Re: More on PEP, AM, average power, etc.
- From: Straydog <asd@xxxxxxxxx>
- Date: Thu, 26 Jan 2006 23:06:08 -0500
On Thu, 26 Jan 2006, Uncle Peter wrote:
"Straydog" <asd@xxxxxxxxx> wrote in message news:Pine.NEB.4.63.0601252142230.9221@xxxxxxxxxxxxxxxxxxx
> 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.
In some cases it is a lot easier to accept what is technically correct, and work backwards to correct erroneous conclusions.
Well, I didn't have trouble seeing that if a scope shows modulation RF peaks that are double the unmodulated carrier, then the instantaneous power at the peak is 4X. What I had trouble with was from the curves that show plate current in tetrodes, pentodes independent of plate voltage. What I did not consider is how screen voltge would affect plate current if the screen voltage were modulated along with plate voltage. Previously, I thought power input could only double but that would leave a deficit since peak output voltage shows on the scope that power at the peak has to be 4X the unmodulated power.
First, a Class C amplifier is driven into grid conduction, almost to the point of plate saturation.
High Level AM modulation is applied to the SCREEN and PLATE, only doubling the plate voltage as in your 6F6 example to show a non linear relationship isn't a valid argument.
Well, it was a 6FG6 (not a 6F6) and what we are trying to do is find out how power _input_ at the peak of a modulation cycle becomes 4X the power input when an unmodulated carrier is being put out. Part of the answer comes from modulating the screen, and one of the guys was talking about some modulation of the control grid through a "grid leak" resistor which was also mentioned in the RCA transmitting tube manual in the front which gives some rudimentary explanations for all of this (and was helpful for me to re-read).
What is the
operating Class of the tube, and did you account for the modulating voltage also being applied to the screen grid?
Well, if you look at all of the curves showing Ip vs Vp, they usually give curves for a fairly large range in control grid voltages (but at only one screen grid, if it is present, voltage) so you can look at how Ip changes for any range in changes in control grid so you can know about what class of amplifier you are running by looking at highest control grid voltage and lowest control grid voltage you want to use and whether you get close to cutoff (where Ip goes to zero or, maybe, close to zero).
To quote Henny: "A linear relation must exist between plate voltage and tank circuit current for good operation... In such a modulated amplifier, the output peak will be four times the unmodulated carrier and the continuous power output with complete modulation is 1.5 times the power at zero modulation."
I understand this, now, and know where it comes from. Fine.
Note that is only
true for a true Class C power amplifier stage, and not for Class A or B.
I can accept this, too.
I doubt that Henny or Tenny based their texts on conjecture or misguided realities.
I can appreciate that in the more advanced treatises on the subject that the guys know more about what is going on. The ARRL handbooks gloss over a lot of this and I always wondered why the FCC changed the rule from measuring simple DC power input (plate volts X plate current), even on a linear for SSB, with a simple D'arsonval movement meter (or a digital bar graph meter that could be made to mimic a mechanical meter), to the rule that PEP output not exceed 1500 Watts. One would have to surely use a scope and I'd prefer not to have to trust these so-called PEP reading meters that are all over the place now.
But, thanks for your contribution.
Pete
.
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