Following is a note I received from a friend who earnestly believes that a
Class C plate modulated final requires approximately 200% (rather than 50%)
of the final power input to achieve 100% modulation. I am interested in any
theoretical arguments surrounding this.
Thanks, Mont - K0YCN
-------------------------------------------
Mont,

Here are the numbers as I calculate them to achieve my goal.

Since Eimac specs are listed in terms of watts output so I will follow that
too.
My goal is to be capable of 130% positive peak modulation with 375 watts of
carrier output.

The Eimac tube manual says a single 4-400 typical operation at 3000 volts on
the plate is 630 watts high level plate modulated carrier output. So, 630
watts X 4 = 2520 watts pep capability.

So, a more conservative 375 watts X 4 X 1.30 = 1950 watts pos. peak.
Dividing that by 2 = 975 watts of peak audio output required from the
positive producing modulator tube to make 1950 pos. peak RF output.

The 4-125 typical operation indicates 330 watts of AB1 is available. This
is a far cry from 975 that I need. On the other hand a 4-400 in AB1 will
deliver 1540 watts.

The negative vs. positive peaks will be controlled by a broadcast quality
Innovonics AM asymmetrical compressor limiter which I am currently using in
the shack.

The simple unexplained rule of thumb I came to understand early in my early
ham days is that a 375 watt transmitter requires 187.5 watts of audio for
100% modulation. So, a 4-125, with 330 watts of capability, should be more
than what is needed. In practical terms this is very misleading. That is
why I been beating a dead horse for three years while two broadcast
engineers could not explain why I could barely attain near 100 modulation.
Little did I know that I was driving the pants of the poor 4-125's while
trying!

Hence, I ordered a custom built filament xfmr that will power two 4-400
modulators. Hope to have all that working in about two weeks.

73,

Soooo...if the modulator is transformer-coupled, how does the DC input
to the RF amplifier increase when modulation is applied? A transformer
can't couple DC...

Not saying the fellow can't do what he wants, but we need to know a LOT
more about the circuit.

Remember, too, that the half-power rule is for sinewave modulation. If
you use square-wave modulation at 100% depth, you need as much power
from the modulator as the DC input to the RF amplifier. The modulated
output is four times the unmodulated power, for half the time, and zero
for the other half, assuming an RF stage that responds linearly to the
modulation voltage.

For a given modulated waveform, it's easy enough to figure out what the
actual power is, and it all has to come from somewhere. If the plate
modulation is linear (current is linear with modulated voltage) and the
modulation is coming through a transformer, then the average current
from the RF deck plate supply must be constant, and the DC power input
is therefore constant. Whatever power you put in the sidebands must
come through the transformer. But maybe the circuit he's proposing DC
couples the modulation voltage, and thus allows an increase in the DC
current going to the RF deck.

Cheers,
Tom

In article .com,
"K7ITM" wrote:

Soooo...if the modulator is transformer-coupled, how does the DC input
to the RF amplifier increase when modulation is applied? A transformer
can't couple DC...

You really don't know the answer to this? Did you actually pass the
Theory Exam? Inquiring minds want to know.........

It is so simple you can't believe it. It's only Ohm's Law.

The Class-C plate modulated final amplifier is driven hard on its
control grid such that it is driven into saturation even when its DC
plate volts is increased to twice its normal DC value.

When saturated, the peak RF plate output volts always swing between
twice the DC supply volts and some very low value.

The final amplifier therefore constitutes a fixed DC load resistance
across the DC supply, which depends on unmodulated RF power output and
amplifier power efficiency.

This fixed resistance is also the value of the audio-frequency load
impedance across the secondary of the modulation transformer.

For 100 percent modulation, the peak RF plate volts across the tank
circuit swing between twice the modulated plate supply DC volts and
zero. (Or some very low value.)

The harder the grid drives the plate circuit into saturation, the
greater the modulating linearity. The drive limit is reached when any
of the tube's electrodes approaches its rated power dissipation.

Eaxactly the same things happen with transistors.
----
Reg.

Thank you, Reg. My reply would have been much less gracious and not
nearly so complete. Probably something like, "Which part of 'constant
DC input' do you not understand?" I appreciate that you took the time
to write such a nice reply, one that the lurkers may well learn
something from.

Another reason, of course, that a tube can't handle modulation peaks is
that it's getting weak (or the initial design was inadequate). Seeing
the plate current meter wiggle during modulation that's below 100% is a
good sign it's time to check the RF output tubes, and as your reply
suggests, the drivers as well, though the grid current meter should
tell you enough about that. If the filament (cathode) no longer has
enough emission, the tube may handle the carrier OK, but not the
positive modulation peaks where the current must be about twice as high
as with just the carrier. Then the DC current will drop during
modulation.

