"Class A" nonsense

[balijukka]

I can not reach to this EIA standard anywhere. What are the exact words? Does it really say at maximum output power? Or maximum clean output?
If it does, that cleans the table and settles this. Could you please type it out, if you have the standard.

All the old books that I have, do not mention any of this. Just the conduction angle and they all meantion the grid alternating voltage (= input signal) as a one of the setting parameters.

jukka

[Steve Conner]

I was in the local Guitar Center today showing off my Tweed Bassman clone, and a fellow stopped me dead in my tracks. He asked me: "Is the amp true class-A". I paused and smiled and thought of this thread. I told him I wasn't really sure, but it probably wasn't, and I'm not sure that any are......

Just tell them "It's Class A-for-Awesome"

[Diablo]

Just tell them "It's Class A-for-Awesome"

Well it was awesome enough that one of the clerks wants to buy the amp. Talk about bringing coals to New Castle...

[mooreamps]

I can not reach to this EIA standard anywhere. What are the exact words? Does it really say at maximum output power? Or maximum clean output?
If it does, that cleans the table and settles this. Could you please type it out, if you have the standard.



jukka

Then consider what they publish on the "data sheet".

http://boozhoundlabs.com/howto/pdf/6V6GT.pdf



-g

[balijukka]

Then consider what they publish on the "data sheet".

http://boozhoundlabs.com/howto/pdf/6V6GT.pdf



-g

There's nothing in that data sheet that answers the question.
I still have not come up with any standard or other "official" clas A definition saying that the measurement are to taken at maximum claimed/unclipped/clean/whatever output.

The ONLY definitions I have found, just require the 360 degree conduction, and the in the original one there are the words " grid bias AND ALTERNATING GRID VOLTAGE".

This again leads to that there is no per se class amplifier, just an operating condition. The rest is marketing hype all the way.

And I truly believe that if someone explains amplifier's behaviour saying for example ".. when going from class A to class AB..." everyone in this board understands what the author means.

And in when it comes to guitar amps a push-pull class AB amp sounds helluva lot better than push-pull class A amp, due to the compression that happens at the load line bend (and to some degree at grid clipping in cathode biased). Class A amp does not sing the way a class AB amp does.

jukka

[Old Tele man]

Class-A = tube is always conducting (360°) and never stops conducting.

OK, so 'how' do you "test" the statement "...always conducting..."?

Answer: by running the amp up to it's maximum rated output...and see if the top "flattens" due to saturation, the bottom "flattens" due to cutoff, or both. If any of these occur, it's not Class-A.

And, how do you "test" that you're at maximum rated output?

Answer: by following the EIA/JEDEC/NEMA/JEDC-guidance found in current (31 January 2001) MIL-STD-1311C (Method 1341) for specifying vacuum tube POWER OUTPUT:

"2. Class B amplifier. ...The total harmonic distortion of the signal shall no exceed 5 percent excluding distortion introduced by the source impedance..."

...and, we know that tube-induced distortion is influenced by BOTH tube type (triode, tetrode or pentode) and configuration (SE or push-pull), so the "combination" of tube+configuation is open to the amp manufacturer.

...TUBES: (a) Triodes have asymmetric transfer functions that commonly produce mostly even-order distortion; (b) Tetrodes/Pentode have symmetric transfer functions which produce mostly odd-order distortion. Some people equate even-harmonics to "rich tone" and odd-harmonics to "bright detailed."

...CONFIGURATION: (a) Single-Ended (SE) operation is asymmetrical while (b) Push-Pull (PP) operation is symmetrical due to OT-cancellation of even harmonics.

...since Class-AB is essentially Class-B that's been adjusted to use "projected (control grid) cutoff" biasing there's always gonna be some small amount of 'crossover distortion' which is the reason behind the 5% THD limit. HiFi amplifiers tend to minimize the THD value, while musical amplifiers tend to start at that value...and go up further as they're driven closer to saturation.

[balijukka]

Class-A = tube is always conducting (360°) and never stops conducting.

OK, so 'how' do you "test" the statement "...always conducting..."?

Answer: by running the amp up to it's maximum rated output...and see if the top "flattens" due to saturation, the bottom "flattens" due to cutoff, or both. If any of these occur, it's not Class-A.

Still, What authority, and where says that it must be maximum rated output.
Are we all supposed to follow "Old Tele Standard",

If the tube goes to saturation, it is still conducting 360 degrees.

jukka

[Chuck H]

Are we all supposed to follow "Old Tele Standard",

If the tube goes to saturation, it is still conducting 360 degrees.

Now your just playing at Devil's advocate .

Of course if the tube only goes into saturation it remains conducting for 360*. But class A "should" be biased at he center of it's operation, which would mean that if it's saturating, it's also going into cutoff on the other end. And if that happens prior to the amps "rated" output power, then it's not class A, is it.?.

Of course any slick "guitar amp" company will call an amp class A even if their amp is not biased at the center of the 360* operating point. As long as the power tubes aren't cutting off prior to the rated output this is basically accurate. But this isn't always the case, and it never will be. So what, are you gonna cry about it?!?

