp is that one device saturates, and the other sticks with its normal bias condition. No, the coupling capacitor charges up as a result of grid current shifting the bias point so that cutoff becomes even easier. There is no circumstance in which I have ever managed to put a class A amplifier output device into cutoff. You haven't tried very hard then. The designs I have used must have been better matched in drive level between the driver and output stages. They tended to limit almost simultaneously so that the drive level to the output stage did not go on rising as the input signal increased, just squared off. As I say, I never saw an output valve go into cutoff. d -- Pearce Consultinghttp://www.pearce.uk.com A driver stage that limp must be fertile ground for Miller, so how's your bandwidth, Don? Andre Jute Lateral thinker and skunk-trapper |
#97
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
"Arny Krueger" wrote: "John Byrns" wrote in message ... In article , "Arny Krueger" wrote: Actually, there is cancellation regardless of whether one side has ceased conducting or not, because the cancellation comes from the fact that the transfer functions of the two sides are identical but opposite. During the portion of the wave where both sides conduct, there may be cancellation of odd order distortion. Are you sure about this? yes. This looks about right: http://dave.uta.edu/dillon/ee5301/lecture11.htm Which type of symmetry discussed in that lecture is suggestive of canceling odd order distortion? Some of the cases presented appear to have a period of T/2 rather than T which confuses the issue. Also I am assuming that the output devices in our idealized amplifier have a single valued input to output function, which somewhat constrains the types of symmetry the output signal can have. In school I was taught the law of half-wave symmetry. Any wave, no matter how distorted, that has matching halves has no even-order distortion. Aren't there two symmetry laws, I can never remember the second one, or is it the first? Thanks for the memory jog. Please see the reference. There are lots of symmertry laws! A wave that is composed of one half of any kind and the other half is zero, has only even-order distortion. Could you say that again please? A wave that is composed of one non-zero half wave of any kind, and the other half is nothing (zero), has only even-order distortion. You took me too literally, I wondered if you would and left the question so it would allow simply saying it again. If you actually mean what you seem to be saying, you are wrong, as a trivial example will easily demonstrate. Or are you talking about the wave from each of the two devices, which is why I asked for clarification? Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#98
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
Eeyore wrote: Patrick Turner wrote: Eeyore wrote: Andre Jute wrote: Poopie Stevenson aka Eeyore wrote: John Byrns wrote: Andre Jute wrote: Historically, the original purpose of Class AB was to annihilate the second harmonic which before made up such a very large part of the THD, while still allowing beam tubes and pentodes to give much larger power than available before. Andre, why was Class AB necessary to annihilate the second harmonic, didn't Push Pull operation already annihilate the second harmonic irrespective of the class of operation? Yes, you are right. It does. The SOLE purpose of AB is to produce larger power outputs using the same tubes. Now this poor dumb slow learner Poopie Stevenson, an embarrassment to everyone who comes into contact with him, claims to know more than the RDH4! Yo, Poopie, in the RDH4 (Newnes 1997) on p 545 we find this nugget of authoritative information by F. Langford-Smith B.Sc. B.E. himself: "Class AB operation indicates overbiased conditions, and is used only in push-pull to balance out the even harmonics." It's the push-pull that cancels those harmonics YOU UTTER CRETIN ! A Class **A** push-pull output stage will do that too. AB operation has NOTHING to do with distortion cancellation. But it does. ONLY because AB working is by design push-pull. The same thing happens in long-tailed pairs. the distortion cancellation is NOTHING whatever to do with AB operation. Well, sure, LTP low 2H output relies on each device having its own 2H cancelling with the other one. They cannot work in class AB as neither of them can cut off, because of the large common cathode resistance. But in a power output stage, there IS in effect a large LTP working until one tube does cut off, ie, have Ia less than 1/10 the idle value, and then the cancelation is zero, but the voltage output is the sum of two devices acting non linearly upon each +ve and -ve 1/2 waves. In fact one can set up an output pair of tubes with a commonened cathode constant current sink, and have the output stage ONLY able to work in class A, and if one tube cuts off there is no output in the anode circuit connected to the OPT with CT. Such a class A output stage has been touted as being the purest form of class A PP by Allen Wright. It can be driven by having SE drive to one grid of the PP pair with the other grid gounded. I have used the principle for driver stages. Patrick Turner. d for the CCS as the common cathode e Graham |
#99
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