Cheers,
Tom

He is not calculating PEP properly. For 100% modulation the plate
voltage must double. That will require 187 watts average power to do
so. A little more with transformer losses.

Average power output from the transmitter will be 375 watts carrier
and 93 watts in each side band for a total of 562 watts average output
with full modulation.
But Peak envelope power will be 1500 watts at 100% modulation. 4
times carrier power.

100% modulation requires the plate voltage to double. Doubling the
plate voltage also doubles the current. So twice the voltage and twice
the current equals 4 times the power for PEP = 1500 watts.

4-125's in AB1 don't produce much power. I would have to look it up
but their AB1 rating is very low compared to some other tube types.

73
Gary K4FMX


On Mon, 24 Apr 2006 02:58:08 GMT, "Mont" wrote:

Following is a note I received from a friend who earnestly believes that a
Class C plate modulated final requires approximately 200% (rather than 50%)
of the final power input to achieve 100% modulation. I am interested in any
theoretical arguments surrounding this.
Thanks, Mont - K0YCN
-------------------------------------------
Mont,

Here are the numbers as I calculate them to achieve my goal.

Since Eimac specs are listed in terms of watts output so I will follow that
too.
My goal is to be capable of 130% positive peak modulation with 375 watts of
carrier output.

The Eimac tube manual says a single 4-400 typical operation at 3000 volts on
the plate is 630 watts high level plate modulated carrier output. So, 630
watts X 4 = 2520 watts pep capability.

So, a more conservative 375 watts X 4 X 1.30 = 1950 watts pos. peak.
Dividing that by 2 = 975 watts of peak audio output required from the
positive producing modulator tube to make 1950 pos. peak RF output.

The 4-125 typical operation indicates 330 watts of AB1 is available. This
is a far cry from 975 that I need. On the other hand a 4-400 in AB1 will
deliver 1540 watts.

The negative vs. positive peaks will be controlled by a broadcast quality
Innovonics AM asymmetrical compressor limiter which I am currently using in
the shack.

The simple unexplained rule of thumb I came to understand early in my early
ham days is that a 375 watt transmitter requires 187.5 watts of audio for
100% modulation. So, a 4-125, with 330 watts of capability, should be more
than what is needed. In practical terms this is very misleading. That is
why I been beating a dead horse for three years while two broadcast
engineers could not explain why I could barely attain near 100 modulation.
Little did I know that I was driving the pants of the poor 4-125's while
trying!

Hence, I ordered a custom built filament xfmr that will power two 4-400
modulators. Hope to have all that working in about two weeks.

73,

The Eimac tube manual says a single 4-400 typical operation at 3000 volts on
the plate is 630 watts high level plate modulated carrier output. So, 630
watts X 4 = 2520 watts pep capability.
So, a more conservative 375 watts X 4 X 1.30 = 1950 watts pos. peak.
Dividing that by 2 = 975 watts of peak audio output required from the
positive producing modulator tube to make 1950 pos. peak RF output.

He'd be better off to consider what the modulator is actually doing.
The modulator is DOUBLING the anode voltage for 100% positive peaks.
Say the anode voltage is 1000 volts and current is 1 ampere. The
modulator drives a load impedance of 1000/1 or 1000 ohms. It has to
produce 1000 volts PEAK, which on a sine wave is 707 volts RMS.

707 volts RMS into a 1000 ohm load is 500 watts of sine wave power.

The modulator SINE WAVE power for 100% positive peak is exactly half
the plate input power. This ASSUMES a triode well into class C for the
PA. A tetrode will actually often use a bit less power because it often
must be modulated on the screen (or control grid) and anode at the same
time in order to be linear. The anode of a tetrode or pentode does not
follow square law power rules like a low mu class C triode does.

You can generally consider for voice even LESS power is required, a
good rule of thumb is about 25% of dc input power. So as a general rule
a 250 watt average audio power modulator will modulate a 1000 watt
transmitter 100% on voice.

I suspect he has something wrong with the PA not following square law
power change with anode voltage changes. In order to be linear with
modulating voltage changes the PA has to be well into class C
(switching very hard) and it must, with a terode, have some portion of
audio power applied to the screen (you could do the drive power
instead, but that's tricky).

He could have an audio mismatch or the modulator system might be
incapable of supplying the necessary power which is half the dc plate
input power for 100% modulation with a sine wave. He could have a PA
that does not follow square law power as voltage is changed. There is
absolutely NOTHING wrong with the rule that a modulator capable of
supplying half the dc input will modulate a properly designed PA 100%.

