"Antonio Vernucci" wrote in message
...
The aim was, guess what? Suppressor-grid modulation! I can
remember
hearing some of these rigs on the air. I don't know just what
percentage of
modulation was possible, but I remember it being pretty weak. Good
strong
signal with very low modulation level.
The aim had been to use what parts one had, and it surely was an
inexpensive
way to join the AM crowd!

Old Chief Lynn, W7LTQ

In the 1957 ARRL Handbook, these particular tubes were also used in a
cathode driven (grounded grid) amplifier.

73, Barry WA4VZQ

Yes, I was aware of the "modified 1625" but for linear amplifier usage.
Now I know that they were also used for suppressor-grid modulated
stages.

But now a doubt come to my mind: the 1625 is a beam-power tube, not a
pentode (actually it is an 807 with 12.6V filament). So, how could it
work in suppressor-grid modulated stages?

73

Tony I0JX

Read Lynn's post again. He said the carrier was strong but the audio was
quite weak. It is quite difficult to get over 95% modulation with a
suppressor modulator. With beam power tubes, you get far less than this.
Also you get lots of distortion because the beam forming plates do not
linearly control plate current, so you have to lower the modulation level
even further to make the audio readable by listeners. Hence the "strong
signal with very low modulation level" makes sense. Fortunately with
pentodes, it is easy to drive the output to zero (negative modulation).
The real problem occurs when the suppressor is driven very far into the
positive voltage region and it starts drawing current. Typically the
suppressor grid can only dissipate a few watts. Since current is drawn
during the positive peaks, an audio driver must present a low impedance.

73, Barry WA4VZQ

Nordic Breeds WA4VZQ wrote:
Read Lynn's post again. He said the carrier was strong but the audio was
quite weak. It is quite difficult to get over 95% modulation with a
suppressor modulator. With beam power tubes, you get far less than this.
Also you get lots of distortion because the beam forming plates do not
linearly control plate current, so you have to lower the modulation level
even further to make the audio readable by listeners. Hence the "strong
signal with very low modulation level" makes sense.

My experience was about 25% modulation, and that was with distortion that
was... well... kind of bad.

I would characterize this as pretty much the worst quality AM ever, worse
than screen grid modulation by a long shot.

Fortunately with
pentodes, it is easy to drive the output to zero (negative modulation).
The real problem occurs when the suppressor is driven very far into the
positive voltage region and it starts drawing current. Typically the
suppressor grid can only dissipate a few watts. Since current is drawn
during the positive peaks, an audio driver must present a low impedance.

And this also quickly becomes a distortion source unless the audio driver
stage is pretty hefty.

Just say no.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Fortunately with
pentodes, it is easy to drive the output to zero (negative modulation).
The real problem occurs when the suppressor is driven very far into the
positive voltage region and it starts drawing current. Typically the
suppressor grid can only dissipate a few watts. Since current is drawn
during the positive peaks, an audio driver must present a low impedance.

73, Barry WA4VZQ

Ah, yes, "(negative modulation)"!

You seem to be "hep" on old stuff, do you remember "negative peak
clipping"?
I thought I was in Heaven in about 1958 with a single 2E26 final on 15
Meters.
In class C, with a pair of 6L6 modulators, push-pull, class AB1, and used a
VR
tube across the modulation transformer secondary clipping the negative
peaks, while
allowing the positive peaks to go "over" 100%.

Technical gurus of the day poo-poo'd the scheme. It looked a little
rough on the
scope, but unsolicited signal reports said it "packed a lot of modulation".
I am thinking it must have been a little like more modern amateur
"speech
processing"

I also seem to remember those modified 1646's getting almost 20%
suppressor
grid modulation after tinkering around with power supply voltages.

Old Chief Lynn, W7LTQ

"coffelt2" wrote in message
...

Ah, yes, "(negative modulation)"!

You seem to be "hep" on old stuff, do you remember "negative peak
clipping"?
I thought I was in Heaven in about 1958 with a single 2E26 final on 15
Meters.
In class C, with a pair of 6L6 modulators, push-pull, class AB1, and
used a VR
tube across the modulation transformer secondary clipping the negative
peaks, while
allowing the positive peaks to go "over" 100%.

Technical gurus of the day poo-poo'd the scheme. It looked a little
rough on the
scope, but unsolicited signal reports said it "packed a lot of
modulation".
I am thinking it must have been a little like more modern amateur
"speech
processing"

I also seem to remember those modified 1646's getting almost 20%
suppressor
grid modulation after tinkering around with power supply voltages.

