[AMRadio] AM power

Mon Jun 20 23:56:04 EDT 2011

Hi Geoff,

Well, your math is different than mine.

I do agree with your 90 watts carrier, 1500 watts PEP and 100% negative
modulation with an SR of 4 (3:1 positive to negative peak ratio)

With 90 watts carrier out and your SR of 4 (= 3:1 positive to negative
ratio) you will have 67 volts of carrier and 207 volts positive of audio
which equals 274 volts total for 1500 watts PEP on 50 ohms.
With the sr of 4 which is a 3:1 ratio you will have around 67 volts of
negative audio for 100% modulation on negative peaks. (207/3 = 69)

If you double your carrier power to 180 watts you will increase the carrier
voltage to 95 volts. With the same SR of 4 (3:1 ratio) you could run 179
volts of audio, again for a total of 274 volts to give 1500 watts PEP.
With an SR of 4 (3:1 ratio) your negative audio voltage would be 179/3 = 60
volts. That would equate to around 63% negative modulation if I am doing
that right.

I don't see where you get a factor of 4 for reduction of the audio?

When you reduce the voltage on your modulators so that it limits the
positive audio peaks somewhat you now have a "high level clipper" with the
modulators clipping the peaks somewhat. This will cause some splatter and
increased bandwidth.
You are doing some of what an audio processor would do for you but in your
case you have no filter to reduce spurious products generated by the
clipping action of the modulators.
Unless you have a capacitor across the modulation transformer secondary.

The Viking 11 transmitter employs a similar high level clipper by having a
slightly mismatched modulation transformer that allows the tubes to go into
slight clipping. They also have a capacitor across the mod transformer
secondary. They call it "building out the transformer" to double as a low
pass filter to limit the spurious products generated by the clipping. They
refer to it as "soft clipping" if I remember right.

I agree that by doing some clipping in the modulator tubes you can run the
audio up higher to get closer to 100% negative and still stay below the 1500
watt PEP limit when you increase the carrier power.

It all comes down to the sum of the carrier voltage and the audio voltage.
The sum needs to be below 274 volts across 50 ohms. (274 volts = 1500 watts)

If you increase the carrier voltage you need to reduce the positive audio
voltage by the same amount as the carrier voltage increased to maintain the
same PEP.

Back to the 375 watts carrier. 375 watts = 137 volts across 50 ohms.
100% positive modulation will yield 274 volts total which is 1500 watts PEP.
Any more carrier and you can not modulate it 100% on the positive peaks and
stay under 1500 watts PEP.

If your modulation audio is symmetrical:
Any less carrier than 375 watts and you will over modulate on the negative
side if you increase the audio to reach the 1500 watt PEP level with
positive peaks.

Without any type of processing and you have an unsymmetrical voice you will
have to do as Geoff does in reducing the carrier level to stay under the
1500 watt PEP limit.

73
Gary  K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Geoff Edmonson
> Sent: Sunday, June 19, 2011 11:31 PM
> To: amradio at mailman.qth.net
> Subject: Re: [AMRadio] AM power
> 
> On 06/19/2011 07:56 PM, Gary Schafer wrote:
> >
> > Hi Geoff,
> >
> > You are the one telling me I am wrong. :>)
> > I was just showing a different angle on the same thing but I think
> that you
> > interpreted it differently.
> 
> Well, let's see if we can agree on something, ok?
> 
> My voice has an SR of 4.  I use a Sure 55-S mic (balanced line out) into
> a 31-band graphic EQ (balanced line in) to a Modified Bogen PA (added
> 20db or so of inverse feedback).  The output of the Bogen comes off the
> plates of the output tubes to the WA5BXO designed solid-state audio
> drive, capable of ~600v-pk audio swing on the grids.  Basically,
> whatever audio is on the plates of the output tubes in the speech amp,
> is directly applied to the grids of the 250TH's.  the modulation
> transformer is out of a T-368 and there's 60HY @ 500mA of inductor
> shunting the DC off of the secondary of the modulation transformer.
> 
> With my SR of 4, in order to keep the negative peaks close to 100% and
> maintain 1500w PEP, I can not run my output power to more than about
> 90~95w.  We agree on that, right?
> 
> If I ran the carrier level up to twice that, I'd have to reduce the
> positive peaks by a factor of 4, to stay around ~1500w PEP, but where
> are the negative peaks?  ALSO reduced by a factor of 4, and therefore I
> would sound like I'm 25% modulated, an I'm only running now ~200w of
> carrier out.
> 
> So, what I do to run a little more carrier, is reduce the voltage on the
> 250TH's so the positive peaks are not as high, and run the carrier up to
> ~175.  I still get an SR of 3, even with half the B+ on the modulators,
> but the negative peaks are close to the baseline this way, as well.
> 
> That's how things are in -my- world.
> 
> 73 = Best Regards,
> -Geoff/W5OMR
> 
> 
> 
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