On Thu, 26 Jan 2006, Doug 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.

Methinks you are way too hung up on the abtract theory of how linear
tubes are.

In practice the majority of AM transmitters rated at 100 watts of
carrier output are indeed putting out 400 watts PEP with 100%
modulation. As another poster pointed out, this is easily proved by
using an oscilloscope or with SOME Peak reading wattmeters.

Well, I'll buy into the scope measurement.

I'll also buy into my prior incomplete understanding of Ip vs Vp
relationships when the tube curves are looked at without realizing that
screens have to be modulated as well as plates.

I don't like wattmeters (with mechanical needles) that claim to indicate
peak watts or have peak watt scales. But, that is my prejudice.

The FCC certainly agrees with the 4:1 ratio. That's why when the
Amateur power levels permitted by the FCC in the USA were raised to
1500 watts output PEP, the net result was that users of A.M had to
REDUCE carrier power to approximately 375 watts output.

I remember some of that. But at that time I was an SSB user only.

Many diehard AM'ers and even the ARRL vigorously protested this net
reduction of power for AM use.
As I remember the FCC grandfathered the old power limit of 1000 watts
DC input to the final amplifier for Am'ers but only did so for a
couple of years.

Back in the good old days, I used to run a Technical Materiel Corp
GPT-750 AM transmitter on 3885KHZ. I ran 1KW DC input on the plate
with 100% modualtion. That required a 500 watt modulator in the
transmitter. The pair of 4-400A's in the rig easily achieved a power
output of 800 watts under class C high level modulation.
Thus I was legally running 3200 watts PEP output power.
The power supply exceeded 3000 volts and was rated at 1.5 amps CCS,
easily achieving the peak power demands..

And what do you do now? If you don't mind me taking a little more of your
time. And, what about all these guys I hear on AM (160 & 75) who say they
are running Johnson "Desks" at 1 kW input, and modified former AM
broadcast transmitters with 833s in the final?

You are sort of beating a dead horse. this was extensively discussed
everywhere in Amateur circles about a decade ago.

A decade ago I was not interested in anything AM. A decade ago I had
nothing that would even come close to putting out any power that would
have even made me have to think about this FCC specification. Recently,
out of pure nostalgia, I partly restored a very chopped up Johnson Ranger
and got it back on the air, on AM, to my considerable delight. Otherwise
it was too chopped up for a full restoration (to include CW). Similarly,
lots of things I was interested in a decade ago, I'm not interested in
any more. And, I spend much more time at the bench now than at the
microphone.

Art, W4PON

Doug/WA1TUT

"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.

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

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

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

Pete

" 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.

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

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

"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.

"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.

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.