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In Reply to: RE: Thinking about running my non-OTL Circlotron again posted by Tre' on August 23, 2011 at 19:26:49
There is no way around this simple truth.
Yes- unless you simulation does not reflect the actual circuit. I have to assume that you have a parameter wrong somewhere. In the old days the simulators themselves had obvious problems - like the model for the triode - but these days I tend to assume that they are reasonably correct.
We covered the A1/A2 thing ad nauseum in the past. We even came to some resolution on the matter- now you seem to have reversed yourself. Tre, I have to point out that when you come to this forum and make your post with the 'better sound' comment included, its obvious to everyone that you are looking for a fight. Since that is the roots of your intention, you will not command much credibility without admitting it.
Follow Ups:
"There is no way around this simple truth.Yes- unless you simulation does not reflect the actual circuit."
Are you trying to say that a tube can conduct in the negative?
This has nothing to do with simulations and it has nothing to do with me or my intentions.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/24/11
Yes- unless you simulation does not reflect the actual circuit."
Are you trying to say that a tube can conduct in the negative?
No, and no idea where you got that idea either.
You said that there was no "flat spot at the bottom of the curve" and maybe there isn't.But the lower half of the waveform would have to be truncated at anything above 5 watts for a M-60.
Majorly truncated at 60 watts.
Whether it's flat or rounded makes no real difference.
The main point is that it's not the same shape or size as the top half of the wave form, it can't be without the tube being able to conduct in the negative.
In a real Class A (1 or 2) circuit both halves of the wave form are the same (or very close to it).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/24/11
Tre': So what term would you use to describe that particular assymetrical waveform above 5W output (because we know that an M60 makes more than 5W)? Is that waveform a hallmark of Class AB or B operation? I do not think so, but enlighten me. Thanks.
If the tubes don't completely cutoff some say that it's Class A.That's what the books say.
But if we read the rest of the chapter we find that there is more to Class A operation than that.
If we judge the class of operation by what is really happening I would have to say the amplifier is "acting" like a Class A/B amplifier.
Please understand that a Class A/B amplifier does not operate "like" a Class A amplifier up to a certain power level and then operate "like" a Class B amplifier.
Class A amplifiers are biased and loaded and driven in a certain way.
Class B amplifiers are biased and loaded and driven in a certain way different than a Class A amplifier.
Class A/B amplifiers are biased and loaded and driven in a certain way
that is different than either a Class A or a Class B amplifier.The three classes of amplifiers are distinct and not to be confused with one another.
What the "A" portion of a Class A/B amplifier has in common with a Class A amplifier is that the current can and does go down by the same amount (depending on the linearity of the tube used) that is goes up, giving a symmetrical wave form.
As the power level goes up the lower half of the wave form gets truncated.
The higher the power level the more (as a percentage of the total) truncated the lower half of the wave form gets.
This is exactly what happens in an M-60 above about 5 watts.
BTW most tubes are not linear near cutoff. The bias can go more and more negative but the current does not go down in a linear way.
This is called the cutoff region.
Once this behaviour starts the tube is said to be "in cutoff".
There are a lot of tube that "refuse" to cutoff completely without an extreme amount of negative bias.
Using one of these tubes does not make a Class A/B amplifier a Class A amplifier.
A properly designed and operated Class A (1 or 2) amplifier should not reach the cutoff region let alone total cutoff.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/24/11 08/24/11 08/25/11
Wrote the above probably at the same time you were posting your last two posts.
By the way, you guys have had this same discussion here within the past few years, and you used the very same figure to support your argument, which is fine.
" By the way, you guys have had this same discussion here within the past few years, and you used the very same figure to support your argument, which is fine."Yea, I did use that same figure.
That figure fits perfectly the full description given in all the books of what Class A operation is.
While it is true that a tube being operated in Class A does not reach cutoff that is not the end of the definition of Class A operation.
That figure is.
BTW Lynn Olsen refers to it as "deep" Class A. I and the books just call it Class A.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/24/11
This is from the MosFet Circlotron pdf from the link you gave.Micheal Rothacher understands Class A.
