Output classes A and AB
 

Arny Krueger wrote:

"Patrick Turner" wrote in message
...

In class AB, each man pulls the saw about 1/2 way across the stroke then
lets go,
and the other guy grabs his saw handle and pulls the saw back the other
way.
Each man only mainly pulls the saw in turn, and applied force is jerky,
and frankly,
a difficult way to work; the Union will be down soon to have a go at the
boss who told
the men to saw the log that way.

So speaks someone who obviously has no experience cutting wood with a
cross-cut saw. You can pull on a saw, but pushing on it can easily cause it
to bend and bind. When 2 men use a cross-cut saw, each man pulls far more
than he pushes.

Its only an analogy. But you should see the saws of the guys who win
sawing constests
at the shows here. These well trained brutes of men can saw a large
hardwood log faster than a guy with a chain saw,
and the push isn't much less than the pull force. Thus the power input
to the saw is maximized.

With a plain single tube, its ability to increase current beyond the
idle value is its "pull" stroke
and much higher than its "push" stroke of turning off current.
The variation in gm for each side of zero gives rise to the 2H.

The use of two such tubes in PP cancels out the uneven abilities to a
large extent,
but also because the gm change between high current and low current is
not even, 3H is generated....


Class AB verging on pure class B is the preferred mode of operation for a
cross-cut saw.

Depends. We have plenty of class A men here in Oz. Maybe youse got only
class B men where youse are.

And we both fergot to mention bow saws which have a thin long tensioned
blade strung tightly between the
ends of a steel tube bow, and where the push cuts more than the pull.

Patrick Turner

Output classes A and AB
 

In article ,
Patrick Turner wrote:


[Snip]

So what happens when we use a 5k load on the same amp?

The same idle current flows, and the same range of Ia variation 0f +/-
45mA will define the
class A **current** swing, ( where the **current** wave THD 5% and
mainly all 2H ).

So the class A load on each tube = 1/2 x 5k = 2.5k, so the maximum class
A V swing at each anode
= 2,500ohms x 0.045A pk = 112.5Vpk = 225pk from anode to anode, or
159vrms across 5k,
giving 5 watts of class A.

But the load value allows for a much larger increase in Ia than the 50mA
of maximum decrease in Ia.

This also means that once the Ia travels below 10% of the idle value,
the gm of the tube cutting off
has diminished to such a low value the other tube turning on harder is
providing virtually all the Ichange x Vchange
across the available load, and is the only device coupled through only
1/2 the OPT primary
to the load, so the RL seen by this tube turning on hard has reduced to
1/2 its class A load,
or 1/4 of the nominal RL a-a, and in this case its 1.25k.

The load is the same as that for a class B amp.

Isn't 1.25k too low a load for getting maximum power from a KT88 in
triode mode, even in class B?

[Snip]

Anyway, the quad of 6550 while working in class A with a 10ohm load
connected
across the mis-labelled 0-4 ohm outlet do sound VERY well.

Those wanting a schematic of what have done may ask as I have a .gif
available.

Its much simpler than the original, and I won't beak ARC rules by
handing out free copies of their
abominable concoctious junk.

What is all this talk about ARC's anyone else's rules that would keep
you from handing out free copies of their abominable concoctious junk,
assuming you drew the schematic your self? You mentioned this same
issue in connection with the ManleyLabs amplifier you modified, my
understanding is that they only have protection for schematics they have
drawn, if you draw your own schematic of the same circuit, they have no
rights with regard to it. Any Lawyers out there care to comment?

[Snip]

The VT100 had a true horror for a PSU and after fitting a CLC B+ filter
and re-locating
earth paths, I finally got hum&noises to be less than 1mV with preamp
gain at max with
open cd input.

Is this on the 4 Ohm tap? If so it is only 66 dB below 1 Watt, or 69 dB
with an 8 Ohm speaker connected to the 4 Ohm tap, not an awe inspiring
result, but apparently typical for many tube amps. But maybe it's not
so bad with the preamp included, did you measure the hum&noises of the
VT100 alone?


Regards,

John Byrns

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

Output classes A and AB
 

Don Pearce wrote:

On Thu, 25 Oct 2007 13:43:56 GMT, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

Can you overdrive a class A amp to cutoff?

Yes, it is painfully easy.

