Pio1980
AK Member
Credit Alan Blumlein for the tapped OPTs and long tail pair.
Examples of Class A push/pull
- Thread starter Donnie2112
- Start date
Donnie2112
New Member
Wow. A lot of this is over my head but I appreciate the responses.
I’m still learning about electronics, but I’m having fun doing it.
I got a Scott 299 right here that I’m listening to. I’ve read some things about this amp, some is probably true, some my own misunderstanding. So I’ll try to explain terms as I understand them so somebody with more knowledge can maybe correct me.
The Scott has 4 6BQ5 output tubes. So that’s push and pull for each of the two channels. My understanding of Class A is if you look at a sine wave, one of the tubes will be pushing above the 0 reference line of the sine wave, the other will be pulling. But they are both operating on both sides of that sine wave signal.
My understanding of class AB is that the same is true up to a point. After a certain voltage there will only be the tube on one side of the output device handling the signal.
So first of all, I’m assuming this here fine HH Scott amplifier is a Class AB, but I’ve not seen that written anywhere either.
The other thing I thought I understood was that these old Scott amps were biased on the hot side. Does this mean that both tubes in the power section follow that sine wave further to the opposite side of that 0 reference line?
If that’s so, does that mean that my if I keep the amp turned down lower, or offer it less signal, that the amplifier is going to remain essentially in Class A? Which is to say, both tubes will be pushing or pulling the signal the whole way positive or negative?
I hope my question makes some semblance of sense.
Thank you all.
I’m still learning about electronics, but I’m having fun doing it.
I got a Scott 299 right here that I’m listening to. I’ve read some things about this amp, some is probably true, some my own misunderstanding. So I’ll try to explain terms as I understand them so somebody with more knowledge can maybe correct me.
The Scott has 4 6BQ5 output tubes. So that’s push and pull for each of the two channels. My understanding of Class A is if you look at a sine wave, one of the tubes will be pushing above the 0 reference line of the sine wave, the other will be pulling. But they are both operating on both sides of that sine wave signal.
My understanding of class AB is that the same is true up to a point. After a certain voltage there will only be the tube on one side of the output device handling the signal.
So first of all, I’m assuming this here fine HH Scott amplifier is a Class AB, but I’ve not seen that written anywhere either.
The other thing I thought I understood was that these old Scott amps were biased on the hot side. Does this mean that both tubes in the power section follow that sine wave further to the opposite side of that 0 reference line?
If that’s so, does that mean that my if I keep the amp turned down lower, or offer it less signal, that the amplifier is going to remain essentially in Class A? Which is to say, both tubes will be pushing or pulling the signal the whole way positive or negative?
I hope my question makes some semblance of sense.
Thank you all.
Donnie2112
New Member
Well if that's the case then I imagine I already know what a Class A push/pull amp sounds like, because with my large, sensitive speakers, I can't bear to turn the thing much past 4. It gets very loud.
Donnie2112
New Member
Thanks for that. I think I understand a thing better now.
PakProtector
AK member
The simplest way of properly using the language is to remember, they are mutually exclusive. As in an AB biased amp( through the interaction of plate dissipation limits, load, and idle current ) is not going to make and class B power, nor will it make any A power. A Class A amp for sure will make no class B power...unless you throw out all the established definitions of terms and start writing your own. Call Lo-Moo-Triode, become a Medwin-ite and get a first class lesson if you wish to expand that particular skill set...
A is one thing, AB is another. The definition of A is not a single, 'both have to be conducting'. It is a biased in the middle of their curves, *ALWAYS* conducting, even at full power. There is good reason this works better. It is harder on tubes( don't care, have plenty), makes less power for the nameplate( don't care, speakers are quite efficient enough). Puts out more heat than output power( don't care, basement stays cool ).
Now why would the marketing departments let anybody build such a thing. Sure does not happen too often, even when 'the marketing department' is just the Builder/Engineer/Service Technician's spouse.