Chuck

[balijukka]

I'm still waiting someone comes up with a standard that says that the conducting measurements should be done at the rated/maximum/maximum clean/whatever power. So far no-one has shown anything in print.

Logically it would be just maximum clean (=unclipped) output, beyond that the amplifier stops being just amplifier and becomes a clipper amplifier.

But is it written down in any standard. The only standard I have seen, is the standard of instantaneous operation, and accordingly.... well you know, blah,blah,blah...



jukka

[soundmasterg]

I haven't bothered to look for it, (because the book is so big and technical) but I wonder if the RDH 4 has anything to say about it?

greg

[balijukka]

I haven't bothered to look for it, (because the book is so big and technical) but I wonder if the RDH 4 has anything to say about it?

greg

The same as all the other old books. Includes alternating grid voltage in the soup.

jukka

[Old Tele man]

...can you say "suffix?" S-U-R-E you can, "A1" and "A2"!


...which, respectively, indicate "1" for control grid NEVER goes positive (just like Class A NEVER stops conducting) and "2" for control DOES get driven positive at somepoint (just like Class AB DOES stop conducting at somepoint).

...because (as the theory assumes) if the grid doesn't clip (suffix -1), the plate shouldn't clip either. So, over-driving a Class-A1 PP amp will make it a Class-AB2 amp (think about it). As they say: "...intended use and actual use may vary..." (quite a bit!).

...it's ALL in the correct use of the "old" terminology.

QUESTION: exactly when does a Class-AB1 amp cross out of "quasi-class-A" operation (ie: both tubes conducting all the time) and actually begin "acting" Class-AB?
..ANSWER: the instant the output signal current equals the idle current, which is another reason WHY class of operation is not determined at the IDLE state.

P.S.--saturation is not the same as cutoff.

[balijukka]

...can you say "suffix?" S-U-R-E you can, "A1" and "A2"!

...which, respectively, indicate "1" for control grid NEVER goes positive (just like Class A NEVER stops conducting) and "2" for control DOES get driven positive at somepoint (just like Class AB DOES stop conducting at somepoint).

according to IRE 1938 standard the suffix MAY ba added to denote....
That means it is not necessary. A can be just A or A1 or A2 if more specification is required.

Earlier you said, that if the top of the ouput clips, meaning grid conduction, it is NOT class A.
Now you say it can be class A2. Still class A, ain't it. We're learning.

QUESTION: exactly when does a Class-AB1 amp cross out of "quasi-class-A" operation (ie: both tubes conducting all the time) and actually begin "acting" Class-AB?
..ANSWER: the instant the output signal current equals the idle current, which is another reason WHY class of operation is not determined at the IDLE state.

WRONG ANSWER,(unless the signal is balanced square wave.)

The output signal current is ac. The idle current is dc.
When cutoff happens, the ac current peak to peak is twice the idle current peak current being the idle current.
For sinusoidal ac current the RMS value, (which we can compare to the dc value), is Ipeak/sqr2. which is abt 0.7 x idle current. The word famous bad efficiency of class A. You'll always spend one third of the B+ power just for heating the environment.

You still have not answered my question. What standard does say that the measurements are to be done at rated/maximum/whatever power.

jukka

[Enzo]

jukka, I may be wrong, but you seem to be fighting to find a "standard" that will tell you no matter what that such and such an amp is definitely a class so and so amp.

Who says there is such a standard? After several pages here, no one can quite agree how to even define the problem. is it a class A amp because of how it is designed? or is it class whatever because of how it operates under some conditions? And what might those conditions be? And what might they not be?

The fact that the EIA might define amp rating methods doesn't mean that is how Marshall came to their ratings. For better or worse. We look at the RCA book and a certain tube is rated for 300v max. Fender runs that tube at 400v. Now is it a 300v tube or a 400v tube?

Can we really say that one can call an amplifier a "class A amplifier?" We can say it is designed to operate in class A, but that doesn't guarantee that it will be run that way. What defines a "race car?" The design or the use? My mom doesn;t think of her car as a race car, but I could certainly take it and race it.

Point being that circuits have classes of operation, but I am not sure amplifiers do. AMplifiers don;t conduct 360 degrees, stages within it do... or don;t. AMplifiers don;t go silent when confused over class.

Is my (X) a "class A" amplifier? WHo knows, but it sounds like this... Is it hype when they call an amp class A? Or are they just mis-informed? DO I want a class A amp because of how it sounds? or do I want to know the class so I know how I should think it sounds?

[Chuck H]

Excellent points Enzo. Let's also consider that some players want a single ended "class A" amp very strictly BECAUSE of it's overdriven tone. And yet it has been argued here that even a single ended amp operating outside of it's intended parameters isn't class A anymore...

Even if there is a defined "standard" I'm not at all sure it should apply to guitar amps since part of the point to many guitar amps is to operate them beyond the limits of their specifications.

Chuck