Old Chief Lynn, W7LTQ

Hi Lynn,

A pair of 6L6's to modulate a 2E26... there was enough audio to drive a
6146 with a some to spare and enough to really overmodulate a 2E26! :-)

Negative peak clipping was based on a diode with its anode connected to
B+ and its cathode connected to the output side of the modulation
transformer. This would allow the final's voltage to drop to only a few
volts during negative modulation peaks. Later refinement was to add a
gaseous voltage regulator tube between the diode's cathode and the output
side of the transformer. In this case, the final's voltage would only
drop to a voltage determined by the regulator tube. Today a silicon
diode might be used, and zeners would replace the VR tube. The result of
this circuit was that the RF would never be completely cut off during
negative modulation peaks. Positive modulation peaks would be passed
allowing higher positive modulation. This is what gave the higher "talk
power." It still caused distortion, but much less than that if the
negative peaks were not clipped.

Everyone's voice is asymmetric. If you look at the voice waveform with
an oscilloscope, you will see that peaks on one side of zero are often
considerably higher that the opposite polarity. The average is still
zero, however. The peak is caused by harmonics in the voice being in
phase with each other. This property can be used to advantage in
amplitude modulation by having the peaks occur on the positive
modulation. Usually all that was needed to put the peak on the right
side was to reverse the connections to the modulation transformer or to
reverse the connections to the microphone.

I am sure Scott Dorsey knows more about this than I do, but CBS produced
two devices called the Audimax and Volumax that shifted the phase of the
audio as a function of frequency. Another term for these devices is
phase rotator. Kahn Communications also was in the market with its
SymmetraPeak. To the ear, the sound was unchanged, but to the
transmitter, the peaks became symmetrical. There is an excellent
discussion of these devices on James Tonne's (W4ENE) website
http://www.tonnesoftware.com/appnote...s/allpass.html and on Gary
Blau's (W3AM) website http://www.w3am.com/8poleapf.html. {A biased
opinion here — Jim's site contains some _excellent_ free software.}

As to being "hep" on old technology, I appreciate the compliment. As an
undergraduate, my university taught tubes and transistors. By the time I
got to graduate school, tubes were no longer taught. By the time I got
out of graduate school, integrated circuits were the "in thing" and
microprocessors had just begun. I do have a good collection of older
engineering books, however. I am constantly amazed by the technology of
the late 1920's and the 1930's. And it was all designed without the
benefit of computers! While I haven't used it in years, I still have my
K&E metal log-log-decitrig slide rule, and the bamboo rule I used while
in high school.

73, Barry WA4VZQ

Nordic Breeds WA4VZQ wrote:
I am sure Scott Dorsey knows more about this than I do, but CBS produced
two devices called the Audimax and Volumax that shifted the phase of the
audio as a function of frequency. Another term for these devices is
phase rotator. Kahn Communications also was in the market with its
SymmetraPeak. To the ear, the sound was unchanged, but to the
transmitter, the peaks became symmetrical. There is an excellent
discussion of these devices on James Tonne's (W4ENE) website
http://www.tonnesoftware.com/appnote...s/allpass.html and on Gary
Blau's (W3AM) website http://www.w3am.com/8poleapf.html. {A biased
opinion here — Jim's site contains some _excellent_ free software.}

The original Audimax/Volumax combination had no phase rotator. I worked
at an AM station that used them, and the chief engineer had installed a
phase reverse switch on the announcer mike and auditioned each announcer
to tell them which position to use. (Apparently they had used figure-8
mikes a year or so before I got there, and the announcers just used the
front of back of the mikes).

A lot of stations using the Audimax/Volumax would also have a phase
rotator in the chain, though. CRL made a popular one, and so did Garron.
Some folks made some boards tht dropped inside the Volumax for it too,
but I never used any of those. I went to the Optimod as soon as I could,
and it has a great phase rotator.

The phase rotator is a hell of a great gadget, it gives you a lot of loudness
without any perceived distortion. Mind you, for communications applications
it's no more effective just than aggressive clipping, but there are folks
who don't want aggressive clipping.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"Scott Dorsey" wrote in message
...
The original Audimax/Volumax combination had no phase rotator. I
worked
at an AM station that used them, and the chief engineer had installed a
phase reverse switch on the announcer mike and auditioned each
announcer
to tell them which position to use. (Apparently they had used figure-8
mikes a year or so before I got there, and the announcers just used the
front or back of the mikes).

A lot of stations using the Audimax/Volumax would also have a phase
rotator in the chain, though. CRL made a popular one, and so did
Garron.
Some folks made some boards that dropped inside the Volumax for it too,
but I never used any of those. I went to the Optimod as soon as I
could,
and it has a great phase rotator.

The phase rotator is a hell of a great gadget, it gives you a lot of
loudness
without any perceived distortion. Mind you, for communications
applications
it's no more effective just than aggressive clipping, but there are
folks
who don't want aggressive clipping.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


Thanks for the corrections, Scott.

