On Thu, 26 Jan 2006, Gary Schafer wrote:

On Thu, 26 Jan 2006 19:31:08 -0500, Straydog wrote:



On Thu, 26 Jan 2006, Gary Schafer wrote:

On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote:


Since my earlier post (dealing with the question of what is peak evelope
power output in an AM transmitter), I've been doing more scrutinizing
of tube Ip/Vp characteristic curves. They are much more non-linear than
the impression you get from just looking at the curves. Also, it is rare
or almost non-existant to find Ip vs screen voltage!

Lets look at the venerable 833 (from my RCA TT-3 transmitting tube
manual). This is a KW input class C triode.

From the curve:
at zero grid volts, 1 kV on the plate gives 175 ma plate current
2 kV 500 ma
That's more than a doubling of Ip for a doubling of Vp

at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current
4 kV 750 ma

looking in my RCA receiving tube manual (RC-20) I found for a 6FG6
a sharp cutoff tetrode that only at zero grid volts was there a near
linear relationship between plate current and plate voltage (meaning zero
current at zero voltage, and a straight line [which actually deviated
slightly from a straight line] with some slope. But at 100 v on plate,
current was 14 milliamps, at 200 v on the plate, plate current was 34
miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode,
and at any of a wide range of grid voltages, plate current could be
doubled with only a 15-20% increase in plate voltage.

My thinking on all of this leads me to claim that anyone who can start
with a 100 watt carrier from an AM transmitter and make a few assumptions
about 100% modulation and come up with a _calculation_ of something like
400 watts of peak power and represent that as having something to do with
reality is pure conjecture.

If anyone wants to put an appropriate oscilloscope on the transmitter output
and measure the RF voltage of unmodulated carrier into an appropriate load
and then measure the peak RF voltage when the carrier is modulated, then
and only then do they have a reasonable _basis_ for making a claim about
peak (instantaneous) output power.

You can't just look at static curves. Consider that with AM modulation
there is usually grid leak bias on the final tube being modulated.
This allows the grid voltage to somewhat follow the modulation and
helps smooth out the non-linearity in the plate.

This was discussed in the RCA transmitting tube manual, but it also
referenced the technical references which go into this in much more
detail. However, if you want to say "you can't just look at static curves"
then you also can't just say "doubling plate voltage also doubles plate
current" either.

If you have access to any of Termans books, as peter said, there is an
excellent section on how modulation works.
He in fact shows that "plate current follows plate voltage almost
exactly with modulation". His words.

He also says that "triodes have considerably less distortion than
screen grid tubes".

I will decline to check this but words and phrases like "almost exactly"
and "considerably less" are unquantitative.

Do you even know who Terman is?

Yep, and I've even looked in his books. But its more than I want to go
into.

I would doubt that you do or you would not make statements like that.
As a matter of fact if you had read any of his work you would not be
making most of the statements that you are in these threads.

I don't have the benefit of reading his works, I'm presuming that you
have, is that correct?

At first I thought that you were interested in learning but I see you
would rather argue for the sake of arguing.

Very early in my comments I brought up the issue of Ip being independend
of Vp in all of the curves (these are facts) for tetrode and pentode
transmitting tubes and receiving tubes and nobody but nobody called
attention to the possibility that this conflict with claims of plate
current doubling with plate voltage doubling could be resolved by
including changes in screen voltage proportional, in some relationship, to
changes in plate voltage. A few of your statements were a little
bit helpful but even the comments in the RCA transmitting tube manual were
weak in dealing with this issue.

What is a further issue is why the FCC decided to drop steady DC input
(easily measured with a plate current meter) in favor of making PEP output
measurement the new criterion by which transmitter power is to be
measured. The only thing I can think of is that there were, in the far
past, some AM amateurs who were running some form of ultra modulation or
super modulation and putting KWs of audio on a 1 KW DC input to the final
signal and the FCC didn't like that. Maybe if any of you have some
background on this, you could mention it.

