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97.95.43.235
For all those that love DHTs and massive power supplies and transformer coupling,this baby has it all.Remember this was back at a time if you wanted a radio or an amplifier audio or RF,you built it because most hams weren't rolling in pesos back then.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 08/16/16Follow Ups:
They must have made a lot of copies Michael... I have the very same edition and have re-read a lot of that book for the better part of 5 years.
The Mind has No Firewall~ U.S. Army War College.
Ken
How do like the information in there?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Was/Is very informative, being a sentimental fluff I enjoyed the advertisements as well ... "grandpa was a stringsaver" gaining insight to a part of our history unknown to myself. The book outlined power supplies in a way I could understand as a novice in DIY. The binding is getting er... rather frayed.
The Mind has No Firewall~ U.S. Army War College.
Check out Pete Millett's fantastic PDF resource linked below. Lots of radio handbooks from ARRL and Editors & Engineers. I've sent him a couple to scan.
Edits: 08/16/16
I always love the adds in the back of these. Used to lust after all that great Ham gear when I was in high school. Imagine a 650 page book for $1.
So I'm a long-time radio amateur. Licensed in 1976, but I was inactive from 1982 - 2015. 33 years. My interest in DIY tube audio grew from the basic tube-based electronic theory I learned as a young amateur. I've haunted the Asylum and some other places for years now. Then early last year, I suddenly became interested in radio again. I found that I could still copy CW (morse code). I found that I knew a bit more about electronics than I did. I found that I could afford to buy some of the old-time gear from back in the day. And I found that I could fix most of the dang stuff.
I'm back on the air 99% CW, and 100% tube. And I really owe it to my phellow audiophools that I am. It's a lot of fun for me. I'm becoming known as one of the cranks that runs antique gear.
Why bother when the internet and mobile phones have the world instantly connected? I can't explain it. But it's a lot of fun for me to know that with 15 or so 50+ year old tubes and a piece of wire in the backyard, I can meet and converse with a stranger 8,000 mile away.
73 de Steve KE4OH dit dit
Sadly,there are quite a few more that have let their license expire because they either didn't want to spend the 7 or 10 dollars to renew it,or they just plain forgot to do it. Now,it cost nothing to renew it and you can do it on line.I also think there are a portion of them that didn't like the fact that the CW requirement was eliminated,when most of us had to pass at least 13wpm to have any voice privileges.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
I was licensed at 14. Two years later, I could copy Morse by ear at 60 WPM. All told, I was a ham nearly 50 years - Novice, General and Extra Class.
The last time I was on the air (about ten years ago), I simply didn't enjoy it. One of my favorite segments had been moved (80M novice band). There was clandestine phone operation all over the CW portions of several bands. I had difficulty finding someone capable of typical Extra Class CW speeds, and fewer still able to operate QSK. I switched to phone and worked some DX, but the thrill was gone.
When renewal of the license came around a few years ago, I let it lapse. It was just a hanger-on from a bygone era. R.I.P.
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Buy Chinese. Bury freedom.
I got into Ham Radio back in Jr. High (1970's). I used to be WN7TTS. I remember the 5 wpm test was a breeze, but the 13 - that had me sweating! As you suggest, I let my ticket expire many years ago - about the time I discovered girls. :-) I think about taking it up again from time to time, so far there seem to be too many other priorities. But I do like the fact that I can now afford the stuff I lusted after way back then. First rig was a Hallicrafters HT40 paired with a Heath General Coverage receiver - man that was hard to tune precisely.
"Classes of amplifiers, Reason for Classifications"The book explains that the reason for classifications is to make it easy to divide types (WRT their intended purpose) without having to explain every time.
Class A is clearly described as an amplifier where the tube is operated in a way that gives the least distortion. Where the tube is operated only in the most linear part of the curve.
I've been trying to explain this, here on the AA, for years.
The short definition of Class A found in most books, "it's Class A as long as the current never cuts off" is NOT the TRUE definition of Class A.
It is true that with Class A the tube never cuts off but that is only a small part of the story. I mean a horse has 4 legs, that doesn't mean that all animals with 4 legs are horses.
The books that give that short definition always go on to describe an operating condition that does fit the true definition of Class A.
This book goes straight to the point.
That point being, A Class A amplifier will give the least amount of distortion (for the chosen tube) because the tube is being operated only in the most linear part of the curve.
Thank you Michael.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/16/16 08/16/16
I know and I agree with your logic 100% as you recall..If class A is being designed to throw away all it's advantages while maintaining its disadvantages,there is no point to doing it that. It would be like buying 710hp Challenger Hellcat with the supercharged Hemi engine and then Dodge says well,we built this monster that is capable of going very fast but we will leave the supercharged hemi in but we are going to detune it to run on 87 octane fuel.Is it still a 710 HP high performance car? NO!By retarding the spark and fuel,you threw away all the advantages of having a high performance 710 hp car.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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Have Fun and Enjoy the Music
"Still Working the Problem"
... "A Class A amplifier will give the least amount of distortion (for the chosen tube) because the tube is being operated only in the most linear part of the curve."
