[Patrick Turner]

On Jul 20, 7:00*pm, "Alex Pogossov" wrote:
"John Byrns" wrote

The claim is that the pull voltage is proportional to the carrier, i.e..
good
for all carrier levels, unlike in Partick's biased detector (with a fixed
pull) which is good only for a certain range of RF carrier.

Are you saying that Partick's biased detector also combats AC/DC loading
distortion?

Sure. Patrick's heavily biased detector combats both slew rate and AC/DC
distortion. But since it is heavily biased it has very low input impedance
at low signal (1.7K, as I showed in part 2). Therefore, Patrick's detector
must have a cathode follower in front.

The capacitor reactance into which the diode charges may have low Z of
1k7. The reactance of 200pF at 455kHz is 1.74k ohms. If it is 100pF as
so commonly used in most old "booooooooring detectors" Alex mentions,
then the high Z IFT circuit has to charge into 3.4k approx, which
means the boooooring detector causes terrible loading effects - one
might think, but waveform displays do not indicate its as bad as Alex
implies. Anyway, my detector sure gives MUCH better detector accuracy,
and the 12AU7 handles it fine. Other suitable triodes are 6DJ8, 12AT7
with higher gm.



In this circuit from RDH4 this extra pull needs to be as small as possible
not to reduce significantly the input impedance at low signal, which would
in turn raise the sensitivity threshold and cause the same distortion by a
different machanism.
Yet the pull must be enough to overcome the effect of the volume control AC
coupled. Thus in this circuit the pull is critical. That is why they made R3
a trimpot.

Indeed.



A problem here is that the pull voltage is very-very "averaged" by the
0.1uF
capacitor across R3, and it will not track signal level well. If a signal
varies quickly or if it composed of a composite multi-tone audiom it will
not work perfectly.

It's not clear to me what you are saying here? *What difference does the
modulation make as long as we are talking a normal AM signal? *When you
talk
about "not tracking the signal level well" what are you talking about?
Normally
the carrier level of a booooring AM signal is constant and doesn't vary,
are you
talking about fading?

You are right. Tracking is not that importamt. Fast fading is virtually
nonexistant for MW broadcasts, especially local stations.


Besides the pull is taken from the primary LC tank of
the IFT, and the IFT transformer ratio is not fixed, but depends on the
accuracy of the radio tuning.

Sounds right, however I would think it would work OK as long as the
compensation
voltage is at least the minimum level required for any given carrier
level.

This circuit increases the effective load on the secondary of the IFT,
effectively lowering the value of the detector load resistor. *An
improvement in
modulation acceptance of the ordinary booooring AM detector could
acomplished by
simply lowering the value of the 500k resistor to provide the equivalent
load,
although it wouldn't get us to 100% as this circuit purports to do.

In the 21-st century you would rather simply use an inverting op-amp
which
would be creating a pull, EXACTLY proportional to the instantaneous
signal
level from the same detector. You need only one diode.

If you have an extra OP Amp why go to all this trouble, why not simply use
the
OP Amp to buffer the output of the detector to provide an AC/DC load ratio
of
unity?

You are right. Use an op-amp as an RF buffer and a biased detector. Or
unbiased detector and an AF buffer, or both. I use an emitter mildly biased
detector and an op-amp buffer after it, another op-amp working as an AGC
with infinite gain.

We need to be honest with ourselves -- SS stuff works so much better in
every and any respect. So much more convenience and versatility.

This is a tube use discussion group, and who gives a **** about SS?

Yet there are some *ppl* who still try build stuff with tubes. These are
like Commmunists, creating problems (hum, noise, drift, low efficiency,
weight, size, distortion, heat, instability, aging, tricky craftmanship,
etc....) and then overcoming the self inflicted problems with enormous
puffing, blowing steam, slogans and chest-pounding....

Steam engines are fun to watch or ride, but we use electric trains,
thanks...

Sorry if I offended anyone.

You'd have to work harder to offend me. But I hate most SS gear. And
the AM tuners full of SS crap don't sound as good as the tube stuff I
repair, re-engineer, or have built for myself.

Humans are fun to watch or ride ( if she's a good looking nice hot
sheila begging for it ), but we now use computers when we jerk off,
because shielas won't **** like they used to.

I like old fashioned telephones and copper wire connections. I will
never get a mobile phone unless I am forced to get one. The ****en
audio in mobile phones is awful, and they keep dropping out. I hated
the audio in my 1998? Metz TV set so much I removed all the speakers.
The analog sound from set top box now goes to a proper old fashioned
SS hi-fi amp and then to two nice hi-fi speakers, and sound is
passable; would be better if tubed though. The Metz was given to me
for "free" for some repair work, nice picture, it was the last CR tube
model, and so heavy that if it fails I just will put a brick through
screen, and break it up to fit into my dumper bin. Then I might return
to not having any TV set, like I didn't for 14 years before the Metz.

I am very glad to keep tube amplifiers alive, and build a few new
ones. So are my customers.

I'm definately not a Communist, or a Nazi, or raging mad Capitalist
despite so many attempts by a few here to label me one way or other.
But I do believe in social justice and maintaining high quality
function from electronics -- AND humans if possible. The world is
turning to PWM amps which are 95% efficient and don't need big
heatsinks. But meanwhile everyone seems to be wanting more of
everything, and in each room of the 27 room house they must have, with
two cars or more in the garage. Sure we have electric trains in big
cities but meanwhile have terrible traffic jams and excessive
consumption of everything possible. So even though we got rid of most
tubes, and nearly all steam trains, and now have efficient amps, we
are polluting the Planet much more than we used to, and also because
we had so many children, who will all want more and ****en more, along
with many future billions of ppl in developing countries.

So Alex, when you say things like ""Yet there are some *ppl* who still
try build stuff with tubes. These are
like Commmunists, creating problems (hum, noise, drift, low
efficiency, weight, size, distortion, heat, instability, aging, tricky
craftmanship, etc....) and then overcoming the self inflicted problems
with enormous puffing, blowing steam, slogans and chest-pounding....""
you are just pushing silly propaganda which everyone here should know
how to ignore.

I am grateful that a tiny minority maintains **WORKING** examples of
past technologies such as steam engines, aeroplanes (such as old WW2
planes) so that today's ppl have a REAL way of understanding the past
so they may know the future. Unfortunately, the world seems to be
drifting toward many more nuclear armed nations and maybe we won't
have to wait too long before some crazy country demonstrates what
happened in Hiroshima somewhere in some middle eastern city to settle
a dispute. So some really awful olf technology seems to be hanging
around when it should have been dismantled for ploughshares and
alternative and sustainable technologies. Probably it is benign that
each stereo SS amp that is finally dumped because it just wasn't worth
fixing is then re-cycled into a 5.1 receiver which will have an even
shorter life.
Its a contradictory world.

Anyway, the human race is heading into unknown territory, and the
whole heap of problems encountered along the way are all of far
greater significance than suffering the presence of a few "communists"
building a few tube amps. But in a way, Alex is right. Tube amps are
made in China, and guess what folks, China is stage managed by the
Chinese Communist Party, which has a rather frightful record on human
rights. I'm not sure what the Chinese nation may change into, but
change happens, and nothing stays the same. Old Buggers clinging to
old ideas about 1,001 things WILL die and New Buggers will take over
and nobody knows how the new world will fare over the next thousands
of years and methinks that considering tube amp making to be a
negative activity is extremely short sighted.


Patrick Turner.