"How much bias current will we need? This amplifier will be conservatively rated for 20 Watts @ 8 Ohms in Class A, so our peak output current will be 2.24A (1.414 * SQRT(20/8). So, we’ll need to set the bias for each output MOSFET around one-half of this value, and we’ll round it up to 1.2A DC."
This is what can't be done in a Circlotron using 8 6as7g's driving 8 ohms above about 5 watts.
One cannot set the idle current at half the needed peak output current to make 60 watts into 8 ohms.
At the idle current normally used in a 8 tube Circlotron OTL the current can only be driven down (as much as up) to an output level of about 5 watts.
If driven to an output wattage above that the "flat spot at the bottom of the curve" starts to show up.The current in Micheal's output devices can be driven down as much as they are driven up, to the full rated 20 watts, with out creating that "flat spot at the bottom of the curve".
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/24/11
(sigh...) MOSFETs lack a grid current window...
You are confusing the Class A1 vs. Class A2 issue with the linear operation issue again.
Linear operation is indicative of Class A (1 or 2) operation.
Non-linear operation (truncated lower half wave form) is indicative of Class A/B or Class B operation.
Just because you can and do run the M-60 into A2 does not mean that the wave forms can get non-linear (non-symmetrical) while still claiming Class A operation.
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."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
You are confusing the Class A1 vs. Class A2 issue with the linear operation issue again.
I'm calling you out on the bullshit (again)- I have no such confusion nor ever did.
The diagram you show is for Class A1. I've seen it many times.
Tre, its obvious to me that I offended you sometime a long time ago as you have had an ax to grind for the last ten years or more. I don't know what I did you offend you, and you apparently are not man enough to talk about it. But one thing is sure- it has nothing to do with audio- it has to do with some human interaction of which I have no idea.
I've apologized for whatever it was in the past, as I have had no intention of offending anybody, but you persist like a rash that won't go away. Again, the persistence has nothing to do with electronics. Its something personal. At this point I don't see the point of arguing with you further as you have also persisted in logical fallacies like the Strawman in an attempt to make me wrong. At this point you have also seemed to have a short memory as well, obviously on purpose, going back through the archives will sort that comment out.
At this point you now owe me an apology. Plain and simple. Otherwise, get the hell out of here and don't come back.
Now you should know as everyone else does that I am simply not known for comments like this in over 20 years of the Internet. Its not that I am mad, its that you are being an ass.
"Its not that I am mad, its that you are being an ass."
I have re-read my posts to you in this thread.
I do not see where I have been an ass.
I have only talked about electronics except one instance where I claimed that you were confusing two different points.
I don't think that rises to the level of "being an ass".
You are, of course, welcome to your opinion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Maybe someone could do an actual experiment, hooking a scope up to an M-60 running at high power but below clipping, and get an actual plot of the current flowing through the tubes? This would be quite instructive, and it could eliminate the debate about whether the simulation is accurate or not.
Chris
Because of the nature of the circlotron circuit, no matter where you hook up the scope you will be seeing the combined waveform from each bank of the push pull. The whole circle is part of a "feedback loop" if you will.Even though each bank's waveform is asymmetrical the combined waveform looks fine. This is normal for Class A/B amplifiers and for Class B amplifiers as well.
To look at the waveform of one bank will not be easy and to look at the waveform at a given power level will be harder.
If you pull one bank of tubes you will be able to see the asymmetrical waveform but you will have changed everything in terms of the feedback and loading.
I have no problem at all understanding that a tube idling at 60ma. can only reduce it's current by 60ma.
If the tube is driven to a current more than twice it's idle current then the lower waveform will be truncated.
If the current is increased to 200ma, that 1/2 current waveform will be of a size that represents 140ma. and will be much larger than the following 1/2 current waveform that can only be a 60ma. current waveform, at best.
200ma. per triode, 8 triodes per bank, one bank is cutoff or close to it.
(however much current is flowing in the other bank will subtract from the output power)200ma times 8 is 1.6 amps peak.
