On Oct 24, 1:37 am, Andre Jute wrote:
Multi-grid wrote:
Class A has nothing to do with dissipation either.
Then Poopie wrote:
The definition of Class A is very simple. It requires that the output
device(s) never cease conducting under any signal condition.

Then Mickhttp://www.nascom.infoandhttp://mixpix.batcave.netwrote:

Correct. A class AB amp fulfills this up to the point where one side
starts to turn off.

I can see what you're getting at, Poopie, and so can Mick apparently,
but I don't think I want to wear your definition where it says "under
any signal condition". That's most misleading, especially with a
newbie on the board who stubbornly keeps claiming signal has nothing
to do with Class A. Any power stage that is hitherto Class A can
driven to cutoff by simply increasing the signal until the output cuts
out at either zero current or zero voltage.

In any and all cases, Class A is defined, though usually implicitly
rather than explcitly, as at a particular, *limited* signal voltage. I
therefore prefer the definition of Class A which says simply:
***Class A operating conditions do not permit the output device to
cease conducting.***
If you insist on describing the signal condition, you could add the
words *at the design or intended or specified signal*.

As for Dougles Multi-grid's silly insistence that signal and
dissipation have nothing to do with Class A operation, thanks for the
giggle, sonny, but you'd better hit the books lots more before you
seek entry to this club.

They don't Andre, no matter how much you claim they do. Class A, is as
simple as you first stated it:

***Class A operating conditions do not permit the output device to
cease conducting.***

It should get the addition that remote cut off behaviour is not
included.

In particular, you should pay attention to
this, which often comes as a bolt from the blue to repair hacks like
you, who tend to assume that the parameters are godgiven and fixed:
the *designer* chooses the plate voltage and negative bias, that sets
the current at quiescence, through which the designer then runs the
loadline at a particular angle when he chooses a primary impedance for
his output transformer, the slope of the loadline determining how far
the quiescent point is from current cutoff (or voltage cutoff...), and
that in turn defines the signal voltage that can be put in for Class A
operation, which in turn defines the output power. Thus the signal
level has everything to do with Class A operation, and in turn defines
maximum possible dissipation from Class A operation. Cruise my
netsite; all this is explained in both words and pictures.


I make no such assumptions. That you'd think I would only shows your
limitations. Stay off this group-killing personal attacking if you
please. Isn't one war named after you enough?
cheers,
Douglas

In article . com,
Multi-grid wrote:

On Oct 24, 1:37 am, Andre Jute wrote:

As for Dougles Multi-grid's silly insistence that signal and
dissipation have nothing to do with Class A operation, thanks for the
giggle, sonny, but you'd better hit the books lots more before you
seek entry to this club.

They don't Andre, no matter how much you claim they do. Class A, is as
simple as you first stated it:

***Class A operating conditions do not permit the output device to
cease conducting.***

Precisely, that is why when the operating conditions of a class AB
amplifier are restricted by limiting the applied input voltage the
amplifier is able to put out class A power at a level that is lower than
the maximum available class AB power.

It should get the addition that remote cut off behaviour is not
included.

Most real world tubes display remote cut off behaviour as the plate
curves become distinctly compressed in the high voltage low current
quadrant. I guess that rules out the possibility of any tube amplifier
operating class A.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

On Oct 23, 8:51 pm, John Byrns wrote:
In article . com,

Multi-grid wrote:
On Oct 24, 1:37 am, Andre Jute wrote:

As for Dougles Multi-grid's silly insistence that signal and
dissipation have nothing to do with Class A operation, thanks for the
giggle, sonny, but you'd better hit the books lots more before you
seek entry to this club.

They don't Andre, no matter how much you claim they do. Class A, is as
simple as you first stated it:

***Class A operating conditions do not permit the output device to
cease conducting.***

Precisely, that is why when the operating conditions of a class AB
amplifier are restricted by limiting the applied input voltage the
amplifier is able to put out class A power at a level that is lower than
the maximum available class AB power.

It should get the addition that remote cut off behaviour is not
included.

Most real world tubes display remote cut off behaviour as the plate
curves become distinctly compressed in the high voltage low current
quadrant. I guess that rules out the possibility of any tube amplifier
operating class A.

John, with respect:

Andre is out of his depth on this one. And his typical response to
being out of his depth is to repeat, attack, repeat, attack, and then
repeat again. But none of that makes what he states true or correct.

