In article . com,
Multi-grid wrote:

On Oct 28, 2:29 am, "Doug Bannard" wrote:
"John Byrns" wrote in message

...

In article ,
Patrick Turner wrote:

John Byrns wrote:

In article ,
Eeyore wrote:

Patrick Turner wrote:

One cannot have distortion cancelling by one tube cancelling that
in
another when one is cut off.

THANK YOU !

Basics do matter.

Indeed they do, but neither Patrick, myself, or anyone else is correct
on every issue. In this case Patrick has vigorously asserted that this
view, which he holds in common with you, is true, but he has failed to
even attempt an argument that might demonstrate its truth. Patrick is
an extremely skilled and talented fellow in the practical aspects of
tube amp design and construction, but he has a very limited
understanding of what is going on behind the scenes in the theory of
tube amp operation.

I think my website might indicate that your are not quite right about my
levels of understanding.

Can you point out where on your website it explains why "One cannot have
distortion cancelling by one tube cancelling that in another when one is
cut off"?

I admit I do not understand this notion that even order distortion can't
be canceled when one tube is cut off, and I would really appreciate an
explanation from someone that understands why cancellation should fail
to occur?

Why I don't understand this is because even order distortion is canceled
in class B amplifiers, where one or the other tube is cutoff over almost
the entire cycle, yet cancellation still occurs! I am having trouble
reconciling this with your claim that cancelation can't occur. Also the
related observation that there is no even order distortion in a sine
wave which has had both the tops and bottoms of the sine wave
symmetrically clipped off.

Please explain.


John:

I think that the problem that Patrick may be having with this concept is
more one of semantics than anything else. It's hard to intuitively view
"cancellation" of even harmonic terms if both tubes are not conducting at
the same time. Let's view it with a very small amount of math thrown in.
Sometimes intuition does not take one far enough.

Take a pure Class B amplifier. Each tube or transistor conducts for 180
degrees (forget about any crossover effects). The current in each half of
the OPT primary, neglecting any magnetizing current is a half sine. Do a
Fourier expansion on this half sine and it's full of harmonics...all even
ones. But as long as the two tubes are well matched, the two half primary
currents are of identical shape and add in the OPT secondary to produce a
full sine, symmetrical about its zero voltage axis.
The even harmonics are gone. Is this cancellation? Well I guess you might
call it that.

Basically any waveform symmetrical about its zero voltage axis will contain
no even harmonics...even a symmetrical square wave. Similarly, if we
symmetrically clip the tops off both the positive and negative half cycles
of our sine we do not see any even harmonic content either for the same
reason. The ideal push-pull output stage using matched devices is not going
to produce even order distortion regardless of its operating class, whether
it be Class A, Class AB or Class B.

If in any doubt, see pages 300-301 of RDH4, or any university level math
text covering Fourier analysis.

Best Regards : Doug Bannard, P. Eng.


That looks like not created v. cancelled from here...

It wouldn't look like "not created" if you used a current probe to
observe the anode current waveforms of each of the two PP output tubes.


Regards,

John Byrns

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