John Byrns wrote: In article , Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? In effect the even order distortions DO re-appear when the amp moves to class AB. Its because of the mismatch between the two tubes, and each has slightly or greatly different gm, so each +ve and -ve half of the waves are amplified by a different amount, and hence you get even order generated in the output from across the whole primary or secondary of the OPT. The odd order is also there, and usually dominates the distortion profile, so that in a typical 45 watt amp using a pair of EL34 in UL and class AB, there might be very low 2H in the first 5 watts, but just below clip its 1%, and with maybe 4% 3H. If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. It sounds like you have become one of Multi-grid's sock-puppets. Not enough ppl have taken their own measurements of typical amplifiers to know as much as what is written in RDH4 by demonstrating it all to themselves. PP amps have much lower 2H at all levels than the same tubes used in SE parallel. But 2H IS produced due to tube mismatches. But I have used tubes which are well matched and 2H is sometimes very low right up to clipping, and usually the 3H is at least 15dB greater. But in old amps where the tubes ahve aged, 2H in PP amps can be as great as 3H, and this is easy to find in such amps as Quad-II, where the old input pair of pentodes has a large differential 2H content, and so does the unmatched KT66, and reversing positions of the input pair will either cancel or add to the 2H and the difference in 2H after such tube swaps can be 15dB. The two non linear current waves in the tubes of the AB pair are summed, and the VOLTAGE total is substantially linear, with a small fraction of the THD of each tube's current wave. And that small fraction is very small indeed, approaching zero to be precise, for the even harmonics. Its magic, but it works for most ppl. It's not magic, it's just math. Its magical luck if you have well matched output tubes. The math is used to describe it only... Where you have a fixed bias PP amps and a pair of 10 ohm R as series cathode resistors, you can see the curent waves on the CRO by monitoring voltage across each 10 ohms. At high AB PO, the wave looks like that of a half wave rectifier. if you connect a 1:1 transformer from cathode to cathode, the secondary voltage looks fine. Its a sine wave but with many harmonics at low level like the anode to anode signal voltage, except that it increases with a lower load and is thus a source of current FB voltage and can be used as positive current FB to reduce Rout. (( There are bothers with a set up like this because this increases THD/IMD, but with NFB used as well the THD/IMD can be much reduced, but Rout made to be very low, or near 0.0 ohms, or even negative..)) Patrick Turner. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#100
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
Arny Krueger wrote: "Patrick Turner" wrote in message ... In class AB, each man pulls the saw about 1/2 way across the stroke then lets go, and the other guy grabs his saw handle and pulls the saw back the other way. Each man only mainly pulls the saw in turn, and applied force is jerky, and frankly, a difficult way to work; the Union will be down soon to have a go at the boss who told the men to saw the log that way. So speaks someone who obviously has no experience cutting wood with a cross-cut saw. You can pull on a saw, but pushing on it can easily cause it to bend and bind. When 2 men use a cross-cut saw, each man pulls far more than he pushes. Its only an analogy. But you should see the saws of the guys who win sawing constests at the shows here. These well trained brutes of men can saw a large hardwood log faster than a guy with a chain saw, and the push isn't much less than the pull force. Thus the power input to the saw is maximized. With a plain single tube, its ability to increase current beyond the idle value is its "pull" stroke and much higher than its "push" stroke of turning off current. The variation in gm for each side of zero gives rise to the 2H. The use of two such tubes in PP cancels out the uneven abilities to a large extent, but also because the gm change between high current and low current is not even, 3H is generated.... Class AB verging on pure class B is the preferred mode of operation for a cross-cut saw. Depends. We have plenty of class A men here in Oz. Maybe youse got only class B men where youse are. And we both fergot to mention bow saws which have a thin long tensioned blade strung tightly between the ends of a steel tube bow, and where the push cuts more than the pull. Patrick Turner |
#101
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
Patrick Turner wrote: [Snip] So what happens when we use a 5k load on the same amp? The same idle current flows, and the same range of Ia variation 0f +/- 45mA will define the class A **current** swing, ( where the **current** wave THD 5% and mainly all 2H ). So the class A load on each tube = 1/2 x 5k = 2.5k, so the maximum class A V swing at each anode = 2,500ohms x 0.045A pk = 112.5Vpk = 225pk from anode to anode, or 159vrms across 5k, giving 5 watts of class A. But the load value allows for a much larger increase in Ia than the 50mA of maximum decrease in Ia. This also means that once the Ia travels below 10% of the idle value, the gm of the tube cutting off has diminished to such a low value the other tube turning on harder is providing virtually all the Ichange x Vchange across the available load, and is the only device coupled through only 1/2 the OPT primary to the load, so the RL seen by this tube turning on hard has reduced to 1/2 its class A