He is not calculating PEP properly. For 100% modulation the plate
voltage must double. That will require 187 watts average power to do
so. A little more with transformer losses.

Average power output from the transmitter will be 375 watts carrier
and 93 watts in each side band for a total of 562 watts average output
with full modulation.
But Peak envelope power will be 1500 watts at 100% modulation. 4
times carrier power.

100% modulation requires the plate voltage to double. Doubling the
plate voltage also doubles the current. So twice the voltage and twice
the current equals 4 times the power for PEP = 1500 watts.

4-125's in AB1 don't produce much power. I would have to look it up
but their AB1 rating is very low compared to some other tube types.

73
Gary K4FMX


On Mon, 24 Apr 2006 02:58:08 GMT, "Mont" wrote:

Following is a note I received from a friend who earnestly believes that a
Class C plate modulated final requires approximately 200% (rather than 50%)
of the final power input to achieve 100% modulation. I am interested in any
theoretical arguments surrounding this.
Thanks, Mont - K0YCN
-------------------------------------------
Mont,

Here are the numbers as I calculate them to achieve my goal.

Since Eimac specs are listed in terms of watts output so I will follow that
too.
My goal is to be capable of 130% positive peak modulation with 375 watts of
carrier output.

The Eimac tube manual says a single 4-400 typical operation at 3000 volts on
the plate is 630 watts high level plate modulated carrier output. So, 630
watts X 4 = 2520 watts pep capability.

So, a more conservative 375 watts X 4 X 1.30 = 1950 watts pos. peak.
Dividing that by 2 = 975 watts of peak audio output required from the
positive producing modulator tube to make 1950 pos. peak RF output.

The 4-125 typical operation indicates 330 watts of AB1 is available. This
is a far cry from 975 that I need. On the other hand a 4-400 in AB1 will
deliver 1540 watts.

The negative vs. positive peaks will be controlled by a broadcast quality
Innovonics AM asymmetrical compressor limiter which I am currently using in
the shack.

The simple unexplained rule of thumb I came to understand early in my early
ham days is that a 375 watt transmitter requires 187.5 watts of audio for
100% modulation. So, a 4-125, with 330 watts of capability, should be more
than what is needed. In practical terms this is very misleading. That is
why I been beating a dead horse for three years while two broadcast
engineers could not explain why I could barely attain near 100 modulation.
Little did I know that I was driving the pants of the poor 4-125's while
trying!

Hence, I ordered a custom built filament xfmr that will power two 4-400
modulators. Hope to have all that working in about two weeks.

73,