In my experience what
happens when you overdrive a class A amp is that one device saturates,
and the other sticks with its normal bias condition.

No, the coupling capacitor charges up as a result of grid current
shifting the bias point so that cutoff becomes even easier.

There is no
circumstance in which I have ever managed to put a class A amplifier
output device into cutoff.

You haven't tried very hard then.


The designs I have used must have been better matched in drive level
between the driver and output stages. They tended to limit almost
simultaneously so that the drive level to the output stage did not go
on rising as the input signal increased, just squared off. As I say, I
never saw an output valve go into cutoff.

A well designed class A SE tube amp will have symetrical clipping where
grid current limits the
increase in anode current at the same load voltage swing amplitude where
load anode current reaches
very near zero. A typical load for a trioded KT88 is 5k.
Two such tubes which are in a PP circuit will behave the same way if
they have the same load.
The total load across the whole PP primary is twice the load of the SE
tube.
So the pair of KT88 will have 10k load a-a.
But with PP the load can be halved, and idle current reduced for class
AB operation and indeed the anode current
is cut right off, because the grid voltage goes to a value so darn
negative.

And the positive voltage swings of the grid can draw grid current which
charges the coupling cap negatively,
so the cut off occurs sooner when the amp is over driven, and amps can
become temporarily paralysed
after being seriously overdriven, because tubes remain cut off until
they re-bias themselves and cap charges leak away
through biasing resistances.

Patrick Turner.

d
--
Pearce Consulting
http://www.pearce.uk.com

Output classes A and AB
 

Andre Jute wrote:

On Oct 25, 1:17 pm, John Byrns wrote:
In article .com,
Andre Jute wrote:



On Oct 25, 7:40 am, Eeyore
wrote:
John Byrns wrote:
Patrick Turner wrote:

Cancelation of even order harmonics occurs in amps working in class AB
during that part of the wave forms which are in class A, ie, the bits
either side of the zero crossing.

But once each tube moves into cut off, nothing is cancelled.

Patrick, I'm surprised to hear you say this. What are you trying to
tell us, that the even order harmonics are only cancelled during those
parts of the cycle when both tubes are conducting, but that the even
order distortion components reappear during those parts of the cycle
when only one tube is conducting? If you actually believe that you
should go back to the books and study the theory of harmonic distortion
more carefully. I hope you didn't get this notion from the RDH4, I
haven't read the RDH4's harmonic distortion explanation, but if this is
what it says I have just lost any respect I had for the book. In a
perfectly balanced PP amplifier the even order harmonic distortion is
completely cancelled even when the tubes are cut off for parts of the
cycle.

I'd love to know how that happens. There's no cancellation of ANYTHING once
one
side has ceased conducting !

Graham

Holy ****! Did I say yet that Poopie is ignorant and incompetent?

Nah, nobody can be that stupid and uninformed about tube basics.

There are a lot of "stupid and uninformed" people around, there are at
least three people involved in this discussion that have expressed this
same belief as Eeyore, they are Multi-grid, Patrick Turner, and Eeyore.

From Eeyore (Poopie Stevenson) I expect only the worst; he says
whatever comes into this head as the opposite of what is said by
someone he dislikes, without any reference to the facts in
electronics; I have made a separate thread to illustrate that Poopie
is joined in this perversity by Pearce and Krueger.

I've given up reading Cuddles Multi-grid's posts; it is too wearing to
think up new ways of explaining the same simple thing to him over and
over again.

Patrick is entitled to a mistake now and again; he makes so few. I saw
the above and knew you would call him on it, so I read on before I
wasted my own time telling Patrick he'd better explain himself -- and
there you were, pointing out the faux pas. Actually, I regret that
Patrick is both honest and not as slippery with words as some we have
seen here, or we could have had a dingdong as he tried to explain it
away without admitting he was wrong. I expect he will just say he
slipped up.

But I don't think I did slip up or make a mistake as John alleged.

See my other replies..

I actually find Patrick's total ignorance of woodsawing the most
amazing thing in this thread -- especially considering that we have a
fellow with real expertise (and ruler braces) on board RAT: Iain
Churches.

I trained as a carpenter and joiner as part of becoming a Builder, a
trade I worked at for 30 years

I got to know all I ever needed to know about sawing wood.

