Output classes A and AB
 

flipper wrote:

On Sun, 28 Oct 2007 13:12:57 GMT, Patrick Turner
wrote:

snip.

I mentioned...

In class A, what one tube does with the load current affect the other
tube.

You said...

Not with pentodes (or any SS device) it doesn't. (Ideal) Pentodes are
essentially voltage controlled current sources and will source that
current regardless of what the 'other' is doing.

But the pentode (or tetrode) gm could be different, and thus if so,
current turn on/off
is different despite the same amplitude of grid input signals and total
summed output voltage being the same for each tube.
So the load each pentode sees is also different.
It does not follow that just because you have PP pentodes instead of
some other devices
such as triodes, bjts, or mosfets, that in PP class A the pentodes will
always be experiencing
the same load as each other.

But even with un-matched pentodes, the amount of class A 2H cancelling
is so substantial
that the 3H is still usually much more than any 2H left due to poor tube
matching.
That one PP output tube never sees an exactly equal load to the other is
of negligible consequence
because the THD in a couple of watts from a pair of tubes capable of 30
watts of class A will be inherently low.


Patrick Turner.

Output classes A and AB
 

In article ,
Patrick Turner wrote:

John Byrns wrote:

In article ,
Patrick Turner wrote:

No, because I endorse what many books tell us such as RDH4.

Yes, and if we read, understand, and apply what the RDH4 tells us, we
understand that even harmonic cancellation does occur in class AB
amplifiers, even when the tubes are cutoff for part of the cycle. That
is what this discussion is all about, Multi-grid, at least I think
Multi-grid was the first, made the claim that even harmonic cancellation
doesn't occur when the tubes in a class AB amplifier are cut off, then
both you and Eeyore jumped in to back up his claim.

I have lost track of whatever it was that Multigrid might have said.

Multi-grid said something to the effect that 2H cancellation can't occur
in a class AB amplifier when one one or the other of the tubes is
cutoff, the same as you are, or at least were previously saying.

But once one tube cuts off, the part of the cycle handled by the other
tube alone
generates a substantially linear output voltage when summed with the
other tube's efforts. IMHO, the summing action of tubes or other devices
in class B does not include the cancelation of harmonics
because of the common mode application of the near identical even
harmonics
of the same phase of both tubes across the whole primary.

But that is just saying in different words that 2H cancellation does
occur even while one or the other tube is cutoff, "common mode
application of the near identical even harmonics of the same phase of
both tubes across the whole primary" is simply the mechanism for
accomplishing the 2H cancellation.

We are saying the same thing in different words.

The class B action means the current wave of the device is a series
of 1/2 sine waves with a flat part and many harmonics are present
if you ever wanted to filter them out and quantify them.
But these harmonics don't appear in the load. Fact.

Exactly, that is the whole point of 2H and even harmonic cancellation in
a PP stage, the odd harmonics do appear in the load however because they
appear across the output transformer as a differential signal, not a
common mode signal.

There isn't mutual cancellation like in class A.
But there is crossover distortion as one device turns off, and the other
turns on.

I don't see the distinction between class A and class B operation, the
2H and even harmonic cancellation mechanism is the same for both classes
of operation, the only difference is the amplitude of the harmonics
generated. In both cases the 2H and even harmonics appear at the output
transformer as a common mode signal and are canceled in the load by the
action of the transformer.

Once one device cuts off, no current flows, so there are no distortion
currents produced
by that device.

That does not follow, all that follows is that the sum of the DC
current, the fundamental current, and the harmonic distortion currents
sum to zero during the interval that the tube is cut off. That is not
the same thing as saying there are no distortion currents flowing while
the tube is cut off.

Mr. Fourier explains this for us.

[Big Snip]

But I see no need to do this labourious and unecessary math and graphing
task which may satisfy
the lofty smartarse acedemics.

Exactly, there is no need for you to do it because the academics have
all ready done it for you, so you already know that the 2H and other
even order harmonics will cancel in a PP stage, be it class A, class B,
or anywhere in between, or even if it is PP class C.

RDH4 does not have too many full descriptions to simply illustrate what
I have been saying.
So what use are the smartarse acedemics? have they earned their dinner?

Certainly they have, you are beginning to sound a little like Ian.

The cancelation thinge is when the tubes are in class A and the 2H is
cancelled, similarly
to in any LTP.

"The cancellation thingy" is also when the tubes are in class AB or even
class B, where the 2H is also canceled.

One could prove almost anything with Fourier math....