load, or 1/4 of the nominal RL a-a, and in this case its 1.25k. The load is the same as that for a class B amp. Isn't 1.25k too low a load for getting maximum power from a KT88 in triode mode, even in class B? [Snip] Anyway, the quad of 6550 while working in class A with a 10ohm load connected across the mis-labelled 0-4 ohm outlet do sound VERY well. Those wanting a schematic of what have done may ask as I have a .gif available. Its much simpler than the original, and I won't beak ARC rules by handing out free copies of their abominable concoctious junk. What is all this talk about ARC's anyone else's rules that would keep you from handing out free copies of their abominable concoctious junk, assuming you drew the schematic your self? You mentioned this same issue in connection with the ManleyLabs amplifier you modified, my understanding is that they only have protection for schematics they have drawn, if you draw your own schematic of the same circuit, they have no rights with regard to it. Any Lawyers out there care to comment? [Snip] The VT100 had a true horror for a PSU and after fitting a CLC B+ filter and re-locating earth paths, I finally got hum&noises to be less than 1mV with preamp gain at max with open cd input. Is this on the 4 Ohm tap? If so it is only 66 dB below 1 Watt, or 69 dB with an 8 Ohm speaker connected to the 4 Ohm tap, not an awe inspiring result, but apparently typical for many tube amps. But maybe it's not so bad with the preamp included, did you measure the hum&noises of the VT100 alone? Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#102
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
Don Pearce wrote: On Thu, 25 Oct 2007 13:43:56 GMT, John Byrns wrote: In article , (Don Pearce) wrote: Can you overdrive a class A amp to cutoff? Yes, it is painfully easy. In my experience what happens when you overdrive a class A amp is that one device saturates, and the other sticks with its normal bias condition. No, the coupling capacitor charges up as a result of grid current shifting the bias point so that cutoff becomes even easier. There is no circumstance in which I have ever managed to put a class A amplifier output device into cutoff. You haven't tried very hard then. The designs I have used must have been better matched in drive level between the driver and output stages. They tended to limit almost simultaneously so that the drive level to the output stage did not go on rising as the input signal increased, just squared off. As I say, I never saw an output valve go into cutoff. A well designed class A SE tube amp will have symetrical clipping where grid current limits the increase in anode current at the same load voltage swing amplitude where load anode current reaches very near zero. A typical load for a trioded KT88 is 5k. Two such tubes which are in a PP circuit will behave the same way if they have the same load. The total load across the whole PP primary is twice the load of the SE tube. So the pair of KT88 will have 10k load a-a. But with PP the load can be halved, and idle current reduced for class AB operation and indeed the anode current is cut right off, because the grid voltage goes to a value so darn negative. And the positive voltage swings of the grid can draw grid current which charges the coupling cap negatively, so the cut off occurs sooner when the amp is over driven, and amps can become temporarily paralysed after being seriously overdriven, because tubes remain cut off until they re-bias themselves and cap charges leak away through biasing resistances. Patrick Turner. d -- Pearce Consulting http://www.pearce.uk.com |
#103
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Output classes A and AB
Andre Jute wrote: On Oct 25, 1:17 pm, John Byrns wrote: In article .com, Andre Jute wrote: On Oct 25, 7:40 am, Eeyore wrote: John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Graham Holy ****! Did I say yet that Poopie is ignorant and incompetent? Nah, nobody can be that stupid and uninformed about tube basics. There are a lot of "stupid and uninformed" people around, there are at least three people involved in this discussion that have expressed this same belief as Eeyore, they are Multi-grid, Patrick Turner, and Eeyore. From Eeyore (Poopie Stevenson) I expect only the worst; he says whatever comes into this head as the opposite of what is said by someone he dislikes, without any reference to the facts in electronics; I have made a separate thread to illustrate that Poopie is joined in this perversity by Pearce and Krueger. I've given up reading Cuddles Multi-grid's posts; it is too wearing to think up new ways of explaining the same simple thing to him over and over again. Patrick is entitled to a mistake now and again; he makes so few. I saw the above and knew you would call him on it, so I read on before I wasted my own time telling Patrick he'd better explain himself -- and there you were, pointing out the faux pas. Actually, I regret that Patrick is both honest and not as slippery with words as some we have seen here, or we could have had a dingdong as he tried to explain it away without admitting he was wrong. I expect he will just say he slipped up. But I don't think I did slip up or make a mistake as John alleged. See my other replies.. I actually find Patrick's total ignorance of woodsawing the most amazing thing in this thread -- especially considering that we have a fellow with real expertise (and ruler braces) on board RAT: Iain Churches. I trained as a carpenter and joiner as part of becoming a Builder, a trade I worked at for 30 years I got to know all I ever needed to know about sawing wood. The worst job of sawing was where two guys have to saw a log along the grain. One is above the log, and the other is below it, but the guy below gets covered in sawdust, but must look up to guide the saw to the line. The guy above is in the sun, and gets hot...... Sawdust can be quite toxic from some woods, and sawyers lungs are badly affected. Thank christ they invented steam saws. But most of the planks used in very old buildings and ships were all cut by hand, push pull mode, and in a variety of classes, A, AB etc. I often heard Harmonic Distortion call out. "Gotta beer for us 'av yer?" Patrick Turner. