[AMRadio] Monitoring Modulation Accurately

rbethman rbethman at comcast.net
Sun Nov 17 09:56:16 EST 2013

I beg to disagree.

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.

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.

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.

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.

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 

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

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

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

I use a Shure 737 microphone fed into the dynamic audio input.

Everything I'm doing has been checked several ways. That includes 
feeding the coaxial output directly to the coaxial input of the SM-220, 
which in turn is fed directly to the TR Switch.

Bob - N0DGN

On 11/16/2013 9:22 PM, Donald Chester wrote:
> Monitoring the signal on a scope by picking it off the receiver i.f. will
> inevitably give a false picture. The highly  selective i.f. amplifier
> stages and receiver selectivity filters will alter the envelope pattern;
> furthermore, viewing another ham's over-the-air signal  through your
> receiver degrades the envelope picture even more, when you add the
> selective fading and other effects of ionospheric perturbations. The only
> way to accurately view the waveform of a transmitter is to sample the
> signal right at the transmitter's  output stage and feed that sample
> directly to the vertical input of the scope. In mine, I by-passed the built-
> in vertical amplifier altogether and feed the sample directly to the
> deflection plates of the scope tube, because I don't trust the linearity of
> the amplifier stages of a cheap Hammy Hambone monitor scope like my HO-10.
> You MIGHT get a worthwhile approximation of your waveform through the R-
> 390A if you use the 16 kc/s bandwidth position, but even then, mechanical
> filters are known to muddle up the phase relationships of the signals that
> lie within the passband.
> Are you sure the Bird is calibrated accurately? Are you running everything
> in the BC-610 within its normal ratings at the proper settings? The BC-610
> is a classic example of a transmitter that SHOULD fully comply with the pep
> nonsense, no questions asked.
> The 250TH in the BC-610 is supposed to run at 2000 volts on the plate @ 250
> MA in AM mode. That's 500 watts input. According to Eimac
> (http://tubedata.milbert.com/sheets/088/2/250TH.pdf ),  the carrier output
> should be 335 watts.  Now, that's the hypothetical plate power output,
> under ideal conditions, as measured right at the plate cap of the tube.
> Considering plate  tank circuit losses, transmission line losses, and
> antenna tuner losses (if a tuner is used), you would be lucky to even get
> 300 watts of carrier to the antenna, probably substantially less. Those
> tube chart figures are kind of like  the EPA mpg fuel efficiency ratings of
> cars.
> The BC-610  modulation transformer total primary to secondary turns ratio
> is about 1.65:1. With that much step-down, using the BC-610's common plate
> supply for the modulator and final amplifier, it is impossible to modulate
> the transmitter significantly beyond 100% positive . It probably won't even
> modulate that high before flat-topping begins to occur. Therefore, with
> that transmitter, unless it has been extensively modified including
> changing out the modulation transformer,  the p.e.p. will actually be close
> to 4 times the carrier power, most likely no more than 1200 watts, or if
> the tank circuit, tuner and feedline were 100% efficient (impossible),
> theoretically 1340 watts.
> If you are loading that transmitter heavily enough to generate 2500W PEP
> (625w carrier output), I'd suggest to be in the market for some spare
> 250THs, because that one isn't going to last very  long...  but I  suspect
> everything in the BC-610 is working normally; the problem is more likely
> that the Bird is not giving an accurate reading, possibly because it isn't
> working into a load close enough to the prescribed 50 Ω, non-reactive.
> If an AM transmitter is running 500 watts DC input or less and you know the
> carrier power is somewhere around 300-375 watts, I suggest relax, give
> yourself the benefit of a doubt and don't worry about it or agonise over
> it. When was the last time one of those periodic FCC enforcement releases
> listed anything about an amateur station, particularly an AMer, being cited
> for "too much p.e.p."?
> "Throttling the speech amplifier" of an AM transmitter to limit the p.e.p.,
> has to be the worst possible way to do it,  inevitably  resulting in an
> anaemic, undermodulated signal. The total sideband power of a full carrier
> AM signal, fully modulated,  is  low enough already; the last thing we need
> is for hams to start making it a practice to run big carriers and
> deliberately modulate less than 100%.
> Don k4kyv

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