73 Tom



The 4-125 typical operation indicates 330 watts of AB1 is available. This
is a far cry from 975 that I need. On the other hand a 4-400 in AB1 will
deliver 1540 watts.

The negative vs. positive peaks will be controlled by a broadcast quality
Innovonics AM asymmetrical compressor limiter which I am currently using in
the shack.

The simple unexplained rule of thumb I came to understand early in my early
ham days is that a 375 watt transmitter requires 187.5 watts of audio for
100% modulation. So, a 4-125, with 330 watts of capability, should be more
than what is needed. In practical terms this is very misleading. That is
why I been beating a dead horse for three years while two broadcast
engineers could not explain why I could barely attain near 100 modulation.
Little did I know that I was driving the pants of the poor 4-125's while
trying!

Hence, I ordered a custom built filament xfmr that will power two 4-400
modulators. Hope to have all that working in about two weeks.

73,

A couple of comments not covered by others.... *****

"Mont" wrote in message
. net...
Following is a note I received from a friend who earnestly believes that a
Class C plate modulated final requires approximately 200% (rather than
50%)
of the final power input to achieve 100% modulation. ... Thanks, Mont -
K0YCN
-------------------------------------------
Mont,
....
My goal is to be capable of 130% positive peak modulation with 375 watts
of
carrier output.

****** WHY?. What could be the possible benefit (thinking on terms of dB)?
This is what, 2 dB more power in the sidebands...? This is 69% more power
in the sidebands and 2dB is 56% more...


The Eimac 4-400...3000 volts...630 watts high level plate modulated
carrier output. So, 630 watts X 4 = 2520 watts pep capability.

***** PEP is a red herring here. I could be wrong because of the contorted
reasoning, but I believe he is confusing Peak power and PEP, but I can't be
sure 'cuz I don't completely understand what he is trying to reason out.
Peak Power is Vpk x Ipk
PEP is V(rms at the peak) x I(rms at the peak)


...375 watts X 4 X 1.30 = 1950 watts pos. peak.
Dividing that by 2 = 975 watts of peak audio output required...

**** This appears to be a location for a logic fault.


The 4-125 typical operation indicates 330 watts of AB1 is available....
*** Noted for later.


The negative vs. positive peaks will be controlled by a broadcast quality
Innovonics AM asymmetrical compressor limiter which I am currently using
in
the shack.

*** This may be a separate issue, but doesn't this suggest that the
modulator AND transformer must have the capability of handling the distorted
waveform from this "asymmetrical compressor " such that the low frequency
response extends low enough to get this extra plate voltage turing the
positive peaks?



... a 375 watt transmitter requires 187.5 watts of audio for
100% modulation. So, a 4-125, with 330 watts of capability, should be
more
than what is needed.

*** No argument here...

In practical terms this is very misleading.
*** paraphrased = " _I_ can't get _my_ circuit to work."


That is
why I been beating a dead horse for three years while two broadcast
engineers could not explain why I could barely attain near 100 modulation.
Little did I know that I was driving the pants of the poor 4-125's while
trying!

*** Are the RF amp and modulator operating properly in the first place?
*** Does he have the proper modulation transformer?
*** Is he even using a modulation transformer?
*** Is he confused between Peak power and PEP?

73, Steve, K9DCI

Hence, I ordered a custom built filament xfmr that will power two 4-400
modulators. Hope to have all that working in about two weeks.

73,

There are a few other factors here. The big one ne is that no real
tube or transistor swings ALL THE WAY to zero volts at full current!
Another is that for the final to be truly ohmic would require that a
near-Class E or F condition, with the grid swinging from cutoff to
saturation almost instantly(as in a square-wave drive, sometimes used
in broadcast AM Class F setups).

The next condition is that the current drawn through the load at 2X
supply voltage must not cause the tube or transistor's bottoming
voltage to more than double! In the real world, this means that the
current(loading) must be backied off from CW conditions for any
particular device, just as the voltage must be. If you load a final
for maximum output at carrier, guess what-you will be lucky to see 30%
upward modulation with MOSFETS or somewhat better with tubes!

As a result, you want to design the final as though you were building a
CW amp to operate at double the supply votage with so short a duty
cycle that dissipation wouldn't overheat the device first. The active
device must not at any time be an impediment to drawing full AC current
through the load.

OK, here's what my experiments with the IRF 510 MOSFET uncovered at MF:

1: this device can make 55W a part at 17V, but for even 90% modulation
you must cut
loading to 37W a pair. Similar current derating should be expectd
from tubes, bipolars,
etc.