On Thu, 26 Jan 2006, Uncle Peter wrote:


" Uncle Peter" wrote in message
news:cufCf.16380$bF.7979@dukeread07...
I doubt that Henny or Tenny based their texts on conjecture or
misguided realities.

Pete



Henny or Terman!! Arggh! Typo.


I knew what you meant. We all make typos from time to time.

On Thu, 26 Jan 2006, Uncle Peter wrote:


"Straydog" wrote in message
.com...


looking in my RCA receiving tube manual (RC-20) I found for a 6FG6
a sharp cutoff tetrode that only at zero grid volts was there a near
linear relationship between plate current and plate voltage (meaning zero
current at zero voltage, and a straight line [which actually deviated
slightly from a straight line] with some slope. But at 100 v on plate,
current was 14 milliamps, at 200 v on the plate, plate current was 34
miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode,
and at any of a wide range of grid voltages, plate current could be
doubled with only a 15-20% increase in plate voltage.

My thinking on all of this leads me to claim that anyone who can start
with a 100 watt carrier from an AM transmitter and make a few assumptions
about 100% modulation and come up with a _calculation_ of something like
400 watts of peak power and represent that as having something to do with
reality is pure conjecture.



In some cases it is a lot easier to accept what is technically correct, and
work backwards to correct erroneous conclusions.

Well, I didn't have trouble seeing that if a scope shows modulation RF
peaks that are double the unmodulated carrier, then the instantaneous
power at the peak is 4X. What I had trouble with was from the curves that
show plate current in tetrodes, pentodes independent of plate voltage.
What I did not consider is how screen voltge would affect plate current if
the screen voltage were modulated along with plate voltage. Previously, I
thought power input could only double but that would leave a deficit since
peak output voltage shows on the scope that power at the peak has to be 4X
the unmodulated power.

First, a Class C amplifier is driven into grid conduction, almost to the
point of plate saturation.

High Level AM modulation is applied to the SCREEN and PLATE,
only doubling the plate voltage as in your 6F6 example to show
a non linear relationship isn't a valid argument.

Well, it was a 6FG6 (not a 6F6) and what we are trying to do is find out
how power _input_ at the peak of a modulation cycle becomes 4X the power
input when an unmodulated carrier is being put out. Part of the answer
comes from modulating the screen, and one of the guys was talking about
some modulation of the control grid through a "grid leak" resistor which
was also mentioned in the RCA transmitting tube manual in the front which
gives some rudimentary explanations for all of this (and was helpful for
me to re-read).

What is the
operating Class of the tube, and did you account for the modulating
voltage also being applied to the screen grid?

Well, if you look at all of the curves showing Ip vs Vp, they usually give
curves for a fairly large range in control grid voltages (but at only one
screen grid, if it is present, voltage) so you can look at how Ip changes
for any range in changes in control grid so you can know about what class
of amplifier you are running by looking at highest control grid voltage
and lowest control grid voltage you want to use and whether you get close
to cutoff (where Ip goes to zero or, maybe, close to zero).

To quote Henny: "A linear relation must exist between plate voltage
and tank circuit current for good operation... In such a modulated
amplifier, the output peak will be four times the unmodulated
carrier and the continuous power output with complete modulation
is 1.5 times the power at zero modulation."

I understand this, now, and know where it comes from. Fine.

Note that is only
true for a true Class C power amplifier stage, and not for
Class A or B.

I can accept this, too.

I doubt that Henny or Tenny based their texts on conjecture or
misguided realities.

I can appreciate that in the more advanced treatises on the subject that
the guys know more about what is going on. The ARRL handbooks gloss over a
lot of this and I always wondered why the FCC changed the rule from
measuring simple DC power input (plate volts X plate current), even on a
linear for SSB, with a simple D'arsonval movement meter (or a digital bar
graph meter that could be made to mimic a mechanical meter), to the rule
that PEP output not exceed 1500 Watts. One would have to surely use a
scope and I'd prefer not to have to trust these so-called PEP reading
meters that are all over the place now.

But, thanks for your contribution.