That's a pretty restrictive "definition". What if the tube is isn't operated in its most linear region so that some other parameter might be optimized but is otherwise never driven to cutoff or saturation? What class might that be?
The ARRL's three paragraphs describing Class A consider more characteristics than distortion alone.
I just simmed a PP 300b amp with 375 volts plate, 560 ohm common cathode resistor giving a bias voltage of -65.
Driving it with 97 volts peak there is still some current flowing in each tube at all times. 8ma minimum.
Are the tubes still operating in Class A?
Driving it with 112 volts peak there's still 1ma.
Still Class A?
Without considering symmetry the "360 degrees conduction" is not much of a definition.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
...this is just like much disparaged class AB operating in class A at low signal levels.
Now wait a minute.
I thought (I could be wrong) you said that the definition of Class A was as long as current is flowing for the entire 360 degree input signal then it's Class A?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"What if the tube is isn't operated in its most linear region so that some other parameter might be optimized but is otherwise never driven to cutoff or saturation?"
First, in a Class A amplifier the tube never reaches the non-linear cutoff region (or the non-linear saturation region) let alone actual cutoff (or actual saturation).
I'm really interested, what other parameter would be optimized by not biasing the tube in the middle of the linear part of the curve?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I'm referring to setting operating parameters such that the device is allowed to operate in the curved portions of the curve and not to bias point. This might be done to maximize power output of a single ended class A audio output stage where high distortion is acceptable. The output stage never reaches cutoff, saturation or g1 current but is not operating in the most linear region at max design output. This is a class A mode of operation.
"This might be done to maximize power output of a single ended class A audio output stage where high distortion is acceptable."
Moving the bias point off-center in either direction limits the swing. How can more power be extracted by limiting the swing?
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Buy Chinese. Bury freedom.
nada aqui
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Buy Chinese. Bury freedom.
I'm referring to setting operating parameters such that the device is allowed to operate in the curved portions of the curve and not to bias point. This might be done to maximize power output of a single ended class A audio output stage where high distortion is acceptable. The output stage never reaches cutoff, saturation or g1 current but is not operating in the most linear region at max design output. This is a class A mode of operation.Steve
That seems like a somewhat feasible argument to be made for a form of class A operation.Nobody denies how text defines class A theory as a device that conducts for 360 degrees of the cycle.All single ended circuits are class A,because they don't share the cycle with an alternate device.You mentioned that some single ended output stages are ok with high distortion and running in the non linear portion of their curves? Now,you just threw away the two major objectives for using class A in the first place. We have already sacrificed half of our power by using this mode of operation.Now,to supposedly gain a fraction of that power back,we decide to discard the other most desirable attribute of Class A operation which is linearity.Doesn't this seem just a wee bit silly for the cost of what we lose in power? I know we are kind of arguing semantics here but class A as it pertains to audio amplification should at the very least, be run in the most linear part of the curve.Many expanded text definitions say this as well.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 08/17/16
...Tre' was asking for an example of where one would sacrifice linearity for some other parameter. And yes, while in this hypothetical example I "threw away some major objectives", I achieved two other objectives: greater output and use of a a single output device . Life is full of compromise.
Bottom line is that while greater relative linearity may be a characteristic of class A operation, it is not the definition of class A operation.
"Bottom line is that while greater relative linearity may be a characteristic of class A operation, it is not the definition of class A operation."
But the greatest linearity (for the chosen tube) is what is shown in all the examples of "good practice" Class A in the books.
So would your hypothetical be a "bad practice" example of Class A but by definition, still Class A? (I'm asking if that is your position)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
...My example would be "good practice" class A for a low cost console amp. It would be "bad practice" class A for a recording studio monitoring amp.
As an interesting aside, consider some of the SET amps reviewed in Stereophile. They are necessarily class A and are described as such. By measurements these are grossly non-linear....to the point that JA has referred to them as tone controls. And yet the subjective review is totally positive.
"It would be "bad practice" class A for a recording studio monitoring amp."
So wouldn't that make it "bad practice" for a DIY audiophile?
And why should/would we spend time dealing with bad practice?
John Wooden taught his players to never practice wrong, it fosters bad habits.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
...A good designer can at will design a low cost console amp or a high quality monitor amp. And yes it probably would be bad practice for an audiophile diyer unless the diyer was called upon to design a low cost amp. Basic definitions like amplifier classes shouldn't be subject to the whims of a fringe hobby. Also the term "practice" as used here isn't the same as practice (repetition) of music.