Its easier to see the mutually cancelling 2H of each tube in a class A
amp.

To me it is easier to see the mutually canceling 2H of each tube in a
class B amp because the amplitudes of the harmonics are larger and as a
result are easier to observe.

But while tubes are cut right off, summed distortion currents in that
tube = zero.

I have to say this does not mean the distortion currents are NOT
PRESENT.
They must be, but all summ to zero amps for the flat line of the tube
when cut off.
If you filter out harmonics of the current wave of a single tube of the
clas B amp,
you'll see the plethera of harmonics, and these are all sine wave
signals
which flow continuously, and without cutting off, but which when added
will give you zero current for
1/2 the cycle wave at fundememtal F.

Exactly, I think you have got it now, the even order distortion currents
cancel even while the tubes are cut off in either class AB or class B.

Work out the 2H current in each tube while in class A of each tube and
see how such currents are applied across the primary.
The reason 2H is low in class A is that the 2H currents are the same
phase at each end of the
OPT primary which cannot produce voltage in the load if the current is
applied in common mode to
both ends of the load.

The same is also true of the even order harmonic currents in class AB
and class B amplifiers!

Well yes, but they are less easy to see, as i explained above.

Your reasoning on this seems backwards, I think we both agree that the
harmonic currents in a class AB stage are larger than those in a class A
stage, I have always found it easier to see large things, within reason,
than to see small things.

In effect we agree then, but practically speaking a tube cut off has
zero
harmonic distortion currents for the time its cut off.

No, a tube cutoff does not have zero harmonic distortion currents for
the time it is cut off, what happens is that the harmonic distortion
currents, along with the fundamental current, and the DC bias current
sum to zero over that interval, which is a completely different thing
than saying the distortion currents are zero during that interval.

You even said this yourself a few paragraphs up from here where you said
"I have to say this does not mean the distortion currents are NOT
PRESENT. They must be, but all summ to zero amps for the flat line of
the tube when cut off."


Regards,

John Byrns

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

Output classes A and AB
 

On Oct 29, 2:14 pm, Patrick Turner wrote:

I have lost track of whatever it was that Multigrid might have said.

I expect so has Cuddles Multi-grid. He's a troll. He says whatever
comes into his head; it doesn't have to make sense, it just has to
stir up someone.

Andre Jute
"You don't need global feedback to build a good-sounding amplifier."
-- Henry Pasternack

If it cancels like a cancellation...
 

If it walks like a cancellation, if it talks like a cancellation, if
it sounds like a cancellation, if it causes less total secind harmonic
like a cancellation, if it *cancels* like a cancellation, it *is* a
cancellation.

I don't care who calls it an addition. That's just algebraic sleight
of hand performed inside the halves of a transformer: a cancellation
by any other name.

Quack. Eeeh. Deeeh.

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
 

I don't see the distinction between class A and class B operation, the
2H and even harmonic cancellation mechanism is the same for both classes
of operation, the only difference is the amplitude of the harmonics
generated. In both cases the 2H and even harmonics appear at the output
transformer as a common mode signal and are canceled in the load by the
action of the transformer.

Fair enough...

Once one device cuts off, no current flows, so there are no distortion
currents produced
by that device.

That does not follow, all that follows is that the sum of the DC
current, the fundamental current, and the harmonic distortion currents
sum to zero during the interval that the tube is cut off. That is not
the same thing as saying there are no distortion currents flowing while
the tube is cut off.

Mr. Fourier explains this for us.

But for ordinary ppl its difficult for them to see that
a number of harmonics flowing continually can sum to give a straight
line signal
with zero current at all.

[Big Snip]

But I see no need to do this labourious and unecessary math and graphing
task which may satisfy
the lofty smartarse acedemics.

Exactly, there is no need for you to do it because the academics have
all ready done it for you, so you already know that the 2H and other
even order harmonics will cancel in a PP stage, be it class A, class B,
or anywhere in between, or even if it is PP class C.

RDH4 does not have too many full descriptions to simply illustrate what
I have been saying.
So what use are the smartarse acedemics? have they earned their dinner?

Certainly they have, you are beginning to sound a little like Ian.

Hmm, some might say that the ordinary man would only feed them porridge
for dinner....

Me?, like Ian? heaven forbid!


The cancelation thinge is when the tubes are in class A and the 2H is
cancelled, similarly
to in any LTP.

"The cancellation thingy" is also when the tubes are in class AB or even
class B, where the 2H is also canceled.

One could prove almost anything with Fourier math....

Its easier to see the mutually cancelling 2H of each tube in a class A
amp.

