Jumping into the middle of a thread with a single, but useful, comment,
Steve, K9DCI says:



"Gary Schafer" wrote in message
...
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!

Probably because the screen and supressor grids are not the main
controlling grids, but are there only to reduce Ip vs. Vp and control
secondary emission off the plate respectively.


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
....

OK OK TWO comments...
I think someone else pointed out that this is a static situation and in a
circuit, the supply voltage and the plate voltage are not the same thing
when you have a tank circuit in there.

73, Steve, K9DCI

"Straydog" wrote in message
x.com...

,...

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.


While possibly (probably) true... DC was easy to measure (by hams) and RF
power difficult to measure in the early days. As time progressed, RF power
became easier to measure and, after all, was the thing which would cause
trouble as far as interference was concerned.

73, Steve, K9DCi

On 1/30/06 2:33 PM, in article , "Steve
Nosko" wrote:


"Straydog" wrote in message
x.com...

,...

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.


While possibly (probably) true... DC was easy to measure (by hams) and RF
power difficult to measure in the early days. As time progressed, RF power
became easier to measure and, after all, was the thing which would cause
trouble as far as interference was concerned.

73, Steve, K9DCi



Even us guys with a commercial ticket used DC measurements to determine
input power; it wasn't a ham thing. Watt meters were used on lower powered
equipment, but it wasn't a requirement. Your "after all" comment is silly.

Don

"Don Bowey" wrote in message
...
On 1/30/06 2:33 PM, in article , "Steve
Nosko" wrote:


"Straydog" wrote in message
x.com...

,...

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...


While possibly (probably) true... DC was easy to measure (by hams) and
RF
power difficult to measure in the early days. As time progressed, RF
power
became easier to measure and, after all, was the thing which would cause
trouble as far as interference was concerned.

73, Steve, K9DCi



Even us guys with a commercial ticket used DC measurements to determine
input power; it wasn't a ham thing. Watt meters were used on lower
powered
equipment, but it wasn't a requirement. Your "after all" comment is silly.

Don


Perhaps a poor expression of intent, but my intent was that it is the output
power which the FCC (or equivalent) is concerned about, not input. As long
as it bacame more feasable to measure output power, why not use that.
Perhaps explicitly stating it was stating the obvious, but it was an attempt
at being complete--sort of a flame insurance.
73, Steve, K9DCI

I understood your post as if you would like to gain some understanding (without having to consult Terman) on why the voltage & current figures you read on your RCA TT-3 manual for the 833 tube do not apparently match the 400-W peak-envelope-power figure commonly quoted for a 100%-modulated 100-W AM carrier.

In my understanding, there are two main issues that you did not consider in your reasoning:

1) the voltage / current pairs you quoted for the 833 tube correspond to a fixed grid voltage (either 0V or -50V). In practice the grid voltage is sinusoidal (at the carrier frequency) and, for a class-C amplifer, it only causes plate current to flow when getting above the interdiction threshold. So, plate current only circulates for a fraction of the RF cycle (180 degrees). Therefore plate current is not sinusoidal and its average value (along half a cycle), i.e. what you read on a DC meter, is closer to the peak value compared to a purely sinusoidal waveform. When plate voltage varies due to modulation, the grid interdiction threshold varies and so the circulation angle does. This turns into a change of the average-to-peak current relationship. That said, it immediately follows that simply taking voltage & current figures at a fixed grid voltage would not make much sense. All what said does not take into account the presence of the plate tank circuit which, as someone else has noted, yields a remarkable effect on actual plate voltage & current figures

2) the modulator is designed to feed a given load resistance (depending on the modulation transformer winding ratio). If the modulator load (i.e. the class-C final stage) does not show a linear current / voltage relationship, the plate voltage will, partially, self-adapt itself along the audio-frequency cycle, thus smoothing the tube non-linear behavior.

In addition to that, there are other effects (like the grid leak bias causing grid voltage to somewhat follow the modulation) that someone else has already pointed out.

The above arguments do not provide a justification of why a class-C tube shows, in practice, a reasonably linear voltage / current behavior, but they should at least give you sufficient evidence why your reasoning is way too simplified to credibly deny the 1:4 PEP ratio of AM-modulated signals.

73

Tony I0JX