"Basic definitions like amplifier classes shouldn't be subject to the whims of a fringe hobby. "
Yet that's exactly what you're doing by attempting to include unorthodox operating conditions under definitions that have been well-understood for decades. It's also exceedingly silly to think that an AB1 amplifier operating at reduced signal levels suddenly fits the definition of Class A.
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Buy Chinese. Bury freedom.
"It's also exceedingly silly to think that an AB1 amplifier operating at reduced signal levels suddenly fits the definition of Class A."
But that's the problem. A Class AB1 amplifier operating at reduced signal levels does meet the short definition of Class A. So does a lot of other amps. I think there's always some leakage current.
That's why I've been trying to explain that there is more to Class A than the short definition.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"What if the tube is isn't operated in its most linear region so that some other parameter might be optimized but is otherwise never driven to cutoff or saturation? What class might that be?"
It's the class that requires the designer to describe why he did that every time he mentions the amplifier's operating characteristics. Your question reminds me of all the times I've heard people say that a Class AB1 amplifier operates in Class A at low signal levels. Nothing could be further from the truth.
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Buy Chinese. Bury freedom.
"Your question reminds me of all the times I've heard people say that a Class AB1 amplifier operates in Class A at low signal levels. Nothing could be further from the truth."
That is correct.
The only thing a Class A/B amplifier running at low power shares with a Class A amplifier is the fact that the tube never reaches cutoff.
With Class A, not only does the tube never reach cutoff (or saturation), it never reaches the non-linear cutoff region (or the non-linear saturation region).
Instead, a tube being run Class A operates only in the most linear part of the dynamic curve.
In a Class A/B amplifier the output tubes are biased just above the non-linear cutoff region and once driven (even just a little) enter the non-linear cutoff region.
Some tubes won't cutoff until you hit them with a hammer. :-) Using those tubes (that won't go to cutoff even with a very large negative voltage applied to the grid) does not make an amplifier Class A.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
nada aqui
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Buy Chinese. Bury freedom.
...so be it.
Class A has historically referred to a single-ended stage operating near the center of its conduction window. Such use may not be mandated by the broader definition you prefer, but it represents the original concept to which the delineation of the various classes was applied. No one 50 years ago would have accepted a stage operating at 1/20 of the midpoint as being Class A. If a design needed such reduced power, a smaller tube would have been used. You can attempt to change the meaning of CLass A to apply to such cases, but those of us who know better will always correct your error.
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Buy Chinese. Bury freedom.
Class A is a few things rolled into one. Most importantly, the final(s) don't get cut off within the power at distortion specification. The x watts Class A, and XX Watts AB is BS. It is an XX Watt, AB amp.
You can certainly cut off a SE stage if you push the distortion spec far enough, and feed it enough signal.
Class A is not just continuous conduction, it is continuous conduction at full power. That this effectively sets the tube operating in the middle of its characteristics is a side effect. Or maybe it is the other way around.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
....You've changed no minds, I've changed no minds, precisely nothing has been corrected . LOL
Have yourself a great day!
Steve
Technical definitions have been altered since forever.It is very common in the automotive technical glossary if you do some comparative analysis of definitions from the 1970s to what we use today.
Now,every definition I have seen,especially when it comes to audio amplifier design says,Class A Amplifier
Class A Amplifiers are the most common type of amplifier class due mainly to their simple design. Class A, literally means "the best class" of amplifier due mainly to their low signal distortion levels and are probably the best sounding of all the amplifier classes mentioned here. The class A amplifier has the highest linearity over the other amplifier classes and as such operates in the linear portion of the characteristics curve.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 08/16/16 08/16/16
"Class A, literally means "the best class" of amplifier "
Well, no it doesn't. Single-ended operation was labeled Class A only because it had been the first invented and was the most common at that time. The others - AB, B, C - came later and so were assigned higher order letters.
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Buy Chinese. Bury freedom.
This is what I meant to copy and post from the link.
To achieve high linearity and gain, the output stage of a class A amplifier is biased "ON" (conducting) all the time. Then for an amplifier to be classified as "Class A" the zero signal idle current in the output stage must be equal to or greater than the maximum load current (usually a loudspeaker) required to produce the largest output signal.
As a class A amplifier operates in the linear portion of its characteristic curves, the single output device conducts through a full 360 degrees of the output waveform. Then the class A amplifier is equivalent to a current source.
Since a class A amplifier operates in the linear region, the transistors base (or gate) DC biasing voltage should by chosen properly to ensure correct operation and low distortion. However, as the output device is "ON" at all times, it is constantly carrying current, which represents a continuous loss of power in the amplifier.