To me it is easier to see the mutually canceling 2H of each tube in a
class B amp because the amplitudes of the harmonics are larger and as a
result are easier to observe.

Well they would be. I built a tunable filter with Q = 50
for all H between 1kHz and 10kHz, maybe I will connect to a cathode one
day
when an typical class AB signal is passing.


But while tubes are cut right off, summed distortion currents in that
tube = zero.

I have to say this does not mean the distortion currents are NOT
PRESENT.
They must be, but all summ to zero amps for the flat line of the tube
when cut off.
If you filter out harmonics of the current wave of a single tube of the
clas B amp,
you'll see the plethera of harmonics, and these are all sine wave
signals
which flow continuously, and without cutting off, but which when added
will give you zero current for
1/2 the cycle wave at fundememtal F.

Exactly, I think you have got it now, the even order distortion currents
cancel even while the tubes are cut off in either class AB or class B.

Its not obvious to most though. A cut off tube has no currents, period.


Work out the 2H current in each tube while in class A of each tube and
see how such currents are applied across the primary.
The reason 2H is low in class A is that the 2H currents are the same
phase at each end of the
OPT primary which cannot produce voltage in the load if the current is
applied in common mode to
both ends of the load.

The same is also true of the even order harmonic currents in class AB
and class B amplifiers!

Well yes, but they are less easy to see, as i explained above.

Your reasoning on this seems backwards, I think we both agree that the
harmonic currents in a class AB stage are larger than those in a class A
stage, I have always found it easier to see large things, within reason,
than to see small things.

In effect we agree then, but practically speaking a tube cut off has
zero
harmonic distortion currents for the time its cut off.

No, a tube cutoff does not have zero harmonic distortion currents for
the time it is cut off, what happens is that the harmonic distortion
currents, along with the fundamental current, and the DC bias current
sum to zero over that interval, which is a completely different thing
than saying the distortion currents are zero during that interval.

You even said this yourself a few paragraphs up from here where you said
"I have to say this does not mean the distortion currents are NOT
PRESENT. They must be, but all summ to zero amps for the flat line of
the tube when cut off."

OK, I am just showing the difference between commonsense perceptions and
theoretical perceptions involving Fourier analysis.

Complementary pairs of transistors which operate close to class B and
spend all their lives
each only half amplifying music create the same perception dilemnas.
One should be able to filter then measure the many harmonic currents in
one of the 0.22 ohm emitter resistors
and get a string of harmonics in a filter which will add up to zero
current for 1/2 the wave.

Then when you measure the other 0.22 ohm R there should be a similar set
of harmonic currents
present which must shunt those of the other bjt, giving a net current
applied to the
SINGLE output terminal which must sum substantially close to zero,
leaving only crossover distortions.

Its never explained this way in the books though.

There are never any graphs pf all the harmonics shown in correct to
scale amplitudes and phase.

Patrick Turner




Regards,

John Byrns

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

Output classes A and AB
 

There is no best load for a class B triode amp. Class B is a horrid way
to build any amp.

That is certainly a broad brush stroke, McIntosh did a nice business in
what were essentially class B amplifiers. Many of the older readers
here enjoyed Rock & Roll music during their teen years delivered via the
class B amplifier at the local AM radio station, without "horrid"
results.

The crock and troll was so full of distortions and screaming gits that
using low distortion
radio gear was wasted on them.


Maybe you meant low bias class AB.

No, I actually meant class B.

Well, class B is where corssover distortion is not pretty....

Do the load line analysis, or have a look at my website pages and print
out a set of curves for 6550
which are virtually the same as KT88.

http://www.turneraudio.com.au/loadma...p-triodes.html

I did the analysis as I earlier had said that I would. 1.25k does seem
to be a reasonable class B load for the KT-88/6550 just as you said.

But only where Ea isn't too high.

But KT88 were run with up to Ea = 800V, in beam mode, and with fixed
bias and for
140 watts and working class AB2, and large PO is available.
TT21 were ideal at this. I forget the load value used.


I asked the question because I have not built any amps with this tube and
am not familiar with it beyond the fact that the Quadraplex VTRs at the
Television Station where I worked as a youth had a couple dozen 6550s in
each VTR. The class B load of 1.25k seemed low to me relative to your
2.5k class A load, but I made two erroneous assumptions in asking that
question. First I didn't realize that ra for the KT-88 is as low as it
is, and second I didn't take into consideration that your class A load
line is dissipation limited rather than voltage limited as with the
class B load line.