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ Andre Jute Visit Jute on Amps at http://members.lycos.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review |
#104
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
Patrick Turner wrote: John Byrns wrote: In article , Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? In effect the even order distortions DO re-appear when the amp moves to class AB. Its because of the mismatch between the two tubes, and each has slightly or greatly different gm, so each +ve and -ve half of the waves are amplified by a different amount, and hence you get even order generated in the output from across the whole primary or secondary of the OPT. You are changing your tune a bit, you originally said "once each tube moves into cut off, nothing is cancelled", now you say "Its because of the mismatch between the two tubes", which is a completely different deal. I stated in several of my posts that the tubes must be identical, of course that is impossible in the practical world, but we want to get as close as possible if we are intent on minimizing distortion. When the tubes are mismatched, the problem is not just confined to class AB operation, it will occur even in a class A amplifier with mismatched tubes. The bias may be able to be adjusted to cancel the 2 nd harmonic at some selected power output, but there is no guarantee that the other even harmonics will also be canceled, even in a class A amplifier if the tubes are mismatched, and if the power output level is changed even the second harmonic can reappear with mismatched tubes. It is the mismatched tubes that are at the root of the problem, not operating in the class AB region with the tubes cut off over part of the cycle. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#105
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Output classes A and AB
"Andre Jute" wrote in message ups.com... AKA Poopie for a good reason. Why don't you share it with us. The reason for your name is surely the one thing you do know. Hmm, "Poopie", like the lie about my academic credentials is another Middiot invention. Obviously, Jute is a Middiot admirer. |
#106
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
"Patrick Turner" wrote in message ... Arny Krueger wrote: "Patrick Turner" wrote in message ... In class AB, each man pulls the saw about 1/2 way across the stroke then lets go, and the other guy grabs his saw handle and pulls the saw back the other way. Each man only mainly pulls the saw in turn, and applied force is jerky, and frankly, a difficult way to work; the Union will be down soon to have a go at the boss who told the men to saw the log that way. So speaks someone who obviously has no experience cutting wood with a cross-cut saw. You can pull on a saw, but pushing on it can easily cause it to bend and bind. When 2 men use a cross-cut saw, each man pulls far more than he pushes. Its only an analogy. Not a very good one. But you should see the saws of the guys who win sawing constests at the shows here. I seriously doubt they are much different from the ones in the Northern hemisphere. Laws of physics and all that. These well trained brutes of men can saw a large hardwood log faster than a guy with a chain saw, Not if people get their choice of chain saw. and the push isn't much less than the pull force. Pushing a saw is only a little bit more productive than pushing on a rope. Are you telling me that this works down there, even though it works up here? Thus the power input to the saw is maximized. No way can two human bodies duplicate the horsepower generated by fuel acting on a high performance engine. With a plain single tube, its ability to increase current beyond the idle value is its "pull" stroke and much higher than its "push" stroke of turning off current. The variation in gm for each side of zero gives rise to the 2H. Sounds like something that either needs negative feedback to correct, or should simply be avoided in a hi fi. The use of two such tubes in PP cancels out the uneven abilities to a large extent, but also because the gm change between high current and low current is not even, 3H is generated.... In fact tubes are exponential devices. The expansion of the exponential series that describes the transfer function of a tube contains both even and odd order terms. No surprise then that unless special care is taken (such as a well-balanced push-pull) tubed amps produce both even and odd order nonlinear distortion. Secondly, it is written nowhere that the current flow in each tube in a P-P output stage need be greater than in a SET. Therefore Patrick, your argument is a straw man argunment. Class AB verging on pure class B is the preferred mode of operation for a cross-cut saw. Depends. We have plenty of class A men here in Oz. Maybe youse got only class B men where youse are. Nahh, men can be class A or B on either hemisphere. And we both fergot to mention bow saws which have a thin long tensioned blade strung tightly between the ends of a steel tube bow, and where the push cuts more than the pull. I didn't forget - its just that my mind didn't wander. |