Due to this continuous loss of power class A amplifiers create tremendous amounts of heat adding to their very low efficiency at around 30%, making them impractical for high-power amplifications. Also due to the high idling current of the amplifier, the power supply must be sized accordingly and be well filtered to avoid any amplifier hum and noise. Therefore, due to the low efficiency and over heating problems of Class A amplifiers, more efficient amplifier classes have been developed.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
...This is closer to one: Class A operation is when the operating parameters of an amplifying device are chosen such that when provided with a 360 deg sine wave input signal, the device conducts 360 deg at all levels up to maximum design output. Characteristics of class A operation are relatively low output signal distortion compared to other classes, relatively low power output and relatively low efficiency. Blah, blah blah...
With all due respect, the definition in the ARRL manual is intended for electrical neophytes who don't know the difference betw an angular degree and a Celsius degree and think sine is a misspelling of sign.
The Radio Handbook 14th Edition page 98 "......Such an amplifier is normally operated in the center of the grid-voltage plate-current transfer characteristic and gives an output wave-shape which is a substantial replica of the input wave-shape."
The Radio Handbook 38th Edition page 65 A Class A amplifier-- "...is one operated so that the wave shape of the output voltage is the same as that of the signal voltage applied to the grid."
Tremaine Audio-Cyclopedia 2nd Edition page 523 "The quiescent plate current is selected for a value in the most linear portion of the plate-current characteristic."
Page 543 "What is a class A amplifier? An amplifier in which the grid-bias voltage is set to approximately one-half the cutoff voltage to obtain linear operation."
Page 604 "When operating as a single tube, class a, little distortion exists because the grid signal operates in the most linear portion of the dynamic characteristic."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The problem with what you present is that in the context of a real-world device you can not unambiguously define "linear" w/o also introducing an arbitrarily chosen measure of linearity "goodness" such as absolute deviation from perfect, % distortion etc. Words such as "substantial", " most linear" (implying imperfect absolute linearity), and "approximately" are open to interpretation and thus not ideal as part of a definition although they may useful as parts of descriptions and characteristics.
As an example of the inherent ambiguity in the stuff you cited, consider the plot used to graphically demonstrate "linearity". What's to prevent someone from extending the "linear" region by +/-10%? To my eye everything is still linear. Maybe you'll agree, maybe you won't - who's correct ? How about to the point just before the line begins to curve? But where exactly does the line begin to curve? How far away from that curve point must one be to remain in the linear region? Without an agreement on what constitutes acceptable deviation from perfect linearity, a measure of linearity goodness, you might pick one point and I might pick another. IOW, the plot and accompanying text are ambiguous and ambiguity really isn't compatible with a definition.
"How about to the point just before the line begins to curve?"
A closer look will show that the most linear part of the curve is really just the middle part as shown in the previous graph.
By the point where the line visibly curves in the first graph we have, in fact, already left the linear part of the curve.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Whilst it may be customary and preferable to design Class A for lowest distortion, that may not be, and in fact mostly won't be half supply voltage.
A designer may chose slightly different operating points for Class A for many different reasons but meet all the criteria of Class A conduction. If such circuits cannot be described as Class A then what what Class do they fall into?
Science and Engineering doesn't allow for huge areas of ambiguity. I was taught that conduction determined the Classes, not linearity or any other arbitrary, non concise parameter.
My 2C,
Naz
Hopefully it was conduction *at rated/maximum power*...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
"Hopefully it was conduction *at rated/maximum power*...:)"
I know what you are saying Doug, and for purposes of accurately describing an amplifier's design I agree 100%.
But from a purely engineering/physics standpoint an amp can operate in class A at low outputs while being unable to maintain/sustain it when more power and therefore larger voltage swings are required.
Does that make it a "class A amp"? IMHO no - but that doesn't prevent the amp from operating in class A under certain conditions. This reminds me of the debate about amplifier power back in the 60s/70s - just because the FTC decided RMS power was what could be advertised doesn't mean that peak power was nonexistent.
Can't run Class A from an AB operating point.
That is the thing, you can't go saying it's A just because it makes one of the criteria for it. Gotta be the whole 9 yards...or it is something else.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
"Can't run Class A from an AB operating point."
Exactly right!
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Buy Chinese. Bury freedom.
How does one differentiate between different possible operating levels. Lets throw the arbitrary concept of maximum power out the door and consider this to be a preamp design. Can a preamp design only be called class A if it is biased to Tre's set of rules?
From a technical perspective I see no issues with a circuit operating class A up to a given level and then class AB above that level. The issues arise here when the ambiguity comes into play and since the transition period from A to AB can vary wildly from design to design. I see the need to further define the grey areas and am fine with doing this on a case by case basis but to amend a technical definition with subjective and ambiguous terms is just bad practice.
dave
again with the cherry picked set of conditions...
Class A is defined by conduction at max power. Cut off and it is an AB design. Could be deep AB, but is still AB.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
What is max power in a linestage or other circuit than a power amplifier?
Plus at some point all class A circuits are going to cut off and I think the issue here is using a softly defined terms. (max power)
dave
"Plus at some point all class A circuits are going to cut off and I think the issue here is using a softly defined terms."
