[AMRadio] AM Transmitter Advice??

Bob Bruhns bbruhns at erols.com
Sat Feb 18 19:21:09 EST 2006

That was Fred, W3PHL.  He built a high level balanced
modulator (called the upside down tube circuit), and
biased it to 600 watts carrier input.  Then he applied
a 2400 watt modulator to it.  He used heavy clipping,
like about 30 dB of clipping, and a low-level low-pass
filter.  At that time, plate power input to an AM
transmitter was measured by the reaings of DC meters
with approximately 0.25 second time constant.  That
time constant would make the meters read average
voltage and current, which smoothed out SSB and
controlled carrier signals.  But on plate-modulated AM
it did not read true power during modulation, because
it was looking at average voltage and average current
separately, while in AM the voltage and current are not
separate, but they increase and decrease together,
resulting in higher power input during modulation.

Fred's X volts and Y milliamps held steady with or
without modulation, looking like 600 watts input, even
though easily 3 kilowatts of power were going to that
final amp during modulation.  The way plate input power
was measured at that time, this was legal...  so the
FCC tried to nail him for overmodulation.

Fred countered that overmodulation was a function of
nonlinear distortion, and since his modulator correctly
handled modulation beyond 100% negative, and it was not
saturating at any point, it could not be said to be
overmodulating.  And if the FCC was going to consider
this to be overmodulation, then what about all of
theose guys on SSB and DSB... they would have to nail
all of them for overmodulation, too!

So the FCC resorted to nailing Fred on splatter out of
band.  It was a cheap trick, because Fred's signal was
pretty clean.  But they railroaded it through, and Fred
lost his license for a year.  When he came back on, he
was using some fantastic Marconi high-level multi-pole
filter, and there was no way they could accuse him of
splattering out of the band.  The FCC had lost interest
by that time anyway, and they left Fred alone after

But that high-level balanced modulator loophole was the
reason that the FCC went for the PEP measurement
technique in 1990.  Theoretically, one could have used
a modulator from a 500,000 watt broadcast station on a
1KW upside-down tube final, and it would have been
legal.  They couldn't have that!  Darn it...

  Bacon, WA3WDR

----- Original Message ----- 
From: "John E. Coleman (ARS WA5BXO)"
<wa5bxo2005 at pctechref.com>
To: "'Discussion of AM Radio'"
<amradio at mailman.qth.net>
Sent: Saturday, February 18, 2006 6:39 PM
Subject: RE: [AMRadio] AM Transmitter Advice??

I have heard rumors of a citation or two but only in
extreme cases such as
10 or 20 KW PEP and I don't know if the rumors are

As I understand it the trick, that the FCC was to
prevent, and someone was
trying to get away with, was to run a 304TL with about
100 Volts on the
plate in a GG configuration.  Forward bias it to a high
plate current like 1
Amp, so that it acts like a switch that is on.  Then
drive it with 5 KW PEP.
It may have been some other some scenario as this but I
think you get the

An Now, For Something Completely Different.

Then there was the trick that a gentleman up in 3 land,
I think, was
going to run the high level double sideband reduced
carrier generator type
rig but he was not reducing the carrier just increasing
the SBs via an extra
upside down tube, as it was commonly called.  The
sideband power would
continue to go up without distortion (if copied on a
proper synchronized
product detector) after the first tube was over
modulated in the negative
direction. The voltage and power would be diverted to
the upside down tube
where sideband power would continue.  There was trouble
with the specific
rule interpretation at the time in the FCC. Of course
any of us today, would
be able to see that the upside down tube's audio plate
current and audio
voltage must also be counted as part of the input
power.  But the FCC was
having trouble deciding, at least as I understand it.
At any rate, I think
they got him for being outside the 40 meter band
limits.  You may remember
more of the specifics on this Don.

John, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of
Donald Chester
Sent: Saturday, February 18, 2006 4:41 PM
To: amradio at mailman.qth.net
Subject: RE: [AMRadio] AM Transmitter Advice??

>My understanding was.
>Under the old rules stage or stages, that provide
output to the antenna,
>total power should not exceed 1000 watts DC input.
This includes the sum
>the driver and final in the case of GG output circuit.
>I think that rule was tested.

I recall that was the rule.  But it seems to me it was
much ado about
nothing.  At most, the feedthrough power would be 10%
of the total output
power.  How much signal strength gain could you get
from increasing your
power 10%?

Of course, back in those days the FCC was very nitpicky
about the ham rules,

as they still are with broadcast stations.  But they
have shifted to the
opposite approach with ham radio.  Riley's efforts have
rooted out the
rottenest of the apples, but I suspect ham radio
enforcement is still pretty

low on the FCC's priority list, as long as the
violations don't cause
interference to other radio services.

Since they changed the power rule, I have never heard
of a SINGLE case of a
ham receiving a citation for running too much p.e.p.



This message was typed using the DVORAK keyboard
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