Pete

On Thu, 26 Jan 2006 22:32:41 -0500, Straydog wrote:



On Thu, 26 Jan 2006, Gary Schafer wrote:

On Thu, 26 Jan 2006 19:31:08 -0500, Straydog wrote:



On Thu, 26 Jan 2006, Gary Schafer wrote:

On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote:


Since my earlier post (dealing with the question of what is peak evelope
power output in an AM transmitter), I've been doing more scrutinizing
of tube Ip/Vp characteristic curves. They are much more non-linear than
the impression you get from just looking at the curves. Also, it is rare
or almost non-existant to find Ip vs screen voltage!

Lets look at the venerable 833 (from my RCA TT-3 transmitting tube
manual). This is a KW input class C triode.

From the curve:
at zero grid volts, 1 kV on the plate gives 175 ma plate current
2 kV 500 ma
That's more than a doubling of Ip for a doubling of Vp

at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current
4 kV 750 ma

looking in my RCA receiving tube manual (RC-20) I found for a 6FG6
a sharp cutoff tetrode that only at zero grid volts was there a near
linear relationship between plate current and plate voltage (meaning zero
current at zero voltage, and a straight line [which actually deviated
slightly from a straight line] with some slope. But at 100 v on plate,
current was 14 milliamps, at 200 v on the plate, plate current was 34
miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode,
and at any of a wide range of grid voltages, plate current could be
doubled with only a 15-20% increase in plate voltage.

My thinking on all of this leads me to claim that anyone who can start
with a 100 watt carrier from an AM transmitter and make a few assumptions
about 100% modulation and come up with a _calculation_ of something like
400 watts of peak power and represent that as having something to do with
reality is pure conjecture.

If anyone wants to put an appropriate oscilloscope on the transmitter output
and measure the RF voltage of unmodulated carrier into an appropriate load
and then measure the peak RF voltage when the carrier is modulated, then
and only then do they have a reasonable _basis_ for making a claim about
peak (instantaneous) output power.

You can't just look at static curves. Consider that with AM modulation
there is usually grid leak bias on the final tube being modulated.
This allows the grid voltage to somewhat follow the modulation and
helps smooth out the non-linearity in the plate.

This was discussed in the RCA transmitting tube manual, but it also
referenced the technical references which go into this in much more
detail. However, if you want to say "you can't just look at static curves"
then you also can't just say "doubling plate voltage also doubles plate
current" either.

If you have access to any of Termans books, as peter said, there is an
excellent section on how modulation works.
He in fact shows that "plate current follows plate voltage almost
exactly with modulation". His words.

He also says that "triodes have considerably less distortion than
screen grid tubes".

I will decline to check this but words and phrases like "almost exactly"
and "considerably less" are unquantitative.

Do you even know who Terman is?

Yep, and I've even looked in his books. But its more than I want to go
into.

His writings are very easy to understand compared to many engineering
books. He leans less on the math and more on practical explanations.
He was one of the most highly thought of professors in the radio
field. Although his books were written in the 30's and 40's, they do
not include some of today's newer discoveries, they are very well
written to explain circuit theory and things like modulation.


I would doubt that you do or you would not make statements like that.
As a matter of fact if you had read any of his work you would not be
making most of the statements that you are in these threads.

I don't have the benefit of reading his works, I'm presuming that you
have, is that correct?

I am not an expert by any means but I often refer to a few of his
books.


At first I thought that you were interested in learning but I see you
would rather argue for the sake of arguing.

Very early in my comments I brought up the issue of Ip being independend
of Vp in all of the curves (these are facts) for tetrode and pentode
transmitting tubes and receiving tubes and nobody but nobody called
attention to the possibility that this conflict with claims of plate
current doubling with plate voltage doubling could be resolved by
including changes in screen voltage proportional, in some relationship, to
changes in plate voltage. A few of your statements were a little
bit helpful but even the comments in the RCA transmitting tube manual were
weak in dealing with this issue.

That is exactly what I told you in my very first post to you. That was
the one that had several different topic headings. The one that you
deleted most of the headings as "incorrect information".