2: MOSFETS tend to lose drive as you raise the supply voltage. Here's
why-in Class C,
You are severely compressing voltage gain. As you add B+ in
modulation, voltage gain
increases. This increases the effect of Miller(reverse transfer)
capacitance). Any non-
neutralized common-cathode device will have this problem-or at the
other extreme
could oscillate on peaks! MOSFETS are lossy in all interelectrode
capacitances, so
even with "unilaterialization" in which both R and C(not just C)
are balanced, drive loss
in the resuloting "bridge" circuit" still rises with the supply
voltage. Since MOSFETS
cannot be driven beyond maximum safe gate voltage, you therfore
will need to modulate
the driver as well, or apply a few volts of gate modulation as an
alternative.

IF YOU DO NOT DO THIS, you will have 100% downward modulation real
easy,
but as little as 30-50% upward modulation, with a real serious
carrier shift problem
and audio that sounds like $%^&.

3: I've heard bipolar transistors also need some base modulation to
follow collector
modulation properly.

4: Tetrodes have a similar problem, but in a different way. Here, the
problem is that plate
voltage and evn bottoming voltage have little effect on plate
current, and real-world
tetrodes and pentodes require some screen modulation to propely
follow the plate
modulation.

5: Triodes with their low plate resistance may well give bottoming
voltage in proportion to
current, and therefore to voltage. This is a desirable condition,
but the only common use
of triodes today is in grounded-grid, where 100% downward
modulation is impossible.

One of the nice things about those Class E and F, true ZVS finals for
AM is that they give a far more ohmic modulation curve!

WSQT wrote:
There are a few other factors here. The big one ne is that no real
tube or transistor swings ALL THE WAY to zero volts at full current!

It does niot have to be at full cuttrent, and it actually cannt be at
full current or the modulation will not be linear.

The power has to follow a square law relationship with voltage change.
meaning current is twice when voltage is twice, current is zero when
voltage is zero.

This is why the anode impedance is stable over a wide range of anode
voltage, and why PEP power is four times the carrier power in a good
clean plate modulated transmitter.

Another is that for the final to be truly ohmic would require that a
near-Class E or F condition, with the grid swinging from cutoff to
saturation almost instantly(as in a square-wave drive, sometimes used
in broadcast AM Class F setups).

Class F is an invention to describe certain circuits. It technically
fits the definition of class C.

RCA and other manufacturers used "square wave" drive in class C
amplifiers in the 40's and 50's. The method of obtaining the square
plate current waveshape was the addition of third harmonic resonators
or traps in the grid and anode circuits of low mu triodes. Allowing the
3rd harmonic inherent in the tube switching from cutoff to positive
grid voltage by not "grounding" the grid or anode at the third harmonic
caused the PA to switch into and out of conduction rapidly. Typical
efficiencies were in the mid 90% range.

It was called class C back then, and it techincally fits the
description of class C.

When the tube switches very hard, the linearity of anode power input
(and RF output) follows the desired square law change very well.


The next condition is that the current drawn through the load at 2X
supply voltage must not cause the tube or transistor's bottoming
voltage to more than double! In the real world, this means that the
current(loading) must be backied off from CW conditions for any
particular device, just as the voltage must be. If you load a final
for maximum output at carrier, guess what-you will be lucky to see 30%
upward modulation with MOSFETS or somewhat better with tubes!

What does that have to do with plate modulated stages?

The anode operating impedance is nearly constant throughout the full
audio cycle, and the ratio of E/I tracks very well regardless of load
setting in low-mu triode class C modulated stages.

The tetrode is a problem only because the anode does not follow a
square law power change as voltage is changed. This is because, as you
pointed out, the anode current is controlled by the screen voltage more
than anode voltage.

In a tetrode or any other screen grided tube, some audio has to be
applied either to drive, control grid, or screen voltage. This is to
ensure anode current tracks a square law relationship with modulation
voltage, plate operating impedance is reasonably constant, and peak
power is four times carrier power.

No matter how I load a class C plate modulated triode, modulation
remains reasonably constant. It is only in multigrided plate modulated
tubes that modulation can be seriously affected, since screen current
and the effects of screen voltage and current change can vary
drastically with load setting.

The issue the orignal poster missed was how the class C PA behaves as
voltage is changed by the modulation transformer. Power output should
square as voltage is doubled, but that's tough to do in a tetrode
unless screen operating conditions are controlled and the circuit
applies audio voltage in the proper relationship to anode voltage in a
grid.

Most of the AM pages I see don't really understand the importance of
that, and think just throwing a high inductance choke in series with
the screen makes the tube follow square law rules as modulation voltage
is changed.


73 Tom