1.6 amps peak across an 8 ohms speaker is 10.24 watts RMS.
With do respect to all parties, I don't really need to do an actual experiment.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/26/11
I am assuming one would insert a very low value resistor in series with the cathode of one of the tubes, and put the scope across that. This would give a direct view of the current flowing through that tube.
I agree that the outcome seems inevitable, since there simply isn't room for anything else to occur. I just feel that one clear trace showing the waveform of the current flowing in a single tube would put a stop to the endless debating about whether it is or isn't class A.
What seems a shame in all this is that it detracts from the much more important point that it is a beautiful amplifier that apparently sounds superb.
Chris
The amp already has the resistors mentioned. So its been easy enough over the years to measure this,
A2 trades greater power for linearity. If the tubes were to cut off without the amp clipping, the result would be a class AB2 amplifier. This is explained in the FAQ section of the website and has been there for over a year.
"I am assuming one would insert a very low value resistor in series with the cathode of one of the tubes, and put the scope across that. This would give a direct view of the current flowing through that tube."I'm not sure it would. I think AC current flows in a circle that includes all the tubes. I could be wrong about that.
"I agree that the outcome seems inevitable, since there simply isn't room for anything else to occur. I just feel that one clear trace showing the waveform of the current flowing in a single tube would put a stop to the endless debating about whether it is or isn't class A."
I couldn't agree more.
"What seems a shame in all this is that it detracts from the much more important point that it is a beautiful amplifier that apparently sounds superb. "
Yes it is a shame and the amplifier does sound great. It's just not Class A in the true sense.
Transcendent Sound, makers of OTL amplifiers, makes it clear that their amplifiers (and most other OTLs) are Class A/B.
The paper from Transcendent, while using some language that I don't totally agree with, makes the same points.
A Class A amplifier needs to idle at 1/2 the total needed peak current [that's 1/2 per bank. If you add the idle current of the two banks the amplifier needs to idle at the total needed peak current].
It takes 3.86 amps peaks to make 60 watts into 8 ohms so there needs to be 1.93amp idle current per bank or 241ma. per triode.
That just can't be done with a 6as7g.
Thanks for your input.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/27/11 09/20/11
Hi Tre,
I don't know why you are trying to stir the pot about what Ralph has said about his amplifiers. I went back in the archives about this subject and it seems that Ralph has always made it very clear about what he is doing. You're implications by bringing up this subject every year or so is that Ralph is misleading us about something and by doing so, you are trying to impune his credibility and reputation. I have know Ralph for 20 years and he has never mis-lead me in any way. On the contrary----if anything, he always takes the conservative approach.
What I find amusing--is that when Ralph calls you on it. You simply respond that it's all about the science. I think that if there is any deception going on---it's on your part by your extremely weak ( it's all about the science ) response.
I must admit that I'm an avid Atma-Sphere user because the equipment just sounds exceptional in all areas. The icing on the cake is that the support that Ralph and his staff give is even BETTER !!
I have the money to just about buy anything I want and over the years I have gone through allot of equipment to come to a point where by comparison----the Atma equipment just sounds better and is even more reliable.
My only assumption is that you are one of those guys that just want some attention and like to get it by posting on this forum. But by doing so it's at the expense of one hell of a nice gentleman that has devoted his life to his public and his company.
I also think you owe Ralph an apology but I really don't think you are man enough to do it. We will just get some weak response about it ain't personal and it's about the science.
Looking at the Atma-Sphere website pages for the S-30, M-60, MA-1, MA-2 and MA-3 the text in all cases says "Class A operation".But that's not true.
Tremaine Audio-Cyclopedia 2nd Edition
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."
None of the output tubes in Ralph's amplifiers operate this way.
What do you think I should do? What would you do?
Way back in the days of the ASOG forum this question came up and was never answered, at least not to my satisfaction.
Ask Kurt Strain about this issue. Paraphrasing Kurt "Tubes almost never cutoff completely. If a tiny leakage current is all it takes to call an amp Class A then all Class A/B amplifiers are Class A."
