[Patrick Turner]

Multi-grid wrote:


That's a bunch of nonsense and drivel, if the cancellation were simply
the result of "the finals are biased so that the change in
characteristic for the opposing phases approximately cancels", then the
amount of odd order distortion could also be changed by PP connection.
The fact is that the even order cancellation is not dependent on the
biasing of the tubes, beyond the requirement that the two tubes are
identical and are both biased the same.

So then John, when one is cut off, what is cancelling?

Look at the current wave in each tube. In AB or B the current wave has
SEVERE distortion spectra.
But the voltage across the WHOLE PP OPT primary is largely free of the
horrendous THD in each tubes current wave.

So its THD reduction by complementary action.

Its like two men sawing a log with a long bush saw, with one man at each
end either pulling or pushing the saw
all the way on each stroke. This is class A, and the differences in
applied forces by each man tend to cancel
and a linear saw action results.

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.

But in electronics, we can switch things on and off with absolute ease,
and there is no
Triode Union to make a boss's life a misery.



and on this: "The fact is that the even order cancellation is not
dependent on the biasing of the tubes, beyond the requirement that
the two tubes are identical and are both biased the same.

And how are two tubes 'the same' when one is in the traditional AB
bias piont? Further, as one starts cutting off( not far from its idle
point ), how is this remotely like what is going on in the other
phase?

I'll give you a hint.....nah, you'll get it eventually.

Do try and keep your answer out of the Nonsense&Drivel category.
cheers,
Douglas

We all get it in the end....

Check your High School Trigonometry book to understand why the even
harmonics cancel while the odd ones don't, it's a simple bit of math
that doesn't depend on bias, only that the two sides of the PP circuit
are identical.

No kidding. Where would one get these magical identical tubes?

Getting a class A amp to cancel its 2nd HD is not a trivial exercise.

It mainly IS a trivial exercize and was invented about 2 days after the
first triode was made.

Any two tubes are never identical, but are often within 90% equal to
each other.

Therfore the 2H cancelation in pure class A PP is so substantial that
90%
of the 2H of either tube is cancelled away leaving far far less than if
the two tubes were
used in SE & parallel for the same class A power.


I suggest you try it on your AB amps, just for kicks.
cheers,
Douglas

Trying to make the driver stage make distortions that will cancel the
output stage's PP distortions
is extremely difficult to achieve, and nobody sets out to do it. I tried
it, I failed.
Trouble is that PP output tube distortions change in amplitude and
perhaps phase and spectra
with the dynamic changes in loads connected. Speakers are not lile a
simple one value resistance.
One can manage to more easily exploit 2H cancelling phenomena in SE amps
between SE driver and SE output tube.
PP amp makers all sensibly try to make the driver linear, and output
stage linear,
and then not have to apply too much NFB to cancel the little amount of
mainly 3H distortion..

Some think this is BS, and build a class B thing with lots of THD/IMD
without any NFB.
Then they apply lots more NFB than the fella using mainly all class A in
his output stage.
The unruly class B amp with a shirtload of NFB in the output stage
McIntosh, emitter follower etc,
and a bootfull of global NFB has become the natural choice of
profit hungry amp makers. If you hunger for best music, you must spend
more
for the inefficiency costs of class A.

The staus quo among all amps is 95% AB with rough working AB / near B
devices switching with lots of NFB.
5% are either mainly class A, or AB tube amps, or the new PWM amps with
whatever correction facilities thay can muster.
In 20 years time, all the SS AB amps will have become extinct like
dinosaurs.
The PWM amps can sound as good as generic low bias SS amps and be far
more efficient and far cheaper and lighter/smaller.
Tubes will remain if the Greenhouse Police are not too zealous, and they
will hold sway amoung a
small minority of listeners actually willing to put their money on the
sound,
rather like there will always be someone who likes sailing yachts around
the bay,
rather than drive around the bay in a stink boat.

Patrick Turner.