Part of the definition of Class A is how it's driven (loaded too, but that's a whole other discussion).
With Class A the input signal is limited to keep the operation within the linear part of the operating curve. That's just part of the package.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Where it printed that you must limit the input voltage in order to achieve Class A operation?
By extension of that concept, any amp you can clip is not a Class A design.
dave
It doesn't have to be printed, it's obvious.Tremaine page 604 "When operating as a single tube, class a, little distortion exists because the grid signal operates in the most linear portion of the dynamic characteristic."
With too much input signal the tube operation will go beyond the most linear portion of the dynamic characteristic [curve] and that little distortion will turn into a lot of distortion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/18/16
I think the big difference here is between a general or first order definition being soft and ambiguous and a more precise model being the same.
The more precise the definition the less ambiguity allowed which is why we create more complex models in the first place and these models must be held to higher standards. Further defining anything with more imprecise terms is counterproductive and it is often these same people with complex definitions based on subjective terms espousing 5 decimal place accuracy to justify their opinions.
Tremaine uses the phrase "most linear" and while that seems like a strong statement, it is actually impossible to define without adding lots of other information / assumptions. Luckily his writings were directed to peers / students where a conceptual understanding of what he is saying doesn't take a huge leap of faith. Fast forward to today where on the interwebs, anyone can be an expert and we have a very different situation.
dave
Soft or ambiguous as it may be, I am quite comfortable ruling out any portion of an AB amp's power output as Class A. I am pretty sure the folks responsible for this were from somebody's marketing department...but given the target of such marketing should also bear some responsibility, I'll lay it on both groups.
For a linestage in particular, I would see no issue waiving the requirement for operation in the middle of the possible load line...could be the dose of 2HD available by running near cut off is desirable...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Where's the "most linear part" of this curve?
I think we have squeezed all the juice out of this orange. :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hmm... Mike is there a Class A+ and Class A-? 8^)
Isn't Class "A" the most wasteful of available power? Least efficient of the usual class of audio amps?
to Triode K about what I meant to copy and paste..
"Is there a class A and class A+?"
Of course.It's a sex ed class with lots hot women in it with only one lucky guy.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
At one point I proposed "Linear Class A" to differentiate between "good practice" Class A and a design that merely didn't allow cutoff but in the end I changed my mind based on the text.
The Radio Handbook 14th Edition page 98 "......Such an amplifier is normally operated in the center of the grid-voltage plate-current transfer characteristic and gives an output wave-shape which is a substantial replica of the input wave-shape."
The Radio Handbook 38th Edition page 65 A Class A amplifier-- "...is one operated so that the wave shape of the output voltage is the same as that of the signal voltage applied to the grid."
Tremaine Audio-Cyclopedia 2nd Edition page 523 "The quiescent plate current is selected for a value in the most linear portion of the plate-current characteristic."
Page 543 "What is a class A amplifier? An amplifier in which the grid-bias voltage is set to approximately one-half the cutoff voltage to obtain linear operation."
Page 604 "When operating as a single tube, class a, little distortion exists because the grid signal operates in the most linear portion of the dynamic characteristic."
I am fully convinced that Class A is running the tube in "as linear a way as possible". That is, right in the middle of the "most linear part of the dynamic curve" and limiting the input signal to keep the tube's operation away from the non-linear cutoff and saturation regions and staying only in the linear part of the curve as shown in the graph above.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Or would there be another graph line with a different slope? Such as the ones you see when running tubes at different anode voltages?
The graph I showed in the previous post is the dynamic curve.The dynamic curve takes into consideration the voltage across the tube, the idle current and the load impedance.
The graph above shows how a dynamic curve is derived.
This one happens to be of a not so linear tube with a poorly chosen operating point.
The plate voltage moves farther as the tube is driven to more current vs. when the tube is driven to less current. That is what creates harmonic distortion.
What one wants to do is operate the tube so the plate voltage moves in an equal way up and down from the idle point for the same amount of grid bias change.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/17/16
Gonna need to break out the old College Physics book to translate.
8^)
Does Class "A" push-pull pentode amps require the kind of global feedback that Class AB1 pentode amps need to reduce distortion?
Thanks!
IMO a Class A PP pentode amp would not require as much NFB as a Class AB PP pentode amp because the tubes a Class A pentode amp are running in a more linear way than the tubes in a Class AB pentode amp.
Having said that, the real reason a PP pentode amps needs NFB is to lower the plate resistance of the pentodes so the damping factor is improved.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
A preamp with only minimal swing requirements could happily run Class A at numerous operating points along the load line whereas a power amp is much more limited. Or are you saying that any amp must be biased to either the lowest distortion (nother can of worms) or approximately half supply (also with it's issues)?
Naz
"A preamp with only minimal swing requirements could happily run Class A at numerous operating points along the load line..."