The worst job of sawing was where two guys have to saw a log along the
grain.
One is above the log, and the other is below it, but the guy below gets
covered in sawdust,
but must look up to guide the saw to the line. The guy above is in the
sun, and gets hot......
Sawdust can be quite toxic from some woods, and sawyers lungs are badly
affected.

Thank christ they invented steam saws.

But most of the planks used in very old buildings and ships were all cut
by hand, push pull mode,
and in a variety of classes, A, AB etc.

I often heard Harmonic Distortion call out.
"Gotta beer for us 'av yer?"


Patrick Turner.

Regards,

John Byrns

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

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

Output classes A and AB
 

In article ,
Patrick Turner wrote:

John Byrns wrote:

In article ,
Patrick Turner wrote:

Cancelation of even order harmonics occurs in amps working in class AB
during that part of the wave forms which are in class A, ie, the bits
either side
of the zero crossing.

But once each tube moves into cut off, nothing is cancelled.

Patrick, I'm surprised to hear you say this. What are you trying to
tell us, that the even order harmonics are only cancelled during those
parts of the cycle when both tubes are conducting, but that the even
order distortion components reappear during those parts of the cycle
when only one tube is conducting?

In effect the even order distortions DO re-appear when the amp moves to
class AB.

Its because of the mismatch between the two tubes, and each has slightly
or greatly different gm, so each +ve and -ve half of the waves are
amplified
by a different amount, and hence you get even order generated in the
output from across the whole primary
or secondary of the OPT.

You are changing your tune a bit, you originally said "once each tube
moves into cut off, nothing is cancelled", now you say "Its because of
the mismatch between the two tubes", which is a completely different
deal. I stated in several of my posts that the tubes must be identical,
of course that is impossible in the practical world, but we want to get
as close as possible if we are intent on minimizing distortion. When
the tubes are mismatched, the problem is not just confined to class AB
operation, it will occur even in a class A amplifier with mismatched
tubes. The bias may be able to be adjusted to cancel the 2 nd harmonic
at some selected power output, but there is no guarantee that the other
even harmonics will also be canceled, even in a class A amplifier if the
tubes are mismatched, and if the power output level is changed even the
second harmonic can reappear with mismatched tubes. It is the
mismatched tubes that are at the root of the problem, not operating in
the class AB region with the tubes cut off over part of the cycle.


Regards,

John Byrns

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

Output classes A and AB
 

"Andre Jute" wrote in message
ups.com...

AKA Poopie for a good reason. Why don't you share it with us. The
reason for your name is surely the one thing you do know.

Hmm, "Poopie", like the lie about my academic credentials is another Middiot
invention. Obviously, Jute is a Middiot admirer.

Output classes A and AB
 

"Patrick Turner" wrote in message
...


Arny Krueger wrote:

"Patrick Turner" wrote in message
...

In class AB, each man pulls the saw about 1/2 way across the stroke
then
lets go,
and the other guy grabs his saw handle and pulls the saw back the other
way.
Each man only mainly pulls the saw in turn, and applied force is jerky,
and frankly,
a difficult way to work; the Union will be down soon to have a go at
the
boss who told
the men to saw the log that way.

So speaks someone who obviously has no experience cutting wood with a
cross-cut saw. You can pull on a saw, but pushing on it can easily cause
it
to bend and bind. When 2 men use a cross-cut saw, each man pulls far more
than he pushes.

Its only an analogy.

Not a very good one.

But you should see the saws of the guys who win
sawing constests at the shows here.

I seriously doubt they are much different from the ones in the Northern
hemisphere. Laws of physics and all that.

These well trained brutes of men can saw a large
hardwood log faster than a guy with a chain saw,

Not if people get their choice of chain saw.

and the push isn't much less than the pull force.

Pushing a saw is only a little bit more productive than pushing on a rope.
Are you telling me that this works down there, even though it works up here?

Thus the power input to the saw is maximized.

No way can two human bodies duplicate the horsepower generated by fuel
acting on a high performance engine.

With a plain single tube, its ability to increase current beyond the
idle value is its "pull" stroke
and much higher than its "push" stroke of turning off current.
The variation in gm for each side of zero gives rise to the 2H.

Sounds like something that either needs negative feedback to correct, or
should simply be avoided in a hi fi.

