[AMRadio] Monitoring Modulation Accurately

Donald Chester k4kyv at charter.net
Sun Nov 17 14:59:15 EST 2013

From: rbethman <rbethman at comcast.net>

>>1) The manual and the equipment gives numbers based on the 110VAC output
of the PE-95 generator.

The voltage at the receptacle runs on average 127VAC. I've measured the
plate voltage with a HV probe, and itt is over the 2KVDC.>>

What is you exact plate voltage, and what plate current do you load it up

>>I've taken measurements using the monitor jack on the front of the Johnson
TR Switch fed directly to mi Tek 5440. I get the same results as reading
from IF outputs.>>

I'm assuming by "measurements" you mean the envelope pattern on the 5440.
What selectivity on the R-390A did you use? The results would be more
accurate in the 16 kc/s position. Are you  getting close to 100% modulation,
both positive and negative, on voice peaks?  

I once hooked my bench test scope to my R-390 and looked at the envelope
patterns of signals delivered through the i.f.  I could tune in a short wave
broadcast station, and I noticed that the percentage of modulation varied
all over the place, along with the QSB. At times the signal would show more
than 100%, as in a DSB reduced carrier signal. At other times the percentage
would  drop off to less than 50%.  I got similar results on the ham bands.
The only possible explanation would be selective fading. 

Monitoring your own signal through the receiver would eliminate uncertainty
due to selective fading, but i.f. bandwidth limitations would still affect
the scope pattern, and phase shifts across the filter passband would still
occur, and these could distort the pattern as well.  A sine wave tone in the
middle of the audio range at something like, say, 1000~ would probably come
out pretty close. But frequencies in the upper audio frequency range  would
be cut, possibly resulting in an erroneously low percentage of modulation on
the display. I  would stick with the monitor jack on the TR  switch for the
signal sample; that way you are assured that anything you see on the display
is what the transmitter is actually putting out, and not artefacts generated
inside the receiver from such things as internal noise, hum and

>>My holding the reins on the audio is achieved by altering the Mic Gain. 
I use no other method. The speech amplifier already has a "clipper" 
built-in, and I haven't fiddled with its setting on either the BC-610 nor
the T-213.>>

What did the envelope pattern on the scope look like when the meter was
indicating 2500 watts p.e.p.?

Are you reducing the audio to the point that the transmitter doesn't
modulate to 90-100%? The purpose of the built-in clipper is to limit the
audio peaks below the level of flat-topping or overmodulation. Why not set
it exactly as per the instructions in the manual, and then verify with the
5440 that the signal isn't flat-topping? If flat-topping on the positive or
overmodulation in the negative direction is evident, then reduce the level
control on the clipper circuit until the display shows that the signal is
clean and undistorted. That way you wouldn't be trying to modulate beyond
the modulation capability of the transmitter. Just talk into the mic and not
worry about being too close or too far away, without having to ride the AF
gain control. A clipper or  limiter like what's in the BC-610's outboard
speech amplifier isn't as good as a broadcast type peak limiter, but when
properly adjusted, it will still satisfactorily do the job it was  designed
to do.

>>2) I've yet to hear from any other station from MI, PA, NY, SC, or FL that
my audio wasn't up to par.>>

I would take over-the-air signal reports only for what they are worth; they
can be very subjective at best. If one is running acceptably clean,
undistorted audio, but modulating only 50%, chances are most people would
still give a good signal report under reasonably good band conditions. The
payoff will be better readability and less strain on the ears of the
receiving operator who is trying to copy the signal under marginal band
conditions and heavy QRM. When  running AM, it should always be a matter of
pride to run as close to 100% modulation as possible, without driving
anything to distortion or splatter

>>3) I have yet to blow a 250TH in 12 years of running these. They don't
even have that characteristic spot that forms on the plate if tuned too

I don't doubt that; I just meant that if you really ran that tube to the
power input level you indicated, it would probably shorten its life. A 250TH
isn't designed to run more than 500 watts DC input on AM with 2000 volts on
the plate. If the BC-610 is still in stock configuration and you are tuning
it according to factory  specifications, THERE IS NO WAY IT COULD POSSIBLY
the a.c. mains voltage runs  high enough to boost the plate voltage well
above 2000v, I would recommend backing off slightly on the loading to
proportionally reduce the plate current to less than 250 MA and keep the DC
input at 500 watts, in the interest of maximum tube and modulation
transformer life.

>>The Bird still shows accurate readings when a Kenwood is on AM and pushed
through an LK-500ZB.>>

"Accurate" what standard of comparison?

>>When I *have* allowed the mic gain to get too high, I have heard critical
audio reports.>>

That's the point. The mic gain should be kept below the point where
objectionable artefacts are heard in the audio. Were you monitoring the
modulation on the scope at the time? What was the percentage of modulation
on voice peaks? Was flat-topping of the positive peaks or other distortion
of the waveform evident on the envelope pattern?

>>The 127VAC vs. the 110VAC does make a significant difference.>>

I seem to recall that the factory specified line voltage was 115 VAC, but
you may be right, so let's assume that to be the case and do the maths. 

Boosting line voltage to 127V would raise the DC plate voltage to 2309
volts. Assuming you didn't re-adjust the antenna loading, that would cause
the 250TH plate current to increase to 289 MA.    2309V X 0.288A = 667 W
input. Assuming the same efficiency as given in the Eimac tube chart (67%),
the carrier output calculates to 447 watts. Multiplying the carrier power
times four to get the p.e.p. , 447 X 4 = 1788 W.   1788 watts is far short
of 2500 watts. And that figure doesn't take into account losses in the tank
circuit, transmission lines, tuner, etc. Let's be optimistic and assume the
overall efficiency of everything between the 250TH plate and the radiating
element of the antenna is 90%. That would indicate an actual p.e.p. of (1788
X 0.9)= 1608 watts.

But, another assumption I made above was that the plate  loading wasn't
re-adjusted to accommodate the higher plate voltage. If you reduced the
antenna coupling to bring the plate  current back down to 250MA, that would
bring the input power to  577 watts, the carrier output power to 387 watts,
the  hypothetical p.e.p. to 1546 watts, and the optimistically assumed  90%
output network efficiency yields 1392 watts.

So Bob, I  wouldn't agonise over the power level. No way is that
transmitter going to exceed the so-called "legal limit".  Just watch the
envelope pattern on the scope and make sure it shows a clean signal.


Don, k4kyv

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