When you say Class A do you mean A. running the tube in the most linear way possible or B. running the tube so that plate current never cuts off?
A tube can't be running in the most linear way possible and at the same time be operating in the non-linear cutoff region.
A tube operating in the non-linear cutoff region (but not cutting off) meets some people's definition of Class A.
My whole point is that Class A is more than that.
Class A is (according to all the examples in all the books) operating a tube only in the most linear part of the curve.
If, in your preamp tube example, the preamp tube's operation is staying in the most linear part of the curve anyway, why not bias it in the middle of the most linear part of the curve? What would be the harm in that?
As you point out, a power tube is more limited. If we want some power and we want the tube to operate only in the most linear part of the curve we pretty much have to bias it in the middle of the most linear part of the curve.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Throwing load lines and distortion into the mix further confuses your definition of Class A. Eg, I know you like CCS loading and for good reason; a more horizontal load line and lower distortion as a result. But this makes it all the less necessary to run an operating point anywhere near the "mid point" unless max swing is necessary. Indeed, a near horizontal load line virtually kills of off a substantial part of your argument.
To include a particular requirement for an operating point in order to meet the definition of Class A also creates more problems than it solves. A set of plate curves may look a lot more linear over a part of the range and indeed may well be for a particular load line if high swing is necessary. However, expand the set of curves and what appears to be the non-linear portion at low plate current suddenly looks very much the same as at high current over the chosen amplifying range. Many designs successfully run tubes in this so called non-linear area and achieve very low distortion and great sound. I'm talking mainly Phono stages here, running plate currents less than 10% of what would be considered to be reasonable current for the particular tube used.
Naz
Naz, I don't necessarily disagree with what you have said but all the examples of Class A in the books follow what you call my "particular requirement[s]".
"Throwing load lines and distortion into the mix further confuses your definition of Class A."
Loading for Class A is different than loading for Class AB.
It is my understand that the higher load impedance (to lengthen and straighten the linear part of the curve) is part of the Class A package.
From all the reading I've done, Class A is not just one thing, it's a whole package with the intent of achieving the least harmonic distortion possible (while still leaving some power in a power stage).
There is no single definition of Class A. If I had to pick one that best describes Class A it would be running the tube only in the most linear part of the curve.
I think if you disagree with that you are disagreeing with most of the technical books.
I didn't write the following;
The Radio Handbook 14th Edition page 98 "......Such an amplifier is normally operated in the center of the grid-voltage plate-current transfer characteristic and gives an output wave-shape which is a substantial replica of the input wave-shape."
The Radio Handbook 38th Edition page 65 A Class A amplifier-- "...is one operated so that the wave shape of the output voltage is the same as that of the signal voltage applied to the grid."
Tremaine Audio-Cyclopedia 2nd Edition page 523 "The quiescent plate current is selected for a value in the most linear portion of the plate-current characteristic."
Page 543 "What is a class A amplifier? An amplifier in which the grid-bias voltage is set to approximately one-half the cutoff voltage to obtain linear operation."
Page 604 "When operating as a single tube, class a, little distortion exists because the grid signal operates in the most linear portion of the dynamic characteristic."
And I didn't create the graph above.
In what way is what I'm saying out of line with the text above or the graph above?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"It is my understand that the higher load impedance (to lengthen and straighten the linear part of the curve) is part of the Class A package.
From all the reading I've done, Class A is not just one thing, it's a whole package with the intent of achieving the least harmonic distortion possible (while still leaving some power in a power stage)."
That's incorrect. If the definition of Class A includes linearity at all, it is only from the perspective that operating a single-ended stage at the midpoint between cutoff and saturation will generally provide low distortion results. At the time this class designation was assigned to its respective operating condition, reducing distortion to the levels we expect today was not a consideration. What you're reciting is the modern use of Class A in an audiophile implementation. It carries an emphasis on reduced distortion - at the expense of output power - that is not a component of the original definition of Class A operation.
--------------------------
Buy Chinese. Bury freedom.
"What you're reciting is the modern use of Class A in an audiophile implementation. It carries an emphasis on reduced distortion - at the expense of output power - that is not a component of the original definition of Class A operation"
But can't it be seen as the natural extension of the original intent?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It seems reasonable to call this a refinement of Class A operation for modern use. It is otherwise unrelated to the intent of those who created the classification.
--------------------------
Buy Chinese. Bury freedom.
The term, 'driver' used to be assigned to the stage preceding the finals is set up to deliver grid current. Now it is just the stage preceding the finals. Not an improvement in language use IMO...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Even without grid current the Miller capacitance still needs to be driven and that takes some current.
Just saying.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
or the essence of what you say as being typically the main attributes of Class A. But taking a line here or there out of a handbook is out of context in that they are not the lines that define Class A, they are merely attributes typical of the class and its configuration as applied to SE power amps in most of your examples.
