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.