#107
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
John Byrns wrote: (Don Pearce) wrote: Eeyore wrote: John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Because if you add an even harmonic to a signal, you have to make it asymmetrical. You always get a peak coinciding with a trough on one half cycle, followed by a peak coinciding with a peak on the next. If you modify the signal to remove any asymmetry, you must by definition remove the even harmonics. Finally a man who understands the theory! But it's not by ** CANCELLATION ** in the case of AB operation beyond the crossover point. That's my issue with the description. It does have that effect but the use of the word *cancellation* is wong IMHO. There should be another way to describe it. Graham |
#108
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
Andre Jute wrote: Eeyore wrote: John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Graham Holy ****! Did I say yet that Poopie is ignorant and incompetent? Nah, nobody can be that stupid and uninformed about tube basics. Poopie must be cracking a joke. For the first time in his life. Good on yer, cobber! If you can't be smart and informed, at least you can try to be a clown, give people a giggle. As an alleged 'wordsmith' you of all people ought to understand what cancellation means. Apparently it went right over your head though. Only to be expected from an ignorant non-technical ****wit. Graham |
#109
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Output classes A and AB
Andre Jute wrote: Coupla superbly detailed posts here, Patrick Slurp, slurp, slurp. My, your tongue IS brown isn't it ? Graham |
#110
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Output classes A and AB
Andre Jute wrote: "Arny Krueger" wrote: "Don Pearce" wrote in message Andre Jute wrote: Now this poor dumb slow learner Poopie Stevenson, an embarrassment to everyone who comes into contact with him, claims to know more than the RDH4! Yo, Poopie, in the RDH4 (Newnes 1997) on p 545 we find this nugget of authoritative information by F. Langford-Smith B.Sc. B.E. himself: "Class AB operation indicates overbiased conditions, and is used only in push-pull to balance out the even harmonics." No such thing in the RDH4 at hand. Do you just lie from habit, Krueger, or are you incapable of using the contents list or the index of a reference book? The reference is from RDH4, Chapter 13, Section 1 (ii) Classes of Operation. Ah yes, you're mired in the past. I suppose all technological development ceased after publication of that tome. Graham |
#111
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Output classes A and AB
Andre Jute wrote: Poopie Stevenson aka Eeyore wrote: Andre Jute wrote: Eeyore wrote: Andre Jute wrote: All of that follows logically from Poopie's absurd redefinition of Class A as a Class in which "the output device(s)never cease conducting *under any signal condition*," (emphasis added). It's ludicrous. It's actually the only accurate definition. I've already demonstrated several times that your words "under any signal condition" make your definition grossly inaccurate. But you're an ignorant **** and what you say is a load of ********. Even when I'm right? You're NOT right. Your ignorance is simply confusing you. HOWEVER, to keep you happy I am happy to modify to modify my definition for clarity. I already posted this once but I suppose you like to argue more than anything. "the output device(s)never cease conducting *under any signal condition* within the rated specification". To be honest, to have to explicitly state "within the rated specification" is really a case of pandering to fools, which certainly describes YOU, Jootikins. Graham |
#112
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Output classes A and AB
Andre Jute wrote: From Eeyore (Poopie Stevenson) I expect only the worst; he says whatever comes into this head as the opposite of what is said by someone he dislikes If there was ever a case of projection ! Graham |
#113
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
Eeyore wrote: John Byrns wrote: (Don Pearce) wrote: Eeyore wrote: John Byrns wrote: Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? If you actually believe that you should go back to the books and study the theory of harmonic distortion more carefully. I hope you didn't get this notion from the RDH4, I haven't read the RDH4's harmonic distortion explanation, but if this is what it says I have just lost any respect I had for the book. In a perfectly balanced PP amplifier the even order harmonic distortion is completely cancelled even when the tubes are cut off for parts of the cycle. I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Because if you add an even harmonic to a signal, you have to make it asymmetrical. You always get a peak coinciding with a trough on one half cycle, followed by a peak coinciding with a peak on the next. If you modify the signal to remove any asymmetry, you must by definition remove the even harmonics. Finally a man who understands the theory! But it's not by ** CANCELLATION ** in the case of AB operation beyond the crossover point. That's my issue with the description. It does have that effect but the use of the word *cancellation* is wong IMHO. There should be another way to describe it. Cancelation is the right word, the two tubes, even when they, "operate beyond the crossover point", generate harmonics which when added, add in phase for odd harmonics, and out of phase for even harmonics, hence the even harmonics "cancel" in the output. This is a very common meaning for the word "cancellation", and I can't think of a better use for the word. If it "does have that effect", then if it walks like a Duck and Quacks like a Duck, it probably is a Duck. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#114