But Class A is a definition applied to all amplifiers and (for example) there is no way that it's necessary to set an operating point anywhere near "one half of the cutoff voltage in a low signal application to achieve performance worthy of Class A even by your high standards:)
The best way I can get my point across is to ask you this question. If I design an amplifier that under normal operating conditions remains in conduction with no grid current, but I do not set the operating point anywhere near the "midpoint" or "the most linear part of the curve" yet achieve the main goal (even using your argument) of low distortion, how would YOU classify it?
Dave also made a valid point about clipping. Does this means that any amplifier capable of overload cannot be classified as Class A?
Naz
"The best way I can get my point across is to ask you this question. If I design an amplifier that under normal operating conditions remains in conduction with no grid current, but I do not set the operating point anywhere near the "midpoint" or "the most linear part of the curve" yet achieve the main goal (even using your argument) of low distortion, how would YOU classify it?"
I think I would just ask you why you didn't set the operating point at the midpoint.
I answered Dave.
A Class A amplifier driven beyond the linear part of the curve is a Class A amplifier that is being abused. Why would anyone want to do that?
I think this horse is dead.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
What Class is it in, it must be in something?
I can give you many reasons why I may not bias for the midpoint in a low signal amp and I'll bet you may too from what I've read.
I also don't appear to have been clear enough in making my points re plate curves and their "apparent" most linear part or CCS loading, which renders the point almost superfluous anyway ... sigh!
Anyway, it seems that we can't all agree on this and I respect everyones right to hold their own beliefs.
Naz
Don't take this personal but I would call it a poor attempt at Class A.
"I also don't appear to have been clear enough in making my points re plate curves and their "apparent" most linear part [see graph above, it's pretty clear to me what part is linear and if you move too far from the center it's not so linear anymore] or CCS loading, which renders the point almost superfluous anyway ... sigh!"
A horizontal load line won't help this tube much. Draw a horizontal load line and then derive a dynamic curve. It's going to be pretty curved with no straight part. (maybe a little between -14 and -18) The spacing starts large on the left and gets skinnier as you go to the right.
"Anyway, it seems that we can't all agree on this [that's clear] and I respect everyones right to hold their own beliefs."
I see some of those beliefs as flying in the face of what is explained in the technical books.
Oh well.
Take care. Nothing personal......
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
> > Don't take this personal but I would call it a poor attempt at Class A < <
Class A none the less?
> > A horizontal load line won't help this tube much < <
Well, actually it will but that's not the point. Are you saying that this tube can't ever be be run in Class A?
Have to be very careful with curves as they can look much worse depending on the scale. Believe it or not the curves look much the same if we expand the region 300-400V up to a max of only 5mA.
Cheers!
Yes, still Class A but not one that follows any of the examples in the books.Is there something wrong with my eyes?
For a 8 volt peak grid swing I get a plate swing of 142.5 one way and a plate swing of 122.5 the other and then there's the problem of the plate resistance increasing as we go to the right.
Let's try this instead.
I get 120 and 125.5 and the plate resistance hasn't changed much at all.But in the end you win. Your example is Class A but bad Class A.
Why in the deuce would we want to spend time talking about a bad Class A?
What would be the point in that?
I pledge from now on I will try to preface my talk of Class A with the word "good" if you will try to remember to preface your talk of what I call bad Class A with the word "bad". OK? :-) (with the understanding the "bad" just means that it doesn't follow the examples in the books and "good" just means that it does)
Or how about when you see me talking about Class A just remember that when Tre' talks about Class A he's always meaning Class A done the way the books show it (silly Tre').
So now we've come full circle. The book Micheal posted the link to says, on page 68 that a Class A amplifier is "An amplifier operated as shown in fig. 306 in which the output waveshape is a faithful reproduction of the input waveshape, is known as a Class A amplifier."
Fig 306 is for all intents and purposes the same as the figure below.
You'll just have to excuse me, I choose to stand on that.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/19/16
I accept that linearity varies from one tube to another. My only points are that linearity will always be better by varying degrees with a more horizontal load line and that to the eye, scale can be deceiving when viewing plate curves.
On the latter, here's a link to a graph of plate curve for a 6SN7 at low voltage and low plate current taken from Stephie's Site. It's not what I was looking for but it will tell the story. This region on a larger scale would normally look non linear. You can Sim curves to explore the effect yourself.
It's clear that good linearity can be gained at many different operating points where power is not a prerequisite.
Naz
"It's clear that good linearity can be gained at many different operating points where power is not a prerequisite."
I think I've left myself misunderstood.
While I think choosing the most linear operating condition is in keeping with the intent of Class A that is a different thing than choosing not to bias in the middle of whatever dynamic curve your chosen load impedance provides.
The sim tool linked below can show you what I mean.
The outside left arrow will change the load impedance and idle current.
The inside left arrow will change the load impedance only and the top white arrow will change the bias point (quiescent plate voltage and current) along that load line.