And as your life's work is somewhere between cleaning up after him,
powdering him and changing his diapers and seeking exceptions and
outliers, I am not surprised that you are joining this discussion as
per usual.

There have actually been some reasonable discussions on tube lore
lately - those from which Andre has stayed away. And, were he to
refrain from delivering (typically incorrect) absolutes he might even
be capable of intiating and even participating in such discussions.

But, it is very hard for a pretender to stick to what he knows and for
a poseur to pronounce with less than "papal" infallibility. Which
makes Andre little more than a charlatan and never-was with delusions
of adequacy. All-and-at-the-same-time, he is a big boy of some 60+
summers and well able to manage his own affairs and discussions... or
it would be so were he mature enough to understand. Infants usually
and eventually become potty-trained. And that is the essential
difference between Andre and an infant, and why your efforts at clean
up after him are futile. He will never change however heroic your
efforts.

If you want an analogy, GM, some time back had a V8 engine that
*could* fire on 4, 6, or 8 cylinders depending on load conditions. But
it was never "other than" a V8, nor could any amount of wishful
thinking or closely held beliefs make it "other than" a V8.

Peter Wieck
Wyncote, PA

Multi-grid wrote:

On Oct 23, 6:01 am, maxhifi wrote:
Multi-grid wrote:
A properly designed Class A amp
never clips because the operating parameters were so chosen that it
runs out of signal from the intended source before it can clip (that
is one reason why the design instructions on my netsite feature the
*design center* process so heavily).

Stay on topic please.

While you're at it, please refrain from making personal attacks, it is
beneath you( I hope ).

Class A has nothing to do with its input signal. The finals clip
symetrically. Or perhaps unsymetrically if it is power limited A, so
to say overbiased given the load and B+. If you have plate dissipation
to spare, it is certainly possible. Matter of fact I am listening to
one such amp now...
cheers,
Douglas

I believe the point it changes from 'class A' to 'class AB' is commonly
accepted to be the point where the output stage begins to draw more
current from the power supply. By commonly accepted, I mean I've read a
ton of magazine reviews of tube amps where they use this logic, stated
almost exactly as Andre has used it.

In other words, the amp is biased a bit hotter than class B, and this
lowers low level distortion a bit at the expense of peak power output.

Stating that "the amplifier is Class A until XXX watts", really, is
telling you how hot the tubes are biased relative to the two extremes of
pure class A (full dissapation), and pure class (cut off).- Hide quoted text -

- Show quoted text -

Class A has nothing to do with dissipation either.

Class A has everything to do with dissipation!!!!

But lets begin with class B.
In class B, there is ZERO anode dissipation because the tubes ( or SS
devices )
are biased at cut off. The amplifier only has device dissipation when
signal flows.
But distortion is notoriously high, and to avoid the worst of it, class
AB is used
where the tubes/devices are biased with idle dissipation well below the
maximum allowable fpr the tube or device.
This allows a small amount of pure class A working power until cut off
begins to occur in tubes/devices
on each alternate wave peak.
Hence a pair of KT88 might be set up with Ea = 600V, and Ia at idle of
30mA, for a Pda at idle
of 18 watts. Maximum allowable for a single KT88 is 42 watts.
The AB power avaliable with 600V is around 100Watts class AB but the
pure class A might be less than 10 watts
for the load where 100Watts AB is possible, which will be a load a lot
less than the load required for
pure class A operation up to clipping.
Its possible to have two KT88 dissipating a total of 36 watts, Ea =
600V, and have a load which ensures all the power
is class A. It is a high ohm load, but achievable if you connect a 16
ohm speaker to an AB amp with an outlet
for 4 ohms; the reflected anode load of say 5k becomes 20k.
And the beam terode class A efficiency will be up to about 45% and so
you'll get a maximum
of 16.2 watts of pure class A power from a pair of KT88 with a total of
36 watts Pda.
If the Pda is raised to 72 watts for the pair by doubling the Ia, then
max class A
also doubles if efficiency is constant, so 36 watts is possible.

The load can be a lot lower as well.

I suggest you learn much more about basic output tube behaviour before
you shoot your mouth off
by saying that " Class A has nothing to do with dissipation either. "



Just because some
marketing group noticed that class A means something good, does not
make it right either. Just because it seems to make sense is no reason
to *******ize the definition. Find some other way to describe it.
cheers,
Douglas

You are looking like the one *******izing something imho.

Please clarify your reasons for whatever your stance really is.

Patrick Turner.