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Output classes A and AB
John Byrns wrote: In article , Patrick Turner wrote: [Snip] So what happens when we use a 5k load on the same amp? The same idle current flows, and the same range of Ia variation 0f +/- 45mA will define the class A **current** swing, ( where the **current** wave THD 5% and mainly all 2H ). So the class A load on each tube = 1/2 x 5k = 2.5k, so the maximum class A V swing at each anode = 2,500ohms x 0.045A pk = 112.5Vpk = 225pk from anode to anode, or 159vrms across 5k, giving 5 watts of class A. But the load value allows for a much larger increase in Ia than the 50mA of maximum decrease in Ia. This also means that once the Ia travels below 10% of the idle value, the gm of the tube cutting off has diminished to such a low value the other tube turning on harder is providing virtually all the Ichange x Vchange across the available load, and is the only device coupled through only 1/2 the OPT primary to the load, so the RL seen by this tube turning on hard has reduced to 1/2 its class A load, or 1/4 of the nominal RL a-a, and in this case its 1.25k. The load is the same as that for a class B amp. Isn't 1.25k too low a load for getting maximum power from a KT88 in triode mode, even in class B? No. If the RL a-a = 5k, then the class B load is 1.25k, and if Ea = 500V, then max Ia at grid current is about 220mA. If you run AB2, you get a heck of a lot more Ia up to around 350mA. KT88 ca easily make 500mA, depending on loads etc. One can get 140W from a pair in AB2 in tetrode. But for hi-fi, about 25 watts max is about right, with RL about 8ka-a. [Snip] Anyway, the quad of 6550 while working in class A with a 10ohm load connected across the mis-labelled 0-4 ohm outlet do sound VERY well. Those wanting a schematic of what have done may ask as I have a .gif available. Its much simpler than the original, and I won't beak ARC rules by handing out free copies of their abominable concoctious junk. What is all this talk about ARC's anyone else's rules that would keep you from handing out free copies of their abominable concoctious junk, assuming you drew the schematic your self? You mentioned this same issue in connection with the ManleyLabs amplifier you modified, my understanding is that they only have protection for schematics they have drawn, if you draw your own schematic of the same circuit, they have no rights with regard to it. Any Lawyers out there care to comment? I fell OK about just letting folks know what they could do to rebuild a Manley or ARC or start from scratch and use the schematic I will be posting at my site. There is nothing I can gain by posting a copy of the original schematic these companies use. Many companies do NOT like ppl posting copies of their schematics on the web, and I have no intention of offending them by doing so. I am free to post alternative schematics used in the cases of their amps though, and Manley and ARC would do very well to copy the schematics for their own use in future. They'd be welcome afaiac, but I betcha they wouldn't dream of doing it, because then they'd appear to those in the know that they don't know more than I do, and if you asked them, they'd get a bit pooey about it all and of course ARC et all know what is the best way to make an amp they'd say. ARC and all these companies would never admit to the follies of their designs. They really don't like ppl questioning why. They like blind praise it seems to me. They say that once you alter anything there isn't any more support. OK, I can cope with that. I improve their amp's function, and company help to support their original design will not be required. Why would I tell my customers that we ought to keep things original to get support to ensure toubles keep happening? Big US companies like to have a stranglehold on service and parts supply after you've bought an amp of theirs. I'd rather not trouble them, and I like to go my own way. I admire their success which I cannot ever attain and I hope they find all I say is food for thought. [Snip] The VT100 had a true horror for a PSU and after fitting a CLC B+ filter and re-locating earth paths, I finally got hum&noises to be less than 1mV with preamp gain at max with open cd input. Is this on the 4 Ohm tap? If so it is only 66 dB below 1 Watt, or 69 dB with an 8 Ohm speaker connected to the 4 Ohm tap, not an awe inspiring result, but apparently typical for many tube amps. But maybe it's not so bad with the preamp included, did you measure the hum&noises of the VT100 alone? After putting in a CLC type B+ filter and re-routing all the earth paths, and fimally reducing hum to less than 1mV, I was satisfied. The original had lots of noise in SE input mode. with both the inputs grounded, the noise of the power amp was very low; and the simple test is that with 89dB/W/M sensitve speakers, you