The Marketing claim is repeated all over the web. It is as divorced from reality as is possible. It is repeated quite often though...
and sometimes even referenced as 'proof' in discussions.cheers,
Douglas
ps...loved the 'make a class AB amp into Class A by claiming the Engineer's Mantle and turning down the volume'....RAOTFLMAO I can tell you. I am going to have to remember that one when I need a humorous engineer story.
Retrovert
Addicted Member
Ummm, not to be snarky, but probably not if you're relying upon the gibberish above posted.
Douglas is completely correct that the utter nonsense being bandied about is not grounded in electrical engineering, let alone reality. If he states that something is true, false, or, for that matter, ridiculous, I suggest you listen.
Let's be clear: conduction angle (the difference between the various amplifier classes) has nothing whatsoever to do with gain (over-simplified to volume level), and the crossover distortion experienced during the zero-crossing, as seen in Class B but not in Class A (for obvious reasons), is completely a separate issue from gain (again over-simplified to volume level). This is not merely first semester of Amplifiers 101, it is the first day of the course.
To learn how a tube amplifier works I suggest obtaining a copy of any basic book on tube amplifiers. Such materials are available as free PDFs. You can look at the diagrams for Class A vs. Class B, Class AB, and even Class C (used for radio transmitters), understand the effects upon power output and the output transformers, and get a good grasp of what's going on. Books like RDH4 will then become more comprehensible. The barrier to entry is not great, but it does exist.
I suggest beginning with the books prepared by the Army during WWII to train soldiers in radio basics to create the necessary volume of repair technicians. The books assume no prior knowledge of electronics. Ten hours of study should give you a good grasp of the basics, and you may build on that instead of unlearning pernicious nonsense posted in online forums.
PakProtector
AK member
Don't forget Telegraphy and Telephony ratings...
Retrovert
Addicted Member
Blumlein is the Nikola Tesla of audio and television.
If only he hadn't been criminally killed by incompetence and stupidity the world of television audio would have been very different, and WWII would have sooner ended.
Pio1980
AK Member
Also elegant, with a simple direct signal path. Many mfgrs used a similar topology, but few did it with the high grade care of Marantz.
Original unrestored 8B.
Donnie2112
New Member
Your idea about teaching myself about the electronics is smart and I’ve been planning on stacking some electronics books on my reading list.
Anyway. I think I understand what you’re saying though.
So basically, we go back to this push pull amp, you’ve got 2 tubes, one has the top half of the sine wave, the other the bottom. What you’re saying is that in a Class A/B that each tube extends a portion below/above that reference line regardless of how much current is applied?
So they will extend half as far if there is half as much gain? It would be easier to draw what I’m trying to say but hopefully it comes across.
Retrovert
Addicted Member
I suggest going to primary sources for this information as you'll find a detailed explanation with diagrams. Requesting a from-scratch reproduction of such information is quite time consuming, and the approximations and summaries are never worth the fighting and quibbling which ensue, nor the wasteland of a locked thread. Certainly not when definitive explanations, embodying many tens or even hundreds of hours of work by authors and reviewers, exist.
As we have seen, many non-engineers have no clue about the theoretical or practical differences between amplifier classes, including differences in efficiency, differences in output transformer design and implementation, difference in loading on the power supply, ability to create a push-pull amplifier using Class A, etc. etc. etc. Instead, the non-engineers quibble and argue and post utter nonsense and generally muddy the water to the point that it's all noise and no signal.
I instead suggest turning to any of the standard learned tomes, as these will adequately explain the classes in great detail, with diagrams. Even Wikipedia would work:
The Army manuals will similarly explain the differences in a comprehensible way with full diagrams. From there you can move to the more technical sources like RDH4.
I suggest avoiding internet forum discussions about amplifier classes given the massive amount of nonsense which may not be easily refuted. One must remember:
Brandolini's Law on the Asymmetry of Bullshit: the amount of energy needed to refute bullshit is an order of magnitude bigger than to produce it.
Harsh words, Retrovert!
