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