Re: More on PEP, AM, average power, etc.



On Thu, 26 Jan 2006, Gary Schafer wrote:

On Thu, 26 Jan 2006 19:31:08 -0500, Straydog <asd@xxxxxxxxx> wrote:



On Thu, 26 Jan 2006, Gary Schafer wrote:

On Wed, 25 Jan 2006 22:09:48 -0500, Straydog <asd@xxxxxxxxx> 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.

.

Relevant Pages

  • Advice on class-C modulated tube
    ... To obtain both plate and screen modulation, I am planning to adopt the well-known circuit whereby the tube screen is fed by the 750V plate supply through a high-power series resistor. ... the EIMAC "Care and feed of power grid tubes" booklet states that the screen resistor must be connected to the "cold" end of the secondary. ... Their argument is that, when plate voltage increases, the screen current decreases and so does the voltage across the screen resistor, thus resulting in a higher screen voltage. ...
    (rec.radio.amateur.boatanchors)
  • Re: Grid #2 in ultralinear mode
    ... > I have not measured the current flowing in the Grid circuit but it must be ... The actual value depends on the tube bias current - so the G1 voltage. ... > mode compare with tying G2 to the Plate with a 100 Ohm resistor? ...
    (rec.audio.tubes)
  • Re: Tube matching?
    ... Take measurments at two bias voltages. ... Nine tubes so far, and all close on umhos, but bias voltage ... varies with plate voltage, screen voltage, and current; ... And, then, 'nominal varies from tube ...
    (alt.guitar.amps)
  • Re: 6SN7 amp: what am I seeing?
    ... When that tube turns off, the voltage swings close to B+, ... as it's pulled up by the plate resistor. ...
    (alt.guitar.amps)
  • Re: More on PEP, AM, average power, etc.
    ... >of tube Ip/Vp characteristic curves. ... 1 kV on the plate gives 175 ma plate current ... >linear relationship between plate current and plate voltage (meaning zero ... Consider that with AM modulation ...
    (rec.radio.amateur.boatanchors)