Well I havn't heard any AM on the HF bands in a long time, but I stand
corrected that some people still use it. Granted it's in the minority
and on "calling frequencies". I don't think those screen grid modulators
found in novice rigs were of much use though. They didn't have
much punch and poor audio. If you are going to go AM, a real plate
modulator is the only way to go. (I shudder to think what a plate
modulation transformer would cost these days....)

Have to agree there Ken as I have had a few screen jobs in the "good old
days". They weren't good for too much only to get on phone for a youngster
who didn't have the bucks for a SSB or plate modulated rig. My old Globe
Chief had the screen modulator but did not perform well at all .

God Bless ....Tom KI3R Belle Vernon PA

You will probably be best off winding your own. My advice is this:
find the core size required to handle an amount of power at 60HZ equal
to the audio power needed, find an old power transformer of that core
size, and unwind it. Count the turns on the 117VAC winding to get turns
per volt, and figure the wire size from the DC input to the modulator
for the primary windings, and from the DC input to the PA(for each
side), and turns from the turns per volt for power. Do not push wire
size the the limit(small) becuase the windings also must handle the
peak currents produced by the audio power, which are higher than the DC
average.

This transformer will have more than enough iron in the core to handle
the AC power, as audio is at a higher frequency and the inductive
reactance higher. DC in the secondary will lower the inductance, but
you should still have enough. If you were trying to handle deep bass
frequencies(not needed nor wanted in a "communications" app as they eat
percentage of modulation), you would use a still larger core, figuring
audio power PLUS the power represented by a 60HZ AC current whose RMS
value is equal to the current drawn by the PA. This "overkill"
transformer would give less distortion on deep bass.

For a ham or any other communications application, frequencies below
300 HZ or so are far less important than they are to a broadcaster, but
be SURE not to allow frequencies to pass the modulator's driver that
the output transformer is not efficient at, or heavy currents will be
drawn and efficiency will suffer badly. If you design a modulation
transformer only to handle a narrowband voice range, the modulator
input transformer should also reject any frequencies lower than the
output transformer can handle, or these frequencies can be filtered out
elsewhere.

Otherwise, just speaking too close to the mike(which produces a deep
bass response) will heat up your modulator.




garigue wrote:
Well I havn't heard any AM on the HF bands in a long time, but I stand
corrected that some people still use it. Granted it's in the minority
and on "calling frequencies". I don't think those screen grid modulators
found in novice rigs were of much use though. They didn't have
much punch and poor audio. If you are going to go AM, a real plate
modulator is the only way to go. (I shudder to think what a plate
modulation transformer would cost these days....)

"Ken Scharf" wrote in message
news
garigue wrote:
You might as well leave out the
AM modulator since it will be useless today. Back in '66-'67 there
were still some people on AM (mostly on 80 and 10 meters), but today
it's completly gone.

OH, Really!?? Snert! - Sporf! - Snurt! TNX for the laugh!

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
Pueblo, Colorado | @ | Jonesy | OS/2 __
38.24N 104.55W | config.com | DM78rf | SK


Yepper ....just like the fellow on 2 meters said .......nobody uses CW
anymore .....its DOA . I just like to put up my feet, lean back, and
open
an Iron City Beer and enjoy the quality of those AM signals ... and this
from a 99.99% CWist. My buddy built on of those rigs in 66 and it worked
fine with his Mor-Gain antenna ...even with the hardware cloth safety
mesh
covering the chassis. Lots 'o fun.

God Bless and 73 all ..... KI3R Tom in Belle Vernon PA


Well I havn't heard any AM on the HF bands in a long time, but I stand
corrected that some people still use it. Granted it's in the minority
and on "calling frequencies". I don't think those screen grid modulators
found in novice rigs were of much use though. They didn't have
much punch and poor audio. If you are going to go AM, a real plate
modulator is the only way to go. (I shudder to think what a plate
modulation transformer would cost these days....)

Ken,

You admit you have NO knowledge of present day ham usage of AM. And
apparently NO knowledge of old timey AM use. Properly designed, built and
used low level modulation works and sounds good.

I get glowing reports with my DX-60B, slightly modified to improve the bass
response. And a Bullet Astatic Mic.

And with the new rice boxes, if you adjust it right, you can get excellent
audio out of them.

With the FCC reduction of power for AM full carrier, plate modulated
(375watts) the big monsters are going the way of the Dinosaur.

Dan/W4NTI

Dan/W4NTI wrote:
"Ken Scharf" wrote in message
news
garigue wrote:

You might as well leave out the
AM modulator since it will be useless today. Back in '66-'67 there
were still some people on AM (mostly on 80 and 10 meters), but today
it's completly gone.

