In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

I have had a chance to think about this a little more, and pending doing
the hard math, I don't believe that using a push pull IF stage will help
with "the 2H distortion so prevalent in a typical SE pentode stage which
is a variable mu type." Push pull stages only cancel even order
distortion products, and the following IF transformer also removes the
even order distortion products, including "2H", so a push pull IF stage
will only help in that it will provide more power to drive the detector
diode, which with your cathode follower scheme is pretty much irrelevant
anyway.

I assure you there are references to IMD caused by IF amp nonlinearities.
Maybe you should inspect the output of a typical pentode IF amp.
You may not easily see less than 2% thd on the modulation waveform on a CRO.
Perahps you ought to measure a typical IFamp, by using a linear detector, as
described.

Read what I said again, I didn't say that there is no "IMD" caused by the
IF amp nonlinearities, what I said was that the even order distortion
products, including "2H", are filtered out by the IF transformer before
reaching the detector.

What mechanism are you suggesting that allows a significant amount of the
even order distortion products make it through the IF transformer?

As far as measuring the distortion at the detector output, I don't have an
AM generator with sufficiently low distortion to do this, I wish that I
did have a generator good enough to measure the effects we are talking
about.

But I am sure that a balanced amp with pentodes, with a common cathode.
each with a lot of 2H when used alone as an SE amp, would display
less thd than the SE case.

This is not a "SE" audio amplifier, this is an IF amplifier, which in a
tube radio is typically followed by a double tuned IF transformer. The
double tuned IF transformer filters out the even order distortion
products, allowing only the odd order distortion to pass, just like your
push pull stage. The only thing the push pull IF stage is going to do is
increase the power available to drive the detector.

They use PP balanced oscillators when a signal output
is required to be cleaner than an SE type.

An oscillator is a whole different matter, we don't want oscillator
harmonics, or RF harmonics, getting into the mixer where they can create
spurious responses.

In FM IF circuits, no need for balanced amps,
because the linearity don't matter much, since the limiter output
is just a 10.7 MHz signal which is devoid of amplitude changes,
and distortion products of 10.7 MHz envelope can't
appear in the audio, since the detector only
detects variations in the distances between the
up/downs of the 10.7 MHz waves.

I'm not an expert in this, but I think this is a fallacy, and the
linearity of FM IF stages is actually quite important.

Nope,

I suggest you read up on your FM theory, there were a lot of good papers
published on this subject in the 1950s.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/