On Thu, 26 Jan 2006, Doug 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!

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.

Methinks you are way too hung up on the abtract theory of how linear
tubes are.

In practice the majority of AM transmitters rated at 100 watts of
carrier output are indeed putting out 400 watts PEP with 100%
modulation. As another poster pointed out, this is easily proved by
using an oscilloscope or with SOME Peak reading wattmeters.

Well, I'll buy into the scope measurement.

I'll also buy into my prior incomplete understanding of Ip vs Vp
relationships when the tube curves are looked at without realizing that
screens have to be modulated as well as plates.

I don't like wattmeters (with mechanical needles) that claim to indicate
peak watts or have peak watt scales. But, that is my prejudice.

The FCC certainly agrees with the 4:1 ratio. That's why when the
Amateur power levels permitted by the FCC in the USA were raised to
1500 watts output PEP, the net result was that users of A.M had to
REDUCE carrier power to approximately 375 watts output.

I remember some of that. But at that time I was an SSB user only.

Many diehard AM'ers and even the ARRL vigorously protested this net
reduction of power for AM use.
As I remember the FCC grandfathered the old power limit of 1000 watts
DC input to the final amplifier for Am'ers but only did so for a
couple of years.

Back in the good old days, I used to run a Technical Materiel Corp
GPT-750 AM transmitter on 3885KHZ. I ran 1KW DC input on the plate
with 100% modualtion. That required a 500 watt modulator in the
transmitter. The pair of 4-400A's in the rig easily achieved a power
output of 800 watts under class C high level modulation.
Thus I was legally running 3200 watts PEP output power.
The power supply exceeded 3000 volts and was rated at 1.5 amps CCS,
easily achieving the peak power demands..

And what do you do now? If you don't mind me taking a little more of your
time. And, what about all these guys I hear on AM (160 & 75) who say they
are running Johnson "Desks" at 1 kW input, and modified former AM
broadcast transmitters with 833s in the final?

You are sort of beating a dead horse. this was extensively discussed
everywhere in Amateur circles about a decade ago.

A decade ago I was not interested in anything AM. A decade ago I had
nothing that would even come close to putting out any power that would
have even made me have to think about this FCC specification. Recently,
out of pure nostalgia, I partly restored a very chopped up Johnson Ranger
and got it back on the air, on AM, to my considerable delight. Otherwise
it was too chopped up for a full restoration (to include CW). Similarly,
lots of things I was interested in a decade ago, I'm not interested in
any more. And, I spend much more time at the bench now than at the
microphone.

Art, W4PON

Doug/WA1TUT