Eeyore wrote:

Multi-grid wrote:

maxhifi wrote:

Stating that "the amplifier is Class A until XXX watts", really, is
telling you how hot the tubes are biased relative to the two extremes of
pure class A (full dissapation), and pure class (cut off).- Hide quoted text -


Class A has nothing to do with dissipation either. Just because some
marketing group noticed that class A means something good, does not
make it right either. Just because it seems to make sense is no reason
to *******ize the definition. Find some other way to describe it.

The definition of Class A is very simple. It requires that the output device(s)
never cease conducting under any signal condition.

It means slightly more than this because tubes don't cut off as sharply
as other devices.

So class A means where PP power tubes do not have Ia on either side of
the PP circuit ever
having Ia fall below 10% of the idle value.
Below about 10% of the idle value, the cut off behaviour becomes
substantially non linear, as opposed to substantially linear prior to
cut off.

Gradual cut off in triodes makes them particularly nice to use in AB
amps.
Minimal switching artifacts are generated. And the gm rises with Ia, so
the
load change reduction happening after cut off of one tube means the
triode compensates for the sudden drop in gain you see in pentodes and
beam tubes.
Its the internal triode NFB at work to give the low Ra.

And if the first 1/2 of output power maximum is class A by my
definition, then is its deemed A,
and the remaining 1/2 is B, and the total action is AB.

Most ppl will not hear any difference if the bias is increased to ensure
all power
up to clipping is pure class A.

The load experienced by one of the tubes in an AB PP pair is NOT a
straight line load
as one would draw across the anode curves of a given tube.
While in class A, each tube sees an almost straight line load, but
always of slightly
different ohm value because the gm of each tube isn't the same.
But the RL for each tube while in class A is approximately 1/2 of
RLa-a.1
Once cut off has begun in earnest during each wave cycle, the load each
tube sees
become its "class B load" = 1/4 RLa-a, and one has to draw a CURVED LOAD
LINE to describe the loading
of each PP output tube to describe what each is really doing.
The load of one tube is affected by the load on the other until cut off
happens,
and then each tube is on its own to deal with the load of 1/4 RLa-a.


Patrick Turner.



Graham

Class A, is as simple as you first stated it:

***Class A operating conditions do not permit the output device to

cease conducting.***

It should get the addition that remote cut off behaviour is not
included.


. Isn't one war named after you enough?
- Show quoted text -

sorry about that, forgot one thing...That is *AT FULL POWER*, if one
gets cut off before that, it is AB, which is neither A nor B.
cheers,
Douglas

I suggest you learn much more about basic output tube behaviour before
you shoot your mouth off
by saying that " Class A has nothing to do with dissipation either. "



- Show quoted text -

The dissipation is only related to the output power. A Class A amp
does not need a particular fraction of plate dissipation to be used in
order to classify as A. Examples are not needed, symbolic notation
will be fine.

The amp is defined by its behaviour. The bias that goes from 360-
degree conduction to something closer to 180 gets labled AB. Iff it
can maintain 360 degree conduction it gets the A.

Since it's usually quality over max power choices that run to class A
designs, the marketers decide to appropriate it for their own in AB
designs. You remember the advertisements general content( reference
Andre's comments on the Vellman ).

B is supposedly 180 degree, but given cut-off bunching, none are that
small. Why not have this discussion about the differences between AB
and B?
cheers,
Douglas

Patrick Turner wrote:

Eeyore wrote:
Multi-grid wrote:
maxhifi wrote:

Stating that "the amplifier is Class A until XXX watts", really, is
telling you how hot the tubes are biased relative to the two extremes of
pure class A (full dissapation), and pure class (cut off).- Hide quoted text -


Class A has nothing to do with dissipation either. Just because some
marketing group noticed that class A means something good, does not
make it right either. Just because it seems to make sense is no reason
to *******ize the definition. Find some other way to describe it.

The definition of Class A is very simple. It requires that the output device(s)
never cease conducting under any signal condition.

It means slightly more than this because tubes don't cut off as sharply
as other devices.

I see what you're saying but I do believe that the definition is unchanged. Obviously
avoiding any region of significant non-linearity is preferable but that in its own right
doesn't change the definition.

Graham

In article .com,
Peter Wieck wrote:

On Oct 23, 8:51 pm, John Byrns wrote:
In article . com,

Multi-grid wrote:

***Class A operating conditions do not permit the output device to
cease conducting.***

Precisely, that is why when the operating conditions of a class AB
amplifier are restricted by limiting the applied input voltage the
amplifier is able to put out class A power at a level that is lower than
the maximum available class AB power.