Note, keep the yellow arrow (grid swing) somewhat minimized to start with or it will limit what you can do with the other controls.
Watch the HD in the readout below as you make your adjustments.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
that there can be many operating points that will provide Class A operation unless max power or swing is a prerequisite.
Whilst I agree there will always be an operating area of lowest distortion, equally you could pick an area that would normally provide slightly higher distortion but achieve better overall results by making the load line more horizontal as your Sim will show. Great tool BTW!
So for clarity I'm saying that the operating point cannot be singled out in the definition of Class A because several factors dynamically interact, meaning that similar distortion characteristics can be obtained at various operating points, within the limits of swing and power we are designing for of course.
To include subjective statements like lowest distortion into the definition would be absurd. Eg, in an SE amp one of the biggest factors determining distortion is the load line so how would one determine whether an SE amp met the criteria of Class A by your expanded description?
Anyway, one thing we both agree on is that best practice should always be used in order to achieve the very best result. So I'll leave it at that.
Thanks for a civil discussion that I'm sure will be of some benefit to some others, particularly newbies.
Cheers
"A preamp with only minimal swing requirements could happily run Class A at numerous operating points along the load line"
So let's take a V8 motor and fill two cylinders on one side with concrete. Now it's a V6, right? It certainly meets the minimalist requirements some here are attempting to apply to amplifiers.
To your comment - no one is discounting a reasonable modification of the operating point as being included in the classification. But operating an AB1 power amp so it never goes to cutoff doesn't qualify. That's not reasonable. An amplifier must be operating near the midpoint of its curve to be Class A. That description is sometimes omitted for the sake of brevity, but it's just as important to the definition as any of the other requirements. Anyone who doesn't like this criteria is welcome to hold a séance and argue the point with those long-dead engineers.
--------------------------
Buy Chinese. Bury freedom.
If a V8's Management system turns of 4 cylinders when not needed what would YOU call it?
It's mode of operation at any given time is what describes it most accurately but only while it is in that mode.
We can argue endlessly about the engine's configuration, capabilities, where it spends the most of it's time etc etc and that's one of the problems ... or opportunities for marketers:)
To me, the engine example is akin to hardware and software/firmware in a microcontroller. Another analogy is SE and PP. This is the configuration, (the hardware, V8 or V6). But PP could run in various classes either permanently or conditionally, akin to the programming in the V8 that can turn cylinders off.
Naz
Your V8/V6 analogy with cemented cylinders is pretty weak. A much better one would be a V8 with cylinder deactivation technology. In this case one could ask the question: If the engine is never pushed hard enough to activate the last two cylinders is it a V8 or a V6? . I know how most would answer this question.
Regarding classes, you state operation "near" midpoint. What is "near"? How far can one deviate from near before it's no longer A? The problem with these definitions is ambiguity and there seems to be a reluctance to address it.
Good thing the question wasn't, 'when is a V8 a V4?' because the side, and entirely correct answer would be when Honda builds one to avoid the cylinder count limit...32 valves, 8 connecting rods, 8 spark plugs and 4 combustion chambers...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
intensional definitions
extensional definitions
demonstrative definitions
precising definitions
ostensive definition
"In classical thought, a definition was taken to be a statement of the essence of a thing."
Just like there is more to the term "definition" than might meet the eye, there is more to Class A than just not reaching cutoff.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"The problem with these definitions is ambiguity and there seems to be a reluctance to address it."The ambiguity is minor and has no impact on our understanding of the definition. There was no need to address it 50 years ago, and none now. Only those who want to materially change the fundamental meaning and intent of "Class A" have a problem with this.
--------------------------
Buy Chinese. Bury freedom.
Edits: 08/18/16
If the intent is to operate the tube in as linear a way as one can then the details aren't all that important.
If the intent is to operate the tube in the non-linear cutoff region while still claiming Class A because "the current never cuts off" the whole intent of Class A is being ignored and some of the potential benefits of Class A (over class AB) are being thrown away.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I have to agree with Naz's line of though here. Adding ambiguous subjective terms to a technical definition is poor practice typically imposed by the marketing division.
All of the semantics being bantered about here are somewhat pointless among those educated in the field but can be counterproductive when put in the hands of someone new to the art or someone with an agenda. It reminds me of the Class A SE Fuzz Box I have been working on :-)
dave
~!
The Mind has No Firewall~ U.S. Army War College.
OK, not so much anymore. ;-)
Not meaning to be contrary, just curious.
Cheers,
91
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Bet Mr. 303 is referencing the political sense of the word. Most amateurs I know are politically conservative. The conservative's currently popular term for liberal is "progressive".
Yeah, you may be right... or perhaps left :^)
Being from a different continent and not hanging out with audio amateurs (or pros) it was totally lost on me.
Cheers,
91
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Lots of info on old transfomers
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