This was under AM TRANSMITTERS.
"Screen grid tubes are not linear in this respect. Plate current is
somewhat independent of plate voltage. That is why you must also
partly modulate the screen along with the plate when using a screen
grid tube in the final. You want to have a linear plate voltage to
plate current relationship."


What is a further issue is why the FCC decided to drop steady DC input
(easily measured with a plate current meter) in favor of making PEP output
measurement the new criterion by which transmitter power is to be
measured. The only thing I can think of is that there were, in the far
past, some AM amateurs who were running some form of ultra modulation or
super modulation and putting KWs of audio on a 1 KW DC input to the final
signal and the FCC didn't like that. Maybe if any of you have some
background on this, you could mention it.

That could have been part of it. It is difficult to tell exactly how
much power is really going out with different types of modulation.

Probably the biggest reason for the change was SSB. Watching the plate
meter kicking up and down was not a very accurate means of measuring
power but in the old day's access to a PEP watt meter was almost non
existent.

Now with a PEP wattmeter it is much easier to read power output than
it is input power.

Another part of the change was to reduce the maximum power that hams
were able to use. As discussed the 1 kw input AM transmitter could
easily have in excess of 3000 watts PEP output and lots more with some
modulation schemes as you refer to.

SSB also could have well in excess of 3000 watts PEP output as well.
The old means of measuring SSB power was the plate current meter on
the final not kicking over the 1 kw DC input level. The meter had to
have a time constant of less than .025 seconds. No sluggish meters
allowed. But the average power in speech is only around 10 to 20% and
that is what was measured.

73
Gary K4FMX

The harder the modulated tube is driven into class-C conditions and
saturation, the more linear does the plate modulation become.

Operation of the tube becomes independent of curvature in the tube's
characteristic. The plate current operating angle is small.
It behaves more like an on/off switch.

The more non-linear it is, the smaller the operating angle, the more
linear is the modulation.
----
Reg, G4FGQ.

The more non-linear it is, the smaller the operating angle, the more
linear is the modulation.
----
Reg, G4FGQ.

Hello Reg:

Does this mean that you are going to have a software program for us soon?
You have written us programs for almost everything else useful in ham radio.
Actually, what I need is a program that designs a maximum legal limit AM rig
using the parts I have, and that then tells me where to get the parts I do
not have with the least work and expense.

73, Colin K7FM

"Reg Edwards" wrote in message
...
The harder the modulated tube is driven into class-C conditions and
saturation, the more linear does the plate modulation become.

Operation of the tube becomes independent of curvature in the tube's
characteristic. The plate current operating angle is small.
It behaves more like an on/off switch.

The more non-linear it is, the smaller the operating angle, the more
linear is the modulation.
======================================

PLATE MODULATION.
It's really all very simple.

Imagine a class-C triode amplifier with very small operating angle and
running nearly into saturation.

The plate load is a tuned tank circuit having a high impedance at
resonance. Or it can be a Pi-tank circuit. Makes no difference!

With the high impedance load, conditions are such that whatever is the
DC plate voltage, the RF plate volts swing down to a very low
plate-to-cathode voltage.

Ideally it should be zero volts. But in practice it cannot fall below
the positive, peak, instantaneous, RF grid volts. This corresponds to
the instant of peak plate current.

The RF voltage across the tank is then very nearly EQUAL to the DC
plate voltage regardless of the tubes characteristic curves. Curvature
doesn't matter. It is obscured by the small operating angle. The tube
is conducting only for a small fraction of the time.

Modulate the DC plate voltage at an audio frequency. With 100 percent
modulation the DC plate voltage swings between a very low voltage and
twice the DC supply volts. And so do the RF volts across the tank.
The job is done. You have an almost perfect linearly modulated
amplifier.

It is necessary only to ensure grid drive is just sufficient to drive
the tube into saturation when the DC plate volts is twice the DC
supply volts. It will then remain saturated at all lower voltages.

With a triode, saturation occurs when the RF plate voltage swings down
to not much more than the peak RF grid volts.

