On Sun, 21 May 2006 18:27:52 -0700, "Lynn Coffelt"
wrote:


"Gary Schafer" wrote in message
.. .

It looks like AM is really becoming a "lost art". Of all the responses
here, only one or two guy knows what you are talking about.

The purpose of the choke inductor like you are describing is to keep
the DC off the secondary of the modulation transformer. This allows
the use of a modulation transformer with much less iron, which will
give better low frequency response. If the finals DC current is run
through the modulation transformer, as is commonly done, there needs
to be an air gap in the transformer to keep the core from saturating
from the DC current. The bigger the air gap the less inductance, the
greater amount of DC current it can handle before saturation and the
poorer low frequency response. To increase the inductance with a
larger air gap requires more iron (larger core) to bring the
inductance back up.

By using a choke and capacitivly coupling the audio from the
modulation transformer to the final plate there is no DC on the
secondary of the modulation transformer to cause saturation. The
modulation transformer then can be any audio transformer. It doesn't
need an air gap. The transformer can have much less iron in it for the
same amount of inductance as one with an air gap. Very good low
frequency response can be had this way.

By the way the screens of the final tubes need to be modulated along
with the plates too. The easy way is to supply the screens through a
dropping resistor from the top of the choke, the same point that feeds
the plates.

THIS IS NOT HEISING MODULATION. It is regular old plate modulation
just like you would do with a conventional modulation transformer.

Yes power transformers can be used quite successfully as a modulation
transformer with this type of setup even though there is no air gap in
the transformer. There is no worry about core saturation because there
is no DC current on the transformer.

A large enough choke for this application can be had by putting
several chokes in series to obtain the required inductance. Regular
power supply chokes work fine. Do not use a swinging choke! It has no
air gap and will saturate.

A TV transformer will not work well as a choke because it has no air
gap and the core will saturate quickly with DC going through it.

Cathode modulation is very similar to grid modulation in performance.
After all audio is placed between the grid and cathode just the same
as it is with conventional grid modulation. The amount of cathode to
plate modulation is minimal.

Screen modulation is similar in performance to grid modulation also.
Efficiency is around 35% carrier efficiency. Very tricky to tune up
properly.

Low level modulation with a linear amplifier behind the modulated
driver also has an efficiency of around 35%. Excellent AM can be
generated this way as is done with some SSB transmitters in the AM
mode. If properly set up one can not tell the difference between it
and a high level plate modulated transmitter except that the plate
modulated transmitter will probably have higher distortion.

73
Gary K4FMX

Oh, my gosh, you're right, Gary! It is not Heising modulation. For 60
years now I've had the mistaken impression that "Heising" implied simply
that a series choke was used in the plate/screen supply to allow ANY
modulation to be applied, either by modulator tube directly, or through a
suitable capacitor.
I am embarrased. "Googling" brought up a whole lot of other aspects of
high level modulation that I didn't know (or hopefully just forgot!)
Does anyone remember negative peak clipping that was the rage about
1958 or so? (Amateur only, as far as I know)...... If the scope wasn't
lying, one of my home brew "high level" AM rigs was capable of modulation of
over 100%. There was an awful lot of controvery at the time, and our nearest
FCC chief engineer said that he didn't care if it did, it was still illegal.
Sigh.
Old Chief Lynn, W7LTQ


Hello Chief Lynn,

You are a couple of years ahead of me. I didn't get started until
around 1960.
I built a small modulator a few years ago and played around with
negative peak clipping. Never could get it to sound good though.
It will allow higher positive peak modulation without over modulating
in the negative direction but the clipping on the negative side
created a lot of distortion.
Broadcast stations regularly run around 120% positive peaks and near
100% negative but they usually do it with clipping at low levels and
clean things up after the clipping.

I remember hearing about the FCC being in a quandary about some of the
super modulation schemes like the Taylor system that could create
upwards of 150% or greater positive modulation.

73
Gary K4FMX