.... but I must agree with the contents. So - but you and I are also part of the "internet forum discussions" as are DF96 and other knowledgeable persons! Let one then say to not accept internet discussions until confirmed by peer information.
But 'much of a muchness'. At least sound advice; even to this day I follow that as well!
.... but I must agree with the contents. So - but you and I are also part of the "internet forum discussions" as are DF96 and other knowledgeable persons! Let one then say to not accept internet discussions until confirmed by peer information.
But 'much of a muchness'. At least sound advice; even to this day I follow that as well!
Retrovert
Addicted Member
Well, It would certainly be a great deal of effort to produce a summary of the issue equal to a fraction of that set forth in even the most basic book, let alone the learned tomes, and the quibbling and arguing would be epic.
Best to directly drink from the fount of All True and Accurate Knowledge than from forum discussions, as these are oft contaminated with the intellectual equivalent of raw sewage, chemical waste, or the Andromeda Strain.
Fair enough. Except that the masses are, alas, very often known to be considerably wrong, as history unfortunately proves on a routine basis. The peer views of geocentric universe, flat earth, luminiferous aether, spontaneous generation of life, bleeding, purging, tetraethyl lead, tobacco, cholesterol, and that mayonnaise is somehow a food (as if!), to name merely a few of the popular delusions, were all once the law of the land. We now know better.
Yes, diyAudio is a wonderful place, and the discussions, debates, and duels between skilled engineers are generally a delight to behold, educational to read, and ignored at one's peril. You and I are readily able to evaluate the highly technical, and often cryptic to outsiders, arguments and counter-arguments proposed, demolished, reformed, and otherwise set forth; most unfortunately cannot perform such analysis, and a novice will certainly have a great deal of trouble, as was above seen. Engineers, furthermore, typically elide the basics, sometimes even the advanced material, because we don't need to discuss the equivalent of the four elements before talking about more advanced topics in materials.
As an alternative to forum banter, I instead suggested two straightforward, uncontroverted, and understandable sources of information (Army manuals and RDH4) along with a less rigorous discussion for an overview of the subject (Wikipedia); vanishingly few internet discussions attain such levels of clarity, cogency, or content. Any summary of such information about amplifier classes would reduce its divinity, and thereby introduce errors, omissions, or confusion.
Plus the endless arguing would be prodigious and time-consuming; such effort would clearly be better spent arguing about sonic bliss from exotic mains cables, the benefits of gluing crystals to transformers, or why the shape of a getter can smother electrons in their crib. (I, for one, welcome our new Crystal Cable Overlords.)
Earlier this year I had previously assembled from my packrat collection of eBooks an archive of PDFs for basic texts in All Thinges Tubes and distributed it for a few weeks via Dropbox. If anyone has interest in such documents, let me know and I'll see if I can resurrect it or, if not, recreate the equivalent.
Bill Ferris
Lunatic Member
Well, FWIW. Though, not a "HiFi" class A tube amplifier, the sixties original, and later on, the reissue Vox AC 30 is a true class A PPP guitar tube amplifier, and is considered by a lot of very good players worldwide, to be the "HiFi" of vacuum tube guitar amplifiers..
Quite a few of those amplifiers have caught on fire from just the wooden cabinet which has been cooked by 4 EL84s and a GZ34 rectifier to combustion temperatures..
But, in decent shape and operating correctly their non-overdriven tone is bell like..
Carry on Folks, just some noise from me, after enjoying the reading of this complete thread, up to this point.
Quite a few of those amplifiers have caught on fire from just the wooden cabinet which has been cooked by 4 EL84s and a GZ34 rectifier to combustion temperatures..
But, in decent shape and operating correctly their non-overdriven tone is bell like..
Carry on Folks, just some noise from me, after enjoying the reading of this complete thread, up to this point.
PakProtector
AK member
Bill Ferris
Lunatic Member
Well, please enlighten me Sir.
I have reworked many of the original versions, and some lesser work on the reissues.
And every piece of audio gear that comes across my bench sees a scope/Tek THD+N/IMD distortion analyzer.
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