With a beam tetrode, saturation occurs when the RF plate voltage
swings down only to something less than the DC screen-grid volts and
100 percent linear modulation cannot be achieved. But 100 percent
modulation is always undesirable because of the risk of
over-modulation.

With a bipolar transistor, modulation can be even more linear because,
with the high impedance tuned tank, the device saturates or 'bottoms'
at very nearly zero RF collector volts. About 0.7 volts.
----
Reg, G4FGQ.

"COLIN LAMB" wrote in message
ink.net...
The more non-linear it is, the smaller the operating angle, the more
linear is the modulation.
----
Reg, G4FGQ.

Hello Reg:

Does this mean that you are going to have a software program for us
soon?
You have written us programs for almost everything else useful in
ham radio.
Actually, what I need is a program that designs a maximum legal
limit AM rig
using the parts I have, and that then tells me where to get the
parts I do
not have with the least work and expense.

73, Colin K7FM
==========================================
Colin,

A real amateur would visit hamfests, look around the junk, take a
selection of the junk home, then sit and think about what he could do
with it.

If you stick out your tonge and pull a funny face, the stall holders
might throw something at you - for free.

The bits and pieces left over can be put towards the next project. ;o)
----
Reg.

On Thu, 26 Jan 2006, Gary Schafer wrote:

On Thu, 26 Jan 2006 22:32:41 -0500, Straydog wrote:



On Thu, 26 Jan 2006, Gary Schafer wrote:

On Thu, 26 Jan 2006 19:31:08 -0500, Straydog wrote:



On Thu, 26 Jan 2006, Gary Schafer wrote:

On Wed, 25 Jan 2006 22:09:48 -0500, Straydog wrote:


Since my earlier post (dealing with the question of what is peak evelope
power output in an AM transmitter), I've been doing more scrutinizing
of tube Ip/Vp characteristic curves. They are much more non-linear than
the impression you get from just looking at the curves. Also, it is rare
or almost non-existant to find Ip vs screen voltage!

Lets look at the venerable 833 (from my RCA TT-3 transmitting tube
manual). This is a KW input class C triode.

From the curve:
at zero grid volts, 1 kV on the plate gives 175 ma plate current
2 kV 500 ma
That's more than a doubling of Ip for a doubling of Vp

at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current
4 kV 750 ma

looking in my RCA receiving tube manual (RC-20) I found for a 6FG6
a sharp cutoff tetrode that only at zero grid volts was there a near
linear relationship between plate current and plate voltage (meaning zero
current at zero voltage, and a straight line [which actually deviated
slightly from a straight line] with some slope. But at 100 v on plate,
current was 14 milliamps, at 200 v on the plate, plate current was 34
miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode,
and at any of a wide range of grid voltages, plate current could be
doubled with only a 15-20% increase in plate voltage.

My thinking on all of this leads me to claim that anyone who can start
with a 100 watt carrier from an AM transmitter and make a few assumptions
about 100% modulation and come up with a _calculation_ of something like
400 watts of peak power and represent that as having something to do with
reality is pure conjecture.

If anyone wants to put an appropriate oscilloscope on the transmitter output
and measure the RF voltage of unmodulated carrier into an appropriate load
and then measure the peak RF voltage when the carrier is modulated, then
and only then do they have a reasonable _basis_ for making a claim about
peak (instantaneous) output power.

You can't just look at static curves. Consider that with AM modulation
there is usually grid leak bias on the final tube being modulated.
This allows the grid voltage to somewhat follow the modulation and
helps smooth out the non-linearity in the plate.

This was discussed in the RCA transmitting tube manual, but it also
referenced the technical references which go into this in much more
detail. However, if you want to say "you can't just look at static curves"
then you also can't just say "doubling plate voltage also doubles plate
current" either.

If you have access to any of Termans books, as peter said, there is an
excellent section on how modulation works.
He in fact shows that "plate current follows plate voltage almost
exactly with modulation". His words.

He also says that "triodes have considerably less distortion than
screen grid tubes".