John, with respect:

Andre is out of his depth on this one.

Peter, I know you are obsessed with Andre, but this is not about Andre,
it is about Multi-grid's silly statement that a class AB amplifier can't
put out a limited amount of "class A power". Multi-grid is confusing
the definition of a PP amplifier's class with it's ability to deliver
some amount of power, less than rated output, without either output tube
cutting off. It has been common usage for as long as I can remember to
call this "class A power" because at this power level both output tubes
are conducting over the full 360 degree cycle, as in a class A
amplifier. An amplifiers class is determined by the operation of the
tubes at the rated power output, however in the case of a class AB
amplifier, a lesser amount of "class A power" is available.

To illustrate how silly Multigrid's claim is, consider a class AB
amplifier with a rated output of 50 Watts in class AB. Further suppose
this amplifier is capable of putting out 15 Watts without either output
tube cutting off, 15 Watts of what Andre, and many other famous
amplifier designers, call "class A power". Now we all agree that this
is a class AB amplifier, but suppose we connect a suitable network of
Zener and ordinary diodes across the primary of the output transformer
which clamp the maximum output voltage at the level necessary to produce
an output of 15 Watts. Has this amplifier now become a class A
amplifier simply because we have placed this clamp circuit across the
primary of the output transformer? If we test it with our sine wave
generator and oscilloscope we are going to conclude that it is a class A
amplifier because both tubes are still conducting over the full 360
degree cycle at the point of clipping. I leave the implications of this
experiment for the student to consider.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

On Oct 24, 6:57 am, flipper wrote:
On Wed, 24 Oct 2007 13:44:13 +0100, Eeyore



wrote:

Patrick Turner wrote:

Eeyore wrote:
Multi-grid wrote:
maxhifi wrote:

Stating that "the amplifier is Class A until XXX watts", really, is
telling you how hot the tubes are biased relative to the two extremes of
pure class A (full dissapation), and pure class (cut off).- Hide quoted text -

Class A has nothing to do with dissipation either. Just because some
marketing group noticed that class A means something good, does not
make it right either. Just because it seems to make sense is no reason
to *******ize the definition. Find some other way to describe it.

The definition of Class A is very simple. It requires that the output device(s)
never cease conducting under any signal condition.

It means slightly more than this because tubes don't cut off as sharply
as other devices.

I see what you're saying but I do believe that the definition is unchanged. Obviously
avoiding any region of significant non-linearity is preferable but that in its own right
doesn't change the definition.

People are talking semantically past each other with some speaking of
the definition of the 'amplifier' class while others are speaking of
it's behavior under a restricted set of operating conditions; and it
is useful to observe that under an appropriately restricted set of
conditions the output tubes conduct 360 degrees as in 'Class A'
operation.

"Useful", indeed, Flipper. Thanks. But I would go further and say that
the signal and dissipation restriction is part of the definition, as
you go on to imply:

For example, if maximum power, or efficiency, were the primary concern
then one might bias more to the 'B' side of the equation while if
fidelity were the primary concern one might bias more to the 'A'.

Precisely. Both Class A operation and Class B operation are inherent
in the nomenclature and definition and their relative importance is
clearly intended to be in the designer's discretion.

Historically, the original purpose of Class AB was to annihilate the
second harmonic which before made up such a very large part of the
THD, while still allowing beam tubes and pentodes to give much larger
power than available before. The "invention" of Class AB as a hi-
fidelity amp is what spurred part of a Olsen's work on perception;
before it wasn't known that odd harmonics are proportionately much
more disturbing than even harmonics. It seems to me that AB amps with
largish parts of their output in Class A is a relatively modern trend,
possibly related to ever less-sensitive speakers.

One might also observe that's likely why it's called Class AB and not
Class insert unique letter.

There is in fact a class between Class A and Class AB with a unique
description: "Limiting Class A1", which is set up so that the
crossover happens when one valve just reaches current cutoff and the
other simultaneously reaches zero bias. It makes for an amazingly
smooth sound but it is a bitch to set up and keep tuned if you want to
keep your circuits simple. I was therefore rather interested in what
Patrick said elsewhere in this thread (in more general sense rather
than specifically about LImitiing Class A1) about within 10 per cent
of conduction angle being imperceptible...

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
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