The use of two such tubes in PP cancels out the uneven abilities to a
large extent,
but also because the gm change between high current and low current is
not even, 3H is generated....

In fact tubes are exponential devices. The expansion of the exponential
series that describes the transfer function of a tube contains both even and
odd order terms. No surprise then that unless special care is taken (such as
a well-balanced push-pull) tubed amps produce both even and odd order
nonlinear distortion.

Secondly, it is written nowhere that the current flow in each tube in a P-P
output stage need be greater than in a SET. Therefore Patrick, your argument
is a straw man argunment.

Class AB verging on pure class B is the preferred mode of operation for a
cross-cut saw.

Depends. We have plenty of class A men here in Oz. Maybe youse got only
class B men where youse are.

Nahh, men can be class A or B on either hemisphere.

And we both fergot to mention bow saws which have a thin long tensioned
blade strung tightly between the
ends of a steel tube bow, and where the push cuts more than the pull.

I didn't forget - its just that my mind didn't wander.

Output classes A and AB
 

John Byrns wrote:

(Don Pearce) wrote:
Eeyore wrote:
John Byrns wrote:
Patrick Turner wrote:

Cancelation of even order harmonics occurs in amps working in class AB
during that part of the wave forms which are in class A, ie, the bits
either side of the zero crossing.

But once each tube moves into cut off, nothing is cancelled.

Patrick, I'm surprised to hear you say this. What are you trying to
tell us, that the even order harmonics are only cancelled during those
parts of the cycle when both tubes are conducting, but that the even
order distortion components reappear during those parts of the cycle
when only one tube is conducting? If you actually believe that you
should go back to the books and study the theory of harmonic distortion
more carefully. I hope you didn't get this notion from the RDH4, I
haven't read the RDH4's harmonic distortion explanation, but if this is
what it says I have just lost any respect I had for the book. In a
perfectly balanced PP amplifier the even order harmonic distortion is
completely cancelled even when the tubes are cut off for parts of the
cycle.

I'd love to know how that happens. There's no cancellation of ANYTHING once
one side has ceased conducting !


Because if you add an even harmonic to a signal, you have to make it
asymmetrical. You always get a peak coinciding with a trough on one
half cycle, followed by a peak coinciding with a peak on the next. If
you modify the signal to remove any asymmetry, you must by definition
remove the even harmonics.

Finally a man who understands the theory!

But it's not by ** CANCELLATION ** in the case of AB operation beyond the
crossover point. That's my issue with the description.

It does have that effect but the use of the word *cancellation* is wong IMHO.
There should be another way to describe it.

Graham

Output classes A and AB
 

Andre Jute wrote:

Eeyore wrote:
John Byrns wrote:
Patrick Turner wrote:

Cancelation of even order harmonics occurs in amps working in class AB
during that part of the wave forms which are in class A, ie, the bits
either side of the zero crossing.

But once each tube moves into cut off, nothing is cancelled.

Patrick, I'm surprised to hear you say this. What are you trying to
tell us, that the even order harmonics are only cancelled during those
parts of the cycle when both tubes are conducting, but that the even
order distortion components reappear during those parts of the cycle
when only one tube is conducting? If you actually believe that you
should go back to the books and study the theory of harmonic distortion
more carefully. I hope you didn't get this notion from the RDH4, I
haven't read the RDH4's harmonic distortion explanation, but if this is
what it says I have just lost any respect I had for the book. In a
perfectly balanced PP amplifier the even order harmonic distortion is
completely cancelled even when the tubes are cut off for parts of the
cycle.

I'd love to know how that happens. There's no cancellation of ANYTHING once one
side has ceased conducting !

Graham

Holy ****! Did I say yet that Poopie is ignorant and incompetent?

Nah, nobody can be that stupid and uninformed about tube basics.

Poopie must be cracking a joke. For the first time in his life.

Good on yer, cobber! If you can't be smart and informed, at least you
can try to be a clown, give people a giggle.

As an alleged 'wordsmith' you of all people ought to understand what cancellation
means. Apparently it went right over your head though.

Only to be expected from an ignorant non-technical ****wit.

Graham

Output classes A and AB
 

Andre Jute wrote:

Coupla superbly detailed posts here, Patrick

Slurp, slurp, slurp.

My, your tongue IS brown isn't it ?

Graham