I will decline to check this but words and phrases like "almost exactly"
and "considerably less" are unquantitative.

Do you even know who Terman is?

Yep, and I've even looked in his books. But its more than I want to go
into.

His writings are very easy to understand compared to many engineering
books. He leans less on the math and more on practical explanations.
He was one of the most highly thought of professors in the radio
field. Although his books were written in the 30's and 40's, they do
not include some of today's newer discoveries, they are very well
written to explain circuit theory and things like modulation.

Fine. Maybe next time I see some of his works at a hamfest, I'll take a
look and see if I might want to delve more deeply.

I might add that I've looked at and own one of the RSGB ham handbooks
which are sometimes more detailed than the ARRL handbook. However, I also
want to keep ham radio a hobby for me rather than a vocation (as, say, an
EE)


I would doubt that you do or you would not make statements like that.
As a matter of fact if you had read any of his work you would not be
making most of the statements that you are in these threads.

I don't have the benefit of reading his works, I'm presuming that you
have, is that correct?

I am not an expert by any means but I often refer to a few of his
books.

Well, there are a lot of gaps in my knowledge, too, and, yes, I know there
are books out there that go very deeply into theory, math, etc.

My other favorite books are the bil Orr (I think W6SAI?) "Radio Handbooks"
which I also think are very nice and cover things differently.


At first I thought that you were interested in learning but I see you
would rather argue for the sake of arguing.

Very early in my comments I brought up the issue of Ip being independend
of Vp in all of the curves (these are facts) for tetrode and pentode
transmitting tubes and receiving tubes and nobody but nobody called
attention to the possibility that this conflict with claims of plate
current doubling with plate voltage doubling could be resolved by
including changes in screen voltage proportional, in some relationship, to
changes in plate voltage. A few of your statements were a little
bit helpful but even the comments in the RCA transmitting tube manual were
weak in dealing with this issue.

That is exactly what I told you in my very first post to you. That was
the one that had several different topic headings. The one that you
deleted most of the headings as "incorrect information".

This was under AM TRANSMITTERS.
"Screen grid tubes are not linear in this respect. Plate current is
somewhat independent of plate voltage. That is why you must also
partly modulate the screen along with the plate when using a screen
grid tube in the final. You want to have a linear plate voltage to
plate current relationship."

Well, you also deleted my response to this, too.


What is a further issue is why the FCC decided to drop steady DC input
(easily measured with a plate current meter) in favor of making PEP output
measurement the new criterion by which transmitter power is to be
measured. The only thing I can think of is that there were, in the far
past, some AM amateurs who were running some form of ultra modulation or
super modulation and putting KWs of audio on a 1 KW DC input to the final
signal and the FCC didn't like that. Maybe if any of you have some
background on this, you could mention it.

That could have been part of it. It is difficult to tell exactly how
much power is really going out with different types of modulation.

Yes, One other thing I was thinking about way back then as to why they
could come up with this way of measuring power was that someone told
someone else in the FCC somethin glike this: since the books say that in
grounded-grid amplifiers, a part of the input drive power gets fed through
to the antenna, maybe someone could build some kind of weird grounded-grid
amplifier where the final has a DC imput of 1 kW and the final is driven
with 5 kW of input drive power and the final puts out, say 0.5 kW and 4.5
kW of drive power feeds through the final and adds to the 0.5 kW from the
final to give 5 kW of output with just 1 kW to the final and goes into the
antenna and its legal. I don't know, just my wild speculation.

Probably the biggest reason for the change was SSB. Watching the plate
meter kicking up and down was not a very accurate means of measuring
power but in the old day's access to a PEP watt meter was almost non
existent.

And, scopes were big and expensive, too. Personally I always just looked
at plate current while saying "ahhhhhhhh" and multiplying by plate voltage
and telling people VxI=watts and that is my "average" power DC INPUT.

And, I've heard, on the air, all manner of misunderstandings of power. I
actually heard one guy say "And, I'm getting 700 watts DC output out of my
linear" and I was wondering how you get DC out of an RF output SO-239
connector off a commercial linear amplifier. :-\

Now with a PEP wattmeter it is much easier to read power output than
it is input power.