I like Ralph and I wish him the best.
My main focus here is to educate people.
I think it is Ralph who owes an apology to everyone who has read his website that says his amps are Class A, when they are not.
The link is to a very short paper I wrote years ago.
P.S. Challenging a claim or theory, in a scientific way, is a normal part of the scientific process.
Defending a claim or theory, in a scientific way, is a normal part of the scientific process.
But Ralph doesn't do that. He changes the subject. He tries to make it about me. ETC.
What would you have me do? I think I stayed on subject and just argued the science without getting personal. What else can I do?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/26/11 08/26/11 08/26/11 08/26/11
Okay, here's everything I know and stated better than I can. John Broskie is right. He wrote that when he also went in and explored the claims. I'll back up John on his essay.But I have an additional comment to John's article. When you use SPICE modeling, it is a small signal condition applied for rp, ip, and vgk calculations. It does large signal harmonic distortion calculations based on 6AS7 triode plate curve modeling, but not for these other parameters as curves for them are not provided in the model.
Therefore, the output impedance calculations are for small signals only and that will operate in the zone of pure class A operation, about a couple watts maximum output. That means that actual output impedance for more signal level than that is significantly higher.
-Kurt
Edits: 08/30/11
but unfortunately that URL only contains the first page of Broskie's treatise on the circlotron. I read the complete piece once upon a time, and I must say I could not/did not follow his argument about circlotron = totem pole (the Futterman topology). I would like to re-read it and try again.
Paul Speltz once made a case that is the exact opposite of what you state. His conclusion was that as power output from the circlotron increases, its output Z goes down. I printed out his post, possibly from the "Tweaks" secion of the old ASOG website, and I saved a hard copy, but I don't have an e-version to post here. I do recall that his analysis was based on his results in driving a signal backwards into the output of his own M60 amplifiers and observing how the signal could "push the output stage around" at different amplitudes. Maybe I can scan it and post it, or maybe Paul is out there somewhere.
But anyway, Tre' has not been making any claims as regards output Z. He is talking about Class A operation, or not.
It is possible to resistively inject a small signal to extract the small signal output impedance (with some error) as that will not disrupt everything.It is not possible to obtain impedance information by messing up the output stage's normal operation by jamming into it a very large signal. The fact that it takes it is a good thing. But that's ruining the amp's own efforts and it is as if, for an analogy, in particle physics he has the worst heaviest probe to measure position while that completely ruins any knowledge of it's velocity.
But this whole thing about all the old claims would never have been an issue if the claims were 100% in line with classic definition, accurate study, and world class measurement techniques including having guardband.
Ralph would never yield to those who already knew what they had in an M60 amp, and so that is a problem with the people who know test and measurement. It makes us want to scream that someone gets to claim something others will not due to better adherence to old definitions and careful factual argument.
There was no personal issue until Ralph saw people who don't like his sloppy methods as some kind of troublemaker. No, they do want accurate information spoken. John Broskie wrote that sarcastic article after losing it when encountered with no yielding to the facts of electrical engineering.
But we CAN as consumers brush it all aside and not give a hoot! The amp is a fantastic performer. It's real measure is how it sounds, and how reliable it is. I can recommend M60 amps like the one I had.
Edits: 08/30/11
Thanks, Kurt.
I really am not qualified to argue Paul's thesis, pro or con. Your post suggests to me that you read his original contribution on this subject in the Tweaks section of the old ASOG website. Is that the case? If not, please don't assume that I accurately described his rationale or his method. That would be unfair to Paul. Anyway, I am a scientist, albeit a biological one, every work day of my life, so in audio, which is "just" a hobby, I feel it is sometimes OK to adopt the Alfred E Newman philosophy: "What? Me worry?"
But just to satisfy my other self, can you describe the rationale that leads you to say the output impedance of a circlotron goes UP as the power output goes up?
To be fair to Paul, this is several years old, and I have no idea whether or not it reflects his current thinking. ("Current" being the operative word.)
