[AMRadio] GG 813 Linear in GE Ham News

W5OMR/Geoff w5omr at satx.rr.com
Thu Sep 29 12:01:42 EDT 2005

Gary Schafer wrote:

> Geoff,
> I am ONLY talking about a linear amplifier here. The drive signal is 
> already modulated. If you are modulating the final stage these 
> efficiency rules discussed here do not apply. Unless you would be 
> talking about grid modulation and then they "do apply".

Yup - you're right ;-)  I'm getting muddled, here, between 
plate-modulating a grounded grid amp, and using the grounded grid amp, 
after a low-level AM source. 

Big difference.  Sorry for the misunderstanding.

> It does not matter what type of signal is modulating the signal when 
> the linear amp is involved. Whether it is a sine wave or speech or 
> anything else. The PEP rule still applies for 100% modulation! You can 
> not exceed 100% positive peak modulation if the carrier is set at any 
> level greater than 1/4 the PEP output capability of the amplifier. 
> Unless you want to operate it in a non linear mode.

I Disagree with that.

> K4KYV said:
> "Overmodulation" is defined not necessarily as exceeding 100%, but 
> exceeding the MODULATION CAPABILITY of the transmitter.  As soon as 
> the final amplifier plate voltage goes past zero, you have exceeded 
> the modulation capability of the transmitter, whether there is still 
> rf output or not. 

That being said, if I were to take a small rig, like a Ranger, or an 
AF-67/8, modify it so that there was plenty of audio available to 
faithfully reproduce my peaked voice, (natural asymmetry) the positive 
peaks -will- reach beyond the 2:1 level, that one would observe when 
looking at an AM carrier (on a scope) modulated to 100% with a sine-wave.  

Here's an example... 
it's a short 1meg video file
The text spoken into the mic is "helloooo test, one, two threee, four."

Note that the unmodulate carrier occupies 2 squares (centimeters) on the 
scope.  The positive peaks are reaching 8 centimeters, pk-to-pk, while 
the negative cycle comes right down to the carrier.  That's a positive 
to negative peak ratio of 4:1.  Let's assume that's a 100w carrier.  4x 
the carrier for PEP would mean 400w PEP output.  That's with a standard 
2:1 ratio.  Since the voltage and current are both being doubled, how 
much power, in PEP do you think a 4:1 ratio is?

(hint:  it ain't 800w PEP)

With this in mind, if I attempt to use this 100w rig with a linear 
amplifier and 8:1 Symmetry Ratio, I'd have to have at -least- an 
amplifier with a pair of 4-1000's in it, to properly handle the maximum 
PEP power out, with a nominal 100w of drive.

The whole purpose of this part of the discussion, Gary, is to 'bust' the 
mythical "375w AM legal limit output carrier power" nonsense, and to 
alert and advise people that while PEP = 4x Carrier leve for 100% 
modulation is true for a sinewave, it ain't true for the human voice.  
Especially the peakedness of the typical male-patterned voice.

I'm not saying that you shouldn't run your AM rig to 500w DC Input 
(which, for a class C amp at 75% is around 375w) or at whatever power 
level you feel comforable with(*), but just be aware of the PEP 'Federal 
Law' that is place.

PEP = 4x the AM Carrier at 100% modulation is true, -ONLY- if you're 
using a sine-wave to modlulate the carrier.

(Operating your AM Rig without a 'Scope, is like driving your car at 
night without headlights!
-Don Chester/K4KYV
Ex. Editor: AM/Px)

73 = Best Regards,

(* = Remember in 1990, before the 1991 ruling that made this a federal 
law, when the phrase "When AM kW's are outlawed, then only Outlaws will 
be running AM kW's" was popular?)

More information about the AMRadio mailing list

This page last updated 21 Feb 2018.