OH, Really!?? Snert! - Sporf! - Snurt! TNX for the laugh!

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
Pueblo, Colorado | @ | Jonesy | OS/2 __
38.24N 104.55W | config.com | DM78rf | SK


Yepper ....just like the fellow on 2 meters said .......nobody uses CW
anymore .....its DOA . I just like to put up my feet, lean back, and
open
an Iron City Beer and enjoy the quality of those AM signals ... and this
from a 99.99% CWist. My buddy built on of those rigs in 66 and it worked
fine with his Mor-Gain antenna ...even with the hardware cloth safety
mesh
covering the chassis. Lots 'o fun.

God Bless and 73 all ..... KI3R Tom in Belle Vernon PA



Well I havn't heard any AM on the HF bands in a long time, but I stand
corrected that some people still use it. Granted it's in the minority
and on "calling frequencies". I don't think those screen grid modulators
found in novice rigs were of much use though. They didn't have
much punch and poor audio. If you are going to go AM, a real plate
modulator is the only way to go. (I shudder to think what a plate
modulation transformer would cost these days....)


Ken,

You admit you have NO knowledge of present day ham usage of AM. And
apparently NO knowledge of old timey AM use. Properly designed, built and
used low level modulation works and sounds good.

I get glowing reports with my DX-60B, slightly modified to improve the bass
response. And a Bullet Astatic Mic.

And with the new rice boxes, if you adjust it right, you can get excellent
audio out of them.

With the FCC reduction of power for AM full carrier, plate modulated
(375watts) the big monsters are going the way of the Dinosaur.

Dan/W4NTI



Well I have to agree with the other poster who also owned a novice rig
with a grid modulator and nobody could hear him. I had the same
experience. The DX-60B may have had a well designed modulator and you
have it adjusted just right. (also you are not trying to over drive
the final). A plate modulator for a low power rig would make sense
if you really want to operate AM and put out a good signal.
I remember in some old ARRL handbooks there were some mobile am rigs
that used a filament transformer as a modulation transformer with a
pair of DS501 transistors (from old car radios) in the modulator.

WSQT wrote:

You will probably be best off winding your own. My advice is this:
find the core size required to handle an amount of power at 60HZ equal
to the audio power needed, find an old power transformer of that core
size, and unwind it. Count the turns on the 117VAC winding to get turns
per volt, and figure the wire size from the DC input to the modulator
for the primary windings, and from the DC input to the PA(for each
side), and turns from the turns per volt for power. Do not push wire
size the the limit(small) becuase the windings also must handle the
peak currents produced by the audio power, which are higher than the DC
average.

This transformer will have more than enough iron in the core to handle
the AC power, as audio is at a higher frequency and the inductive
reactance higher. DC in the secondary will lower the inductance, but
you should still have enough. If you were trying to handle deep bass
frequencies(not needed nor wanted in a "communications" app as they eat
percentage of modulation), you would use a still larger core, figuring
audio power PLUS the power represented by a 60HZ AC current whose RMS
value is equal to the current drawn by the PA. This "overkill"
transformer would give less distortion on deep bass.

For a ham or any other communications application, frequencies below
300 HZ or so are far less important than they are to a broadcaster, but
be SURE not to allow frequencies to pass the modulator's driver that
the output transformer is not efficient at, or heavy currents will be
drawn and efficiency will suffer badly. If you design a modulation
transformer only to handle a narrowband voice range, the modulator
input transformer should also reject any frequencies lower than the
output transformer can handle, or these frequencies can be filtered out
elsewhere.

Otherwise, just speaking too close to the mike(which produces a deep
bass response) will heat up your modulator.


A hint on rewinding transformers. When you disassemble a power transformer
you will note that the laminations alternate (IE: open end of even numbered "E"s
point left, odd numbered ones right). If you took apart a filter choke you
would find that ALL the laminations were oriented the same way. For an
audio transformer, arrange the laminations in groups, and alternate the
groups. Divide the total laminations into 3-10 groups.

Why? In the case of the choke, which must carry DC, we don't want the magnetic
flux to saturate or the inductance will start to drop at some current.
Having all the laminations go in one way leaves a nice air gap in the core
which prevents core saturation. In the case of a power transformer which
carries only AC we want the magnetic flux to be tight to keep efficiency up.
For an audio transformer which must carry some dc (tube plate current),
and ac (audio), a mix of these works out best. Note that in the case of
a push pull primary (modulation or output transformer) the balanced dc
current cancels out and we don't get much saturation of the core. A single
ended output transformer WILL have this problem, as will the secondary
winding of the modulation transformer.

A swinging filter choke should have some core saturation, probably it uses
two or three groups of laminations.