Paul did a fine job in his article you showed us. Basically his 11 ohm result is what I also got. And he also explains that this number is for one condition and that it moves from there.The output impedance change is directly related to the lack of class A operation going on. At 0.6 amp quiescent bias, the amp operates class A to 1.44 watts into 8 ohms (0.6 amp peak current into 8 ohms). During this time, there are two tube "banks" (phases) driving the output impedance, even if Paul is not fully convinced.
When at max power, 60 watts into 8 ohms, the peak current has to be 2.74 amps for one phase and 0 for the other. When operating in this outside zone, only one tube bank contributes and the output impedance now comes from the cathode follower impedance of one phase bank of parallel 6AS7s. I don't have that formula right now, but it's easy to look up.
The 6AS7 is not fully linear, and so when one bank cuts off then the Zout is moving with the changing plate curve point for it. And this might help it or hurt it, depending. But for sure it has left the help of the other phase.
So the output impedance slides continuously over many changing values instantaneously during a full sinewave signal cycle.
There is instantaneous and average full cycle output impedance. Most people measure average on meters, few will do the complete job and measure the full plot of instantaneous output impedance over phase and over varying loads. This is the problem you get into when you try to get one number with highly assuming assumptions about the measurement that is not fully understood. Small signal offers a decent complete number but large signal is nonlinear in many ways, especially in this case, and there is no real good measurement technique for that, although Paul tried hard.
I hope this helps explain things.
Edits: 08/31/11 08/31/11
Isn't that approximated by the term 1/Gm? If so or if not, how to combine the two terms (the foregoing plus the output Z of the bank of output tubes that is not in cut-off) to calculate a net output Z for the amplifier?
1/gm is another small signal parameter. gm changes when you move the operating point's position from one place to another. Thus the output impedance must change.
The problem is that most want a single number in ohms for the average, not a graph of the output impedance shifting with phase of an inputted sinewave. The same goes for output power. People want the average power outputted over the full cycle, not a plot of instantaneous power. We know it's a number with large signal distortion involved.
The issue here is: can we calculate and then measure the average integrated impedance over the full cycle at every power level, knowing the larger signals are more distorted? Math can calculate almost anything once you get the idea of a good method.
It's really quite a complex effort, and it would need a DC element input for the large signal Rout=Vout/Iout when stopped there, and then add small signal wiggling sinewaves for the small signal zout at that position. Then move around 360 degrees in "DC" steps measuring all small signal output impedances at those positions. Then integrate for a mean value and check the standard deviation while you're at it. This is then the mean Zout with a now known distorted deviation.
how many manufacturers state in print that their amplifiers are "Class A", when their products do not in fact operate in Class A up to full power output? The answer, I think, is nearly ALL of them. One company makes a big deal out of the fact that their hideously overpriced linestage operates in "Class A" and uses "no feedback". I hope you are an equal opportunity gadfly.
I have never read the Atma-sphere website in detail. I did not even know that there is a claim to full Class A operation. (Does it really say that? Or does it say "Class A", without specifics?) I really don't give a flying big one what you call the mode of operation, and I have long been aware of the phenomenon that limits Class A power, which you pointed out here for the second or third time in the last several years. Still, Atma-sphere amplifiers are the best sounding tube OTLs I have heard during my 40 years in this hobby.
The world we live in is full of much more consequential false claims that need to be debunked.
"Still, Atma-sphere amplifiers are the best sounding tube OTLs I have heard during my 40 years in this hobby."
I have never disputed that and I have a track record of stating that the Atma-Sphere OTL is a great sounding amplifier.
"The world we live in is full of much more consequential false claims that need to be debunked."
Very true.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
My main focus here is to educate people.
Here is the deal,this is Ralph OTL site!If your are not going to say good thing about his amps dont post.
I think it is Ralph who owes an apology to everyone who has read his website that says his amps are Class A, when they are not.
Goodluck,the Amps are Class A to Ralph an others that pay Class A Money!
I think it funny that Most who buy Ralphs amp an preamp say there the best,For $20k what would you say there fair?