5k is a typical RLa-a for a pair of KT88.

I have seen many with 3.2k.
When wound up to full power with a sine wave they might smoke.

Even 4k was used with Quad-II for a pair of KT66.

So when the amp works AB, the B part of the operation means each tube
sees only 1k
on alternate wave crests.

I quite like Ea = 500V and RLa-a = 8k for KT88/6550.

This means the B load is 2k min, and distortion in UL or CFB isn't too
much.

The change in load from the class A load of 1/2 RLa-a to class B load of
1/4 RLa-a
means that there is a dynamic gain reduction on each wave peak leading
to
3H and other odd H, 5H especially. Class AB beam and pentode amps are
worst, triode AB amps are better,
and make the best AB amps. UL is between the two with regard to
harmonics
generated by the cut off and load changes.


Patrick Turner.




Regards,

John Byrns

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

If it cancels like a cancellation...
 

Andre Jute wrote:

If it walks like a cancellation, if it talks like a cancellation, if
it sounds like a cancellation, if it causes less total secind harmonic
like a cancellation, if it *cancels* like a cancellation, it *is* a
cancellation.

I don't care who calls it an addition. That's just algebraic sleight
of hand performed inside the halves of a transformer: a cancellation
by any other name.

Politicians can sum and cancel almost any economic strategems,
mostly leaving behind them an odd order of broken promises,
and gaining no realization they were ever wrong.

Of some engineers, similar could be said, but for the man in the street
unused to Fourier, a cut off tube is as silent, harmless, dead, and
as distortion current free as a dead politician. Even better than a dead
cat.

We have a Federal Election in a few weeks and the BS is really flowing
well,
and all sorts of arguments happening.

The Liberal pollies can't see that reducing wages and putting up
interest rates really hurts ppl
all the while saying they are not doing either.

The pollies would tell us that income and expenditure currents of many
types flow and sum together to cancel
flows and cut offs in the next person along to give a single wonderful
income and fundemental wellbeing
to all. A Mr Furious dreamed up the theory of frequent political
expedients.

I heard that a candidate Blue Party candidate, Mr Phark Yutu is looking
to get a large vote.


Anyway, back at the amplifier, too much switching and stop start
amplifying is dreadful for the
music. The less we rely on Fourrier, the better.

Patrick Turner.










Quack. Eeeh. Deeeh.

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
 

"John Byrns" wrote in message
...
: In article .com,
: Andre Jute wrote:
:
: On Oct 25, 7:40 am, Eeyore
: wrote:
:
: 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.
:
:
: Regards,
:
: John Byrns

it seems the error made is in using the wrong mental model, that is
seeing the transformer primary as a resistance.
sure, PP operating on a pure R load would cease to
cancel when one side would be cutoff, as it would no longer
partake in the transfer curve for PP !
with a transformer, however, the transfer curve is a function
of the tight coupling of the primary halves, so always PP ;-)
(not the current, but the core flux is what matters)

Rudy

Output classes A and AB
 

Rudy wrote:

"John Byrns" wrote in message
...
: In article .com,
: Andre Jute wrote:
:
: On Oct 25, 7:40 am, Eeyore
: wrote:
:
: 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.
:
:
: Regards,
:
: John Byrns

it seems the error made is in using the wrong mental model, that is
seeing the transformer primary as a resistance.

To the vacuum tubes, RLa-a IS a resistance.

One R set up set up with a CT so any signal voltage applied between the
end and the CT
is applied in opposite phase to the other end, courtesy of the action of
the transformer.

Its RL a-a.

This transformed single R appears to the tubes somewhat variably.
In class B, while a tube is conducting, the load on the tube is 1/4
RLa-a,
and when both conduct in class A the load on each tube is 1/2RLa-a.

The secondary R load is always reflected to the two tubes as RLa-a, with
a CT.
The Ra of the tubes seen at the secondary varies.

Say you have Ra = 1.2k for EL34 in triode, and 6k:6 ohms OPT. ZR =
1,000:1
In class A, Ra-a = 2.4k and is transformed to 2.4ohms by the OPT.
In class B, only one tube is connected via 1/2 the primary, so only one
Ra = 1.2k
and OPT Z ratio has changed to 1.5k:6, because 1/2 the OPT primary isn't
operating
because one tube is cut off, so turn ratio is halved, Z ratio Ra in
class B = 1.5k/6 = 250,
so Ra at the output = 1.5k/250 = 6 ohms.

