Re: More on PEP, AM, average power, etc.
- From: " Uncle Peter" <radioconnectionNO@xxxxxxxxxxx>
- Date: Thu, 26 Jan 2006 21:22:53 -0500
"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.
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. What is the
operating Class of the tube, and did you account for the modulating
voltage also being applied to the screen grid?
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." Note that is only
true for a true Class C power amplifier stage, and not for
Class A or B.
I doubt that Henny or Tenny based their texts on conjecture or
misguided realities.
Pete
.
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