Well, I'd rather not trust the needle meters. At least I'd want to check
it against a scope with bandwidth high enough to measure those voltage
peaks under voice modulation (as I've done with my Ranger).

Another part of the change was to reduce the maximum power that hams
were able to use. As discussed the 1 kw input AM transmitter could
easily have in excess of 3000 watts PEP output and lots more with some
modulation schemes as you refer to.

Yes, and it all seems so silly to me. I always had the feeling that
talking (in the old days) about 2 kW PEP when 1 kW input was the max was
more of a ego hype ploy to make people feel they had something when, from
a practical point of view, the S-meter was going to be responding to the
average power which was easier to measure anyway. But from this one can
argue in lots of branching directions.

SSB also could have well in excess of 3000 watts PEP output as well.
The old means of measuring SSB power was the plate current meter on
the final not kicking over the 1 kw DC input level. The meter had to
have a time constant of less than .025 seconds. No sluggish meters
allowed. But the average power in speech is only around 10 to 20% and
that is what was measured.

And, you could have some weird speech waveform with funny transients in it
that spiked up, too. '

Anyway, 73

73
Gary K4FMX

Actually, when the tube(or any other active device) is delivering
power into a load, in a class C amp, you have either high Vp and Ip cut
off, or LOW Vp and heavy Ip flowing. When the tube is turned on, the
voltage should only be the tube "bottoming" voltage. for the 6146 I
know best this will be between 75 and 100V, depending on Ip(peak).

The part of the tube curves that determines modulating waveforms,
efficiency, and maximum useable loading voltage is the part of the
curve where full(peak) Ip is flowign at only a LOW voltage. For the
6146, for instance, that woudl be currents of 400-800ma flowing with
only 75-125V. If you were drawing full tube current with all your Vp
across the tube, your efficiency and power output would be zero!

Where does the rest of the Vp go? it appears across the load. If the
tube was a hypothetical perfect switch, bottoming voltage would be
zero, and in Class C would be either all the way on or all the way off
at all times. The 120 degree conduction angle common in Class C is used
to minimize switching voltage and therefore overlap of Vp and Ip. That
overlap and bottoming voltage are the source of the heat dissipated by
the tube. Class E and F amps use different tuned circuits to get zero
switching votage and efficiency as high as 90%!

If the tube were a perfect switch, and the conduction angle were
constant with respect to Vp, the resulting amp would give perfect
linear plate modulation! This is becuase the load should be a pure
ohmic resistance at resonance. The reason it does actually work this
way is that bottoming voltrage runs up fast as Ip is increased,
cutting the efficiency of the tube and cutting the percentage of Vp
that appears across the load. If doubling the plate voltage only raises
the voltage applied to the load by 90%, it will only increase current
by 90% at most, meaning the power increase cannot be more than about
81%!

This is why a little grid modulation added to plate
modulation(ESPECIALLY WITH TETRODES/PENTODES!) does so much for the
modulation characteristic of the amp. This is also true to a very
severe extent of MOSFETS, BTW. Solid state AM is utter hell to
modulate and I suspect will never sound as good as tubes. you know,
just like transistor guitar amps sound like garbage(but for different
reasons).

To design an AM final for plate modulation, begin by figuring desired
PEP, which is four times desired carrier power. Now select a tube(s)
and operating point that will make this power at a reasonable
efficiency. If the amp can run key-down at this level, great, but this
is not necessary in any way. In teh real world, this would always be
over its thermal dissipation rating.

In other words, if you ran it without modulation at double plate
modulation, it should run with good class C efficiency unit it
overheats from being too small in dissipation rating! It must NOT be
overloaded to the point that efficiency is dropping. This sets the
loading used on the amp as built, tuned, and run.

When operated at carrier, it will be at a rather light loading compared
to the CW ratign for the same device. It could make more power, but
you'd never modulate it. Do NOT increase the loading. At carrier, it
will still make somewhat over 1/4 the PEP power, due to greater
efficiency at the lighter current. Becuase your efficiency waqs good to
begin with, this difference will not be severe.

