On Thu, 26 Jan 2006, Uncle Peter wrote:


"Straydog" wrote in message
.com...


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