The Rout of the amplifier at the sec is Ra transformed, and notice the
change in Rout
between A and AB. Its responsible for major 3H, 5H and other
distortions.



sure, PP operating on a pure R load would cease to
cancel when one side would be cutoff, as it would no longer
partake in the transfer curve for PP !
with a transformer, however, the transfer curve is a function
of the tight coupling of the primary halves, so always PP ;-)
(not the current, but the core flux is what matters)

If one runs a class B PP amp with only ONE output tube,
you will get a badly distorted signal like the one you'd get
with a pure un-magnetically coupled R between B+ and the anode,
only at each end of the OPT pri there would be two phases of the same
distorted signal.
The load with one tube and OPT is the same as with a single R taken to a
suitably higher B+
than used with an OPT.

Fourier could describe what harmonics flow all the time despite cut off.

Furious arguemnts have followed about him.

Patrick Turner.


Rudy

Output classes A and AB
 

"Patrick Turner" wrote in message
...
:
:
: Rudy wrote:
:
: "John Byrns" wrote in message
: ...
: : In article .com,
: : Andre Jute wrote:
: :
: : On Oct 25, 7:40 am, Eeyore
: : wrote:
: :
: : 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.
: :
: :
: : Regards,
: :
: : John Byrns
:
: it seems the error made is in using the wrong mental model, that is
: seeing the transformer primary as a resistance.
:
: To the vacuum tubes, RLa-a IS a resistance.

for the purpose of loadline analyses, that is a fair approximation.
for analyzing the actual working mechanism, it lacks accounting for
the element of time, as the transformer actually stores energy
comparable to a fast rotating heavy object storing rotational impulse
so in case a tube is cutoff, it's plate will happily follow to wherever
that winding will take it.
(well, not so happy when certain V's are exceeded, perhaps:)

the output result will just be more or less symmetrically compressed,
3H, 5H, etc.

this is distinctly different from a transistor PP, mostly apparent
when you think of asymmetrical signals (music) driving it ;-)

the load each PP tube sees is not suddenly changed at cutoff,
it is in fact only balanced somewhat is a small region around the
bias point, as with larger swing the dynamic plate resistance
will vary more between the tubes, 3H will start to appear

a measure of 'true class A' power of an AB pp, meaning
the 3H would be low without nfb, would be the region where
current is below twice the bias and cutoff AND dynamic
plate R of the output tubes doesn't change more than
"X" percent.

.... to be discussed,
heh,

Rudy

:
: One R set up set up with a CT so any signal voltage applied between the
: end and the CT
: is applied in opposite phase to the other end, courtesy of the action of
: the transformer.
:
: Its RL a-a.
:
: This transformed single R appears to the tubes somewhat variably.
: In class B, while a tube is conducting, the load on the tube is 1/4
: RLa-a,
: and when both conduct in class A the load on each tube is 1/2RLa-a.
:
: The secondary R load is always reflected to the two tubes as RLa-a, with
: a CT.
: The Ra of the tubes seen at the secondary varies.
:
: Say you have Ra = 1.2k for EL34 in triode, and 6k:6 ohms OPT. ZR =
: 1,000:1
: In class A, Ra-a = 2.4k and is transformed to 2.4ohms by the OPT.
: In class B, only one tube is connected via 1/2 the primary, so only one
: Ra = 1.2k
: and OPT Z ratio has changed to 1.5k:6, because 1/2 the OPT primary isn't
: operating
: because one tube is cut off, so turn ratio is halved, Z ratio Ra in
: class B = 1.5k/6 = 250,
: so Ra at the output = 1.5k/250 = 6 ohms.
:
: The Rout of the amplifier at the sec is Ra transformed, and notice the
: change in Rout
: between A and AB. Its responsible for major 3H, 5H and other
: distortions.
:
:
:
: sure, PP operating on a pure R load would cease to
: cancel when one side would be cutoff, as it would no longer
: partake in the transfer curve for PP !
: with a transformer, however, the transfer curve is a function
: of the tight coupling of the primary halves, so always PP ;-)
: (not the current, but the core flux is what matters)
:
: If one runs a class B PP amp with only ONE output tube,
: you will get a badly distorted signal like the one you'd get
: with a pure un-magnetically coupled R between B+ and the anode,
: only at each end of the OPT pri there would be two phases of the same
: distorted signal.
: The load with one tube and OPT is the same as with a single R taken to a
: suitably higher B+
: than used with an OPT.
:
: Fourier could describe what harmonics flow all the time despite cut off.
:
: Furious arguemnts have followed about him.
:
: Patrick Turner.
:
:
: Rudy