A small amount of grid modulation is now introduced. On a triode, the
use of grid leak bias has this effect, as grid current falls with
rising plate voltage durign the switchin time of a real tube. On a
screen grid tube,you put a choke or tertiary winding in the screen
supply. If you just rran the screen all trhe time at enough voltage for
the PEP condition, you would little too much carrier power.

The point of the grid modulation is this: it reduces efficiency just
enough at carrier to be the same as the efficiency at PEP. This, in
turn, is one of the reasons the PEP efficiency must be good-otherwise
you now always have poor efficiency.

Suppose you are designing a final for the amateur limit of 1500 wats
PEP output. You are NOT going to design a 325 watt(the carrier power)
amp! Thermal loading will be on a sine wave signal about that of a 437
watt class C amp, but the amp must not clip or round off the current
pulses at 1500 watts output and double plate voltage. Essentially, your
tubes must be able to efficiently MAKE 1500 watts, even if they can
only get rid of 200 watts of heat in the process.

I learned about this the ahrd way, on MOSFETS for a MW application. Two
IRF 510's could make 50W carrier but wouldn't modulate worth garbage.
To get acceptable modulation requred backing power off all the way to
27 watts and then still needed an assymetrical modulating voltage.

Viewing the modulating voltage on the scope proved the modulator to be
adequate when wound for a normal syymetrical output waveform, leavign
the Class C amp as the culprit. To fix it right too four of those
little MOSFETS, and 2V or gate modulation from a tertiary winding. Just
forward biasing to handle the current didn't cut it. This gave 45W
carrier, with still a shortfall in PEP, but now only the shortage
predicted by the power supply voltage sagging about 3/4V out of 16V.
The heatsink runs very cool at carrier, and doesn't get too hot even
with the modulator running flat-out.

A pair of 6146B's of course, with a pair of 6550 audio tube as a
modulator, would have been able to pound out several times that much
power! Still, they would never, ever be able to make four times the CW
rating from the tube manual simply by applying twice the CW plate
voltage. Similarily, to run the same current at carrier as in a CW rig,
they would need a lot more drive, as at PEP the plate current would be
higher, as high as the current drawn in a low-impedance VHF rig.

Straydog wrote:
Since my earlier post (dealing with the question of what is peak evelope
power output in an AM transmitter), I've been doing more scrutinizing
of tube Ip/Vp characteristic curves. They are much more non-linear than
the impression you get from just looking at the curves. Also, it is rare
or almost non-existant to find Ip vs screen voltage!

Lets look at the venerable 833 (from my RCA TT-3 transmitting tube
manual). This is a KW input class C triode.

From the curve:
at zero grid volts, 1 kV on the plate gives 175 ma plate current
2 kV 500 ma
That's more than a doubling of Ip for a doubling of Vp

at minus 50 grid volts, 2 kV on the plate gives 50 ma plate current
4 kV 750 ma

looking in my RCA receiving tube manual (RC-20) I found for a 6FG6
a sharp cutoff tetrode that only at zero grid volts was there a near
linear relationship between d a straight line [which actually deviated
slightly from a straight line] with some slope. But at 100 v on plate,
current was 14 milliamps, at 200 v on the plate, plate current was 34
miliamps. Definitely NOT a linear relationship. For the 6EM7 a triode,
and at any of a wide range of grid voltages, plate current could be
doubled with only a 15-20% increase in plate voltage.

My thinking on all of this leads me to claim that anyone who can start
with a 100 watt carrier from an AM transmitter and make a few assumptions
about 100% modulation and come up with a _calculation_ of something like
400 watts of peak power and represent that as having something to do with
reality is pure conjecture.

If anyone wants to put an appropriate oscilloscope on the transmitter output
and measure the RF voltage of unmodulated carrier into an appropriate load
and then measure the peak RF voltage when the carrier is modulated, then
and only then do they have a reasonable _basis_ for making a claim about
peak (instantaneous) output power.