Regarding AM modulation, regardless of the modulation scheme or method, 100%
pos and neg modulation is 100% modulation. The methods employed to attain
this make absolutely no difference in the PEP value of power. Of course
there is a difference an that is in the efficiency of the mode, largely the
PA stage is usually less, where in a Class C modulated stage the efficiency
can approach 75%, but more realistic 65%.
I have a modified Heath DX-60 where my published results, schematics and
description exists on one of the AM sites. This method provides some 120%
pos modulation and yet retains 100% or less neg modulation. This
transmitter sounds as good, actually better, than most Class C high level
modulation methods. It he performance measurements also prove this to be
true. The method and modification uses the exact same tubes of the original
Heath design. I did change the "modulator design" to configure it as a
screen modulation method. No tubes were added and no holes were drilled in
the chassis. With negative feedback, and an the enhanced internal audio
path, the frequency response is limited by the value of the screen bypass
capacitor on the 6146. The THD+n was measured at some <3% at 100%
modulation The noise floor is limited by the noise generated in the 12AX7
first audio stage, yet improved slightly with the use of a 7025 in its
place. The point, do not poo poo modulation schemes that are other than
Class C high level modulation. In many cases they can yield actually better
results.
As to driving an amp such as the Centurion, the modified DX-60 does and
excellent job yielding a carrier output about 175 watts. Keep in mind that
since the DX-60 is capable of 125% modulation then the ratio of 4 times the
carrier value no longer applies. Since the Centurion is rated at 1200 watts
PEP, the drive power must be such that the maximum positive modulation does
not exceed 1200 watts PEP.
As to the duty cycle of the Centurion, it is rated at 50%. As long as the
transmit time is about equal the receive time, there is no issue. Likewise
the same holds true when driving any linear amp with any of today's modern
radios that offer the AM mode. The drive, the tuning and loading must be
such that the amp remains linear all the way to its rated output.
73
Bob, K4TAX
----- Original Message -----
From: <bcarling@cfl.rr.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Saturday, August 02, 2014 3:43 PM
Subject: Re: [TenTec] Centurion and AM
I have read this through twice and still can't figure out what the point is
that you are making
- sorry - my bad!
I also have the two year AEET degree. Thanks to Georgia Tech. I haven't
worked on very
many commercial AM broadcast transmitters, but I understand a number of
them use
modulation methods that the expert hams find anathema, such as screen grid
mod or low
level mod. with a linear amplifier.
The transmitters don't worry about some 3 dB loss or other, they just do
their job, and are not
smart enough to go around looking worried! Hence my 422 is still not
complaining to me.
On 2 Aug 2014 at 11:55, Wade Staggs wrote:
*I will try to explain AM Transmitting while using an Amplifier designed
for SSB, as taught to me by my old Elmer((( SK. )))*
* Since SSB only outputs power when you
speak into the microphone. The SSB mode of operation averages out to be
about 30% Duty Cycle where the Tubes and Power Supply are concerned. This
allows an Amplifier built for SSB Service to employ a much lighter duty
power supply. And also the Power Rating for the Tubes and The Cooling
System, can be much higher. BUT, AM operation with Full Carrier is 100%
duty Cycle. Running an Amplifier in AM at Full Carrier Output will soon
kill the tubes and power supply. When I run my SB-220 Amplifier on AM.
All
I want is about a 200 to 250 Watt Carrier at most. This is done by
reducing
the drive on the radio. With a 200-250 Watt Carrier from an Amp rated for
an easy 1000 Watts you will now say..... what's the use? Well, when you
modulate an AM signal, the sidebands go up to 4 times the carrier output.
You are effectively going from 200 Watts to about 800 Watts PEP ((( Peak
Envelope Power ))) ...... The reason I chose 200 Watts carrier from a
1000
Watt amplifier is that this works out to about 20% duty cycle. Then the
100% Modulated AM Signal does the real work. We can't speak as to Alpha
and Ten Tec Amps but would bet that this rule still applies. An RMS type
Meter will not show this power increase but a Peak Reading Watt Meter
will.
*
* My Friend, there are people here with more
Degrees than a Thermometer. While I only have the 2 years of College to
fall back on. But, I have worked on some of the Biggest and Smallest
Transmitters in the World. So, if I am wrong about this in my tried and
true simple explanation. There will be others onboard soon to correct me.
This is however the way we run our equipment. *
* 73 for now from Wade/KJ4WS*
On Sat, Aug 2, 2014 at 11:03 AM, Brian Carling <bcarling@cfl.rr.com>
wrote:
> Yes they can. At least they did this morning in a FB QSO on 7293 KHz
> from
> FL to KY.
> Omni VII and a mighty 422 Centurion.
>
> Best regards - Bry Carling, AF4K
>
>
>
> > On Aug 1, 2014, at 8:46 PM, Barry N1EU <barry.n1eu@gmail.com> wrote:
> >
> > On Sat, Aug 2, 2014 at 12:28 AM, george fritkin via TenTec <
> > tentec@contesting.com> wrote:
> >
> >> 200 to 250 watts AM results in 800 to 1000 watts PEP at 100%
> >> modulation.
> >> Three 811 tubes can not handle that power.
> >
> > How about two 3-500Z's?
> >
> > 73, Barry N1EU
> > _______________________________________________
> > TenTec mailing list
> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
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