K7ITM wrote:

Low level modulation is a huge waste of RF power?? Hey, _AM_ is a huge
waste of RF power, and of spectrum to boot! Except with unusual
modulating waveforms, 2/3 of the power is in a useless carrier. Now,
compared with plate modulation of the PA, low level AM modulation
followed by a linear amp may be somewhat less efficient (though a waste
of DC power, not RF power), but remember, the modulator, presumably
being a linear audio amplifier, isn't all that efficient either. In
addition, there ARE ways to generate AM with low level modulation and
efficient RF amplification--they have been used in AM broadcast
transmitters.

For the cost of a good high-level modulator, how many kilowatt-hours
can you buy?

Cheers,
Tom

The only modulation method that I've ever found to be of any real efficiency
in generating AM is "outphasing" or "ampliphase". You can use the /most/
effective PA designs - optimised FET-based Class E stages can be over 90%
efficient! This approach completely obviates all the problems with
modulation transformers or big chokes. Haven't tried it with valves
("tubes" - US) - perhaps that should be a future project.

Bob

--
Everything gets easier with practice, except getting up in the morning!

Tony,

I did exactly what you are describing. I used a solid state audio amp,
and drove a tube type audio output transformer backwards to step up the
impedance from 8 ohms to several thousand ohms. I used a 4 uf oil capacitor
to couple that across a HV choke that carried the RF final amp plate
current. For a choke I used a 125 watt Stancor poly-pedance modulation
transformer where I used as much winding on the primary as available. It
worked very well.

It might be easier to consider cathode modulation, and in this case the
transformer turns ratio will be much less, and the HV problem will also be
less. Maybe a 110/220 vac to 12vac 5 amp filament transformer will work out
here. This is just a guess, but it might work. Cathode modulation is a
combination of plate and grid 1 modulation, so the % eff is somewhere
between 66% (plate) down to 33% (grid) , and might end up at about 50%.
Consider the 220 vct winding where the grid return goes to the CT, and the
cathode goes to the top of the 220 v winding, and the bottom of the winding
goes to ground. Your turns ratio need may vary, but the concept proposed is
workable.

Jim
WD5JKO


"Tony Angerame" wrote in message
...
I've acquired a high power 50 watts plus tube type hifi output
transformer. I'd like to try modulating a pair of 6146's with a hifi amp.
The circuits I see use a modulating inductor in series with the B+ to the
final to keep single ended dc off the transformer secondary.

Here's the rub. Where do you find a 30-50 henry inductor for 800+ volts?
I was thinking of checking out the secondary of a tv transformer? (Not
many left used to be common whatever happened to those tv's).

Anyone have any ideas on winding one? Seems to me taking an old
transformer apart and scramble winding as much wire as one can afford
might do the trick?

Ideas?


Thanks,


Tony,

WA6LZH

On Thu, 18 May 2006 19:56:21 -0700, Tony Angerame
wrote:

I've acquired a high power 50 watts plus tube type hifi output
transformer. I'd like to try modulating a pair of 6146's with a hifi
amp. The circuits I see use a modulating inductor in series with the B+
to the final to keep single ended dc off the transformer secondary.

Here's the rub. Where do you find a 30-50 henry inductor for 800+ volts?
I was thinking of checking out the secondary of a tv transformer? (Not
many left used to be common whatever happened to those tv's).

Anyone have any ideas on winding one? Seems to me taking an old
transformer apart and scramble winding as much wire as one can afford
might do the trick?

Ideas?


Thanks,


Tony,

WA6LZH

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

"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

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

Gary Schafer wrote:

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.

Correct, and the distortion can be pretty nasty, too.

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

It was the /only/ type of AM I used at first, as I was unable to get big mod
transformers. It worked well enough, though it was difficult to get close
to 100% mod. Lots of envelope feedback cured the non-linearity
distortions.

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.

Low-level mod with a linear is very inefficient (with respect to DC input),
but is sometimes the only really practical way to proceed. Again, lots of
envelope feedback can sort out the distortions caused by less than perfect
PAs!

Bob
--
Everything gets easier with practice, except getting up in the morning!

On Mon, 22 May 2006 19:47:14 GMT, biascomms
wrote:

Gary Schafer wrote:

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.

Correct, and the distortion can be pretty nasty, too.

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

It was the /only/ type of AM I used at first, as I was unable to get big mod
transformers. It worked well enough, though it was difficult to get close
to 100% mod. Lots of envelope feedback cured the non-linearity
distortions.

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.

Low-level mod with a linear is very inefficient (with respect to DC input),
but is sometimes the only really practical way to proceed. Again, lots of
envelope feedback can sort out the distortions caused by less than perfect
PAs!

Bob

Plate modulated transmitters are not all that low distortion in
general. It is much easier to get a very clean low distortion AM
signal by doing it at low levels and amplifying with a linear amp,
provided the amplifier is run properly.
At low levels modulation can be done with class A modulation stages as
opposed to typical class B high level modulators and non linear
modulation transformers.

Even with 33% carrier efficiency of the linear amp the total amount of
AC power required from the power company isn't much different. With
the linear amp there are no large modulator tube filaments to light,
screen, bias and plate supplies to run the modulator.

With the new FCC rules limiting the peak envelope power to 1500 watts
it makes more sense to use low level modulation and an amplifier than
it did with the old rules. Now plate efficiency does not matter, you
can still put the same amount of power into the antenna no matter what
the efficiency of the transmitter is. In the past you wanted the
highest plate efficiency that you could muster to get the most power
into the antenna.

I believe that the thinking of the past that "plate modulation is the
only way to go" has more to do with not coming to grips with the fact
that it is about how much power we can put into the antenna and not
how much plate input we run as in the past.

73
Gary K4FMX

I believe that the thinking of the past that "plate modulation is the
only way to go" has more to do with not coming to grips with the fact
that it is about how much power we can put into the antenna and not
how much plate input we run as in the past.
================================
Question : Do the new FCC rules, eg 1500 W PEP mean ,transmitter output
power or power to the antenna , the latter meaning that any feeder
losses may be compensated by a higher max. transmitter output ?

The latter is the case pertaining to the UK amateur radio licences.
With a maximum output of 26dBW = 400 W PEP (on most bands) , in the case
of 3dB feeder loss ,the permitted transmitter power is 800 W PEP.

Frank GM0CSZ / KN6WH (who has NOT read as yet the 'new' FCC max
power rules)

The FCC allows broadcast stations to compensate for losses before the
antenna, but not amateur stations. You are legally limited to 1500
WPEP output at the transmitter.

Glenn AC7ZN