don't hear ANY noise unless you place your ear against a speaker, and all there should be is a slight hiss, and perhaps barely detectable background buzz. My DVM reads 0.00Vrms when placed on the outputs, and with the preamp connected and in SE feed to the power amp, with gain turned up fully, noise was so low I had to put an ear to the spekers to hear any. The DVM reads 0.00Vrms which occasional flicker reading to 0.001Vrms. Noise may increase somewhat when music flows in the amps, but not much at the few watts which will be used. If noise is say 0.5mV, and average power levels are 1Vrms, the unweighted SNR = -66dB. This seems like a hopelessly poor figure, but its a typical figure better than what many commercial SS and tube amps make. At 30Vrms, if noise is still 0.5mV, then the SNR gets a lot better at -96dB approx. What people really want is an amp so quiet that NO noise is present even when they walk over close to the stand to adjust volume etc. This allows them to listen at low levels. The loudest noise I get is from the darn power transformer in the VT100, audible from 3 feet away. Patrick Turner Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#115
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John Byrns wrote: Eeyore wrote: John Byrns wrote: (Don Pearce) wrote: Eeyore wrote: I'd love to know how that happens. There's no cancellation of ANYTHING once one side has ceased conducting ! Because if you add an even harmonic to a signal, you have to make it asymmetrical. You always get a peak coinciding with a trough on one half cycle, followed by a peak coinciding with a peak on the next. If you modify the signal to remove any asymmetry, you must by definition remove the even harmonics. Finally a man who understands the theory! But it's not by ** CANCELLATION ** in the case of AB operation beyond the crossover point. That's my issue with the description. It does have that effect but the use of the word *cancellation* is wong IMHO. There should be another way to describe it. Cancelation is the right word, ********. the two tubes, even when they, "operate beyond the crossover point", When one tube has ceased conducting, there's NOTHING TO CANCEL, you IGNORANT ****WITTED ****. CANCELLATION IS THE *** WRONG WORD ***. In fact it's ADDITION of waveforms, not cancellation. Graham |
#116
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
John Byrns wrote: In article , Patrick Turner wrote: John Byrns wrote: In article , Patrick Turner wrote: Cancelation of even order harmonics occurs in amps working in class AB during that part of the wave forms which are in class A, ie, the bits either side of the zero crossing. But once each tube moves into cut off, nothing is cancelled. Patrick, I'm surprised to hear you say this. What are you trying to tell us, that the even order harmonics are only cancelled during those parts of the cycle when both tubes are conducting, but that the even order distortion components reappear during those parts of the cycle when only one tube is conducting? In effect the even order distortions DO re-appear when the amp moves to class AB. Its because of the mismatch between the two tubes, and each has slightly or greatly different gm, so each +ve and -ve half of the waves are amplified by a different amount, and hence you get even order generated in the output from across the whole primary or secondary of the OPT. You are changing your tune a bit, you originally said "once each tube moves into cut off, nothing is cancelled", One cannot have distortion cancelling by one tube cancelling that in another when one is cut off. But the SUM of the joint action of each tube in class AB with very non linear currents gives a linear voltage outcome. now you say "Its because of the mismatch between the two tubes", which is a completely different deal. In class A, you often do not get perfect 2H cancellations, since tubes are not matched, so 2H is present with 3H etc. The effect of missmatched tubes gets worse when the amp moves to AB. more 2H with more of everything else. Summing of the output voltage still occurs while little cancellation happens. I stated in several of my posts that the tubes must be identical, of course that is impossible in the practical world, but we want to get as close as possible if we are intent on minimizing distortion. When the tubes are mismatched, the problem is not just confined to class AB operation, it will occur even in a class A amplifier with mismatched tubes. That is what I have said all along. PP isn't perfect. Tubes and bjts and mosfets ain't perfect. SS pn and pnp devices are often about as similar as using a 6L6 and EL34 in a tube PP amp; ie, not very well matched. Doesn't madder, add piles of NFB and she'll be right.... The bias may be able to be adjusted to cancel the 2 nd harmonic at some selected power output, but there is no guarantee that the other even harmonics will also be canceled, even in a class A amplifier if the tubes are mismatched, and if the power output level is changed even the second harmonic can reappear with mismatched tubes. It is the mismatched tubes that are at the root of the problem, not operating in the class AB region with the tubes cut off over part of the cycle. Matched tubes don't stay matched. One should not be too dependant on matched tubes. In a class AB amp with 50% of the power in class A before AB commences, and with tubes with 10% gm variation, the amount of 2H in the first few watts is way less than if the tubes were operating in SE mode. One does not