Maybe
But Ralph doesn't do that. He changes the subject. He tries to make it about me. ETC.
What would you have me do? I think I stayed on subject and just argued the science without getting personal. What else can I do?
He,s The OTL king, Well he thinks so.
Tre' Thanks for the info
And now we can clearly see that your agenda has nothing to do with science.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/28/11 09/02/11
Here, let me show you why I made the assumption:Tre', 8-26-11: "My main focus here is to educate people."
GFL, 8-27-11: "My main focus here is to educate people."
* * *
Tre', 8-26-11: "I think it is Ralph who owes an apology to everyone who has read his website that says his amps are Class A, when they are not."
GFL, 8-27-11: "I think it is Ralph who owes an apology to everyone who has read his website that says his amps are Class A, when they are not."
* * *
Tre', 8-26-11: "But Ralph doesn't do that. He changes the subject. He tries to make it about me. ETC."
GFL, 8-27-11: "But Ralph doesn't do that. He changes the subject. He tries to make it about me. ETC."
* * *
Tre', 8-26-11: "What would you have me do? I think I stayed on subject and just argued the science without getting personal. What else can I do?
GLF, 8-27-11: "What would you have me do? I think I stayed on subject and just argued the science without getting personal. What else can I do?
* * *
Now that I look back on it, maybe GFL really is someone else, and he was quoting you without giving any indication that's what he was doing.
You were pretty pissed when I accused you of duplicity, when it may well have been a misunderstanding on my part. Hmmmm. Isn't that what you're doing to Ralph? Except that you've made a years-long crusade out of it.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
Edits: 08/28/11
Notice that in every case the duplicated sentence was authored first by Tre' and then by GFL. (That was pretty good detective work, incidentally.) I believe that GFL is just stealing Tre's phrases, because he has nothing else to say. Whereas Tre' is in earnest. I don't agree with Tre's approach, but I believe he is sincere.
You have to scroll down a little.
Since this is explained on the site and has been so for some time (and also as it has been in the past), I take umbrage to Tre's comments that I am lying. At best he seems to not allow that class A2 is class A while I maintain the A1 and A2 are both class A.
The problem that I have is if I can't call it class A2, then what is it?? In an AB amplifier, whether AB1 or AB2, the tubes go into cutoff at some point. Class C, D, G and H do not apply. Class A2 is about all there is.
Thanks for the link, Ralph.
It makes no sense to me that Tre' would crusade so vehemently over whether or not "Class A2" can legitimately be called "Class A" (and duh, it's obviously a type of "Class A"). That is certainly NOT worth calling someone a liar over!
So I do not think Tre's vendetta is about amplifiers or even about precise definitions of terms; rather, I think it's about a personal agenda. My guess is that this crusade is just Tre's ego trip, rather than him actually having some personal grievance against you and your company.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
The personal agenda thing had not occurred to me and it makes more sense...
I get your point.Has there been a (technical) misunderstanding on my part?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/28/11
As i sead, Dont F++k with King Ralph,This is an has been A DEAD Site!
An Most here Like Lew an others have nothing to add,just the same thing over an over.I love My OTLs That it.
There was at one time Vary good info here on OTLs but looks like the King did not like it so he just put a end to it. Just like NOW.With help from others that have nothing to add to what your are saying.
Just let the site go back to The same crap, talking about how great the Kings Amps an preamps are,Theres no more to say but you can bet there well be more crap!That All thay Got.
The Amps Are not an never well be CLASS A.But thin what is True Class A
.I no what it is not,but your telling us what is,Thanks
PS. If Ralph was a true OTL KING he would have his on Site!Must be $$$
Would you like to step outside, GFL?
Lew are you a Bully?
"The diagram you show is for Class A1."
What in that diagram says it's A1 only?
Zero bias tubes like 805's require positive grid voltages and grid current but can still have a dynamic curve much like the one shown and still have both halves of the wave form be symmetrical.
I know you think this is something personal but it's not.
It's just about science and physics.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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