Aggressive clipping creates a ton of distortion unless the voice signal
is split into several bands, each processed and filtered, and then
combined. The phase rotator theoretically produces no amplitude
distortion, and due to the way the human ear works, the shifting of the
phases is not heard. I read once that the cochlea and its nerves
perform physiologically something akin to a mathematician performing a
Fourier analysis. I find it amazing that we process sound, for the most
part, on the amplitude versus frequency information, and ignore the phase
versus frequency information.

With modern operational amplifiers, it is fairly simple to produce a good
phase rotator using cascaded all-pass networks. I would hate to have to
manufacture the original SymmetraPeak with its inductor-capacitor network
lattices.

Well, we are pretty far from the original subject, but I have enjoyed the
discussion. However I do have a Boatanchor question.

I remember seeing ads in QST in the 1960's for a device I think was
called "Echoplex." It was supposedly used on commercial and military
voice communications circuits. I never heard one of these in use by a
ham, probably because their cost could buy several Collins S-Line
stations. Doing a Google search brings up lots of echo-effects
processors for guitars and such, but I found nothing for communication
usage. Do any readers here remember the device and its manufacturer and
how it worked?

73, Barry WA4VZQ

Nordic Breeds WA4VZQ wrote:

Aggressive clipping creates a ton of distortion unless the voice signal
is split into several bands, each processed and filtered, and then
combined. The phase rotator theoretically produces no amplitude
distortion, and due to the way the human ear works, the shifting of the
phases is not heard.

Right. I think for communications use, though, the ton of distortion can
actually help intelligibility of consonants under bad conditions. Certainly
it gives you a distinctive sound in a pileup.

I remember seeing ads in QST in the 1960's for a device I think was
called "Echoplex." It was supposedly used on commercial and military
voice communications circuits. I never heard one of these in use by a
ham, probably because their cost could buy several Collins S-Line
stations. Doing a Google search brings up lots of echo-effects
processors for guitars and such, but I found nothing for communication
usage. Do any readers here remember the device and its manufacturer and
how it worked?

I have only heard of the echo-effect box. "Everything I use must have
X in it, like sex and echoplex" says Lee Scratch Perry.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article ,
says...

I remember seeing ads in QST in the 1960's for a device I think was
called "Echoplex." It was supposedly used on commercial and military
voice communications circuits. I never heard one of these in use by a
ham, probably because their cost could buy several Collins S-Line
stations. Doing a Google search brings up lots of echo-effects
processors for guitars and such, but I found nothing for communication
usage. Do any readers here remember the device and its manufacturer
and how it worked?

I have a feeling it's another name for Lincompex, developed by the
British Post Office in 1966. Bell labs in the US apparently then came up
with something very similar - which I seem to remember was named
"Echoplex". Could be wrong - it was all a long time ago.....

Lincompex stands for LINKED COMPRESSOR and EXPANDER. The audio is
heavily compressed, and the pre-compression amplitude variations are
used to frequency modulate a 2900Hz control tone. This is combined with
the compressed audio (300-2700Hz) and fed to the TX. At the other end of
the circuit, the control tone is used by an expander to restore the
original amplitude variations.

Lincompex was used widely on international point to point SSB and ISB
voice circuits - and may still be in use (?)

The equipment was made by Marconi, and possibly ST&C (as was, before
Nortel bought them up).

You are both right.

On a 1966 issue of QST magazine, I found the advertisement of "Echoplex" by Kahn
Research Laboratories. It sold for more than 300$, which was not cheap at those
times.

As to Lincomplex, I remember a friend of mine working for Page Europe who told
me having installed Lincomplex on HF transmitters in Africa.

73

Tony I0JX
Rome Italy

"Antonio Vernucci" wrote in message
...
You are both right.

On a 1966 issue of QST magazine, I found the advertisement of
"Echoplex" by Kahn Research Laboratories. It sold for more than 300$,
which was not cheap at those times.

As to Lincomplex, I remember a friend of mine working for Page Europe
who told me having installed Lincomplex on HF transmitters in Africa.

73

Tony I0JX
Rome Italy

Thank you, Tony.

Somehow, I think you are talking about Leonard R. Kahn of Kahn Research
Laboratories in Freeport, Long Island, NY, and not A. Q. Khan of Khan
Research Laboratories in Kahuta, Pakistan (Pakistan's main nuclear
weapons laboratory as well as an emerging center for long-range missile
development). Leonard Kahn is best known for his paper: L.R. Kahn,
“Single Sideband Transmission by Envelope Elimination and Restoration,”
Proceedings of the IRE, Vol. 40, July 1952, pp. 803–806., and for his
work on AM stereo. Google somehow doesn't know the difference...

73, Barry WA4VZQ