have to worry if the TD in an AB a tube is below 0.1% at 2 watts. The 2H and 3H will be dominant, and not cause undue IMD. if the same amp makes 50 watts in AB, and THD is 0.5%, then its OK, still not to bad, but nodody will notice it. I like having less than 0.05% for everything below average levels; if for an instant during a drum beat power rises to an instant 50 watt level, and the THD leaps to 0.5%, I am not concerned. Most of my PP amps have less than 0.25% at clipping and 0.03% at average levels. I have heard a VAC amp with 4 x 300B in PP for each channel to make 56 watts max, and its usable with/without global NFB. There is no difference to the sound that I am aware of when altering NFB from zero to having 9db applied. This means that if the THD is 0.1% with zero NFB, applying 9dB to reduce it to about 0.03% makes no audible difference to me. Patrick Turner. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#117
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
Patrick Turner wrote: John Byrns wrote: In article , Patrick Turner wrote: This also means that once the Ia travels below 10% of the idle value, the gm of the tube cutting off has diminished to such a low value the other tube turning on harder is providing virtually all the Ichange x Vchange across the available load, and is the only device coupled through only 1/2 the OPT primary to the load, so the RL seen by this tube turning on hard has reduced to 1/2 its class A load, or 1/4 of the nominal RL a-a, and in this case its 1.25k. The load is the same as that for a class B amp. Isn't 1.25k too low a load for getting maximum power from a KT88 in triode mode, even in class B? No. If the RL a-a = 5k, then the class B load is 1.25k, and if Ea = 500V, then max Ia at grid current is about 220mA. If you run AB2, you get a heck of a lot more Ia up to around 350mA. KT88 ca easily make 500mA, depending on loads etc. One can get 140W from a pair in AB2 in tetrode. But I was asking about the best load for a class B triode amp, is 1.25k too low for a KT-88? I guess I will have to see if I can find the triode plate curves for the KT-88, or maybe I can substitute the 6550 curves. What is all this talk about ARC's anyone else's rules that would keep you from handing out free copies of their abominable concoctious junk, assuming you drew the schematic your self? You mentioned this same issue in connection with the ManleyLabs amplifier you modified, my understanding is that they only have protection for schematics they have drawn, if you draw your own schematic of the same circuit, they have no rights with regard to it. Any Lawyers out there care to comment? I fell OK about just letting folks know what they could do to rebuild a Manley or ARC or start from scratch and use the schematic I will be posting at my site. There is nothing I can gain by posting a copy of the original schematic these companies use. I was not talking about the original schematic drawn by these companies, I was talking about a schematic of the same circuit that you or anyone else may have drawn, it is my understanding that there is nothing to prevent you from legally posting such a schematic, illustrating the same circuit as the company circuit, you just can't post the schematic drawn by the company. Many companies do NOT like ppl posting copies of their schematics on the web, and I have no intention of offending them by doing so. I am not suggesting that you should do it, but it is my understanding that they have no say in your posting a schematic you drew of their circuit. Of course you are going to offend them by doing that, and they may retaliate by denying you access to replacement parts. I am free to post alternative schematics used in the cases of their amps though, You are also free to post your rendition of the schematic for their original circuit. As I understand the situation they only have rights to and control over their drawing of the original circuit, you are free to create and distribute copies of a new drawing of the circuit that was drawn by you. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#118
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
Patrick Turner wrote: One cannot have distortion cancelling by one tube cancelling that in another when one is cut off. THANK YOU ! Basics do matter. Graham |
#119
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
Patrick Turner wrote: But the SUM of the joint action of each tube in class AB with very non linear currents gives a linear voltage outcome. And should not be confused with genuine CANCELLATION of distortion by Class A push-pull operation. Graham |
#120
Posted to uk.rec.audio,rec.audio.tubes
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Output classes A and AB
In article ,
Eeyore wrote: Patrick Turner wrote: One cannot have distortion cancelling by one tube cancelling that in another when one is cut off. THANK YOU ! Basics do matter. Indeed they do, but neither Patrick, myself, or anyone else is correct on every issue. In this case Patrick has vigorously asserted that this view, which he holds in common with you, is true, but he has failed to even attempt an argument that might demonstrate its truth. Patrick is an extremely skilled and talented fellow in the practical aspects of tube amp design and construction, but he has a very limited understanding of what is going on behind the scenes in the theory of tube amp operation. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
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