[AMRadio] AM Amps


Gary Blau gblau at w3am.com
Tue Jan 11 23:45:41 EST 2005


Hey Geoff:

Since there's already a screen at hand, why not use it to your advantage
in this case by making it variable to allow clean adjustment of output
power?  It just seems like what the doctor ordered for driving a linear,
that's all.  


As for your second question, I should have clarified that I was not
thinking of a simple phase inversion aimed at impressing the highest
voice peaks to the negative modulation direction, (the primary idea of
the article) but taking it to an extreme of radically reducing the
positive peaks by some means (such as very agressive positive peak
clipping, or unplugging the positive tube in your push pull
modulator!).  Of course, simply inverting the phase in and of itself
will not hurt quality in any way.  But while aggressive positive
limiting of some sort would allow higher carrier power before reaching
the PEP limit of 1500watts, it will also increase distortion.  How bad
or tolerable it might be depends on a lot of variables and the limiting
techniques employed.  If you just want the most intense 'communications
quality' result possible from the rig at hand, then it might make some
sense.   

But in the real world, how much of a potential benefit is at stake
here?  Even wildly asymmetrical voices aren't going to buy more than
several dB relative difference between positive and negative peak
voltages, an amount that can easily be made up for with modest audio
limiting.  Since some sort of negative peak limiting should be used
anyway to protect from carrier pinch off, some amount of that asymmetry
is going to be given up right there.  Finally, if your voice doesn't
happen to be wildly asymmetrical, you're out of luck anyway.  
A lot of AM hams don't seem to use any negative peak control other than
the mic gain pot, and many don't even have a scope to monitor for
carrier pinch off, so a lot of this is like counting pixies on the head
of a pin.  

FWIW my prejudice is looking at this as a broadcast engineer, which may
be a bit different than some AM ops.  Not better, just different.  That
prejudice steers me toward high audio quality, consistently very high
average modulation levels (loudness) being almost always more useful
than modest increases in carrier power, and a paranoid fear of
negatively overmodulating.  I admit to impatience with low power
stations that do not agressively modulate to make up for it, which is
common.  Sorry.  My object is to rattle the speaker on the other end,
and make the station easy to listen to no matter what power level is in
use.  

But, clearly hams can operate successfully without concerning themselves
with any of this and still have a ripping good time.  We're all looking
for our own buzz.

g      


Geoff wrote:
> 
>
>...I'll be the first one to admit that I'm 'weak' when it comes to
> pentode/tetrode design/operation.  I like triodes.  Their easier
> to work with, and require fewer power supplies.  Less can go wrong.
> 
> 
> ...Why wouldn't it sound as good?  You've just reversed the 'phase'
> of the audio if you, say, switch the grid caps on the modulators,
> or switched the plate caps on the modulators, even reversing the
> polarity of the microphone would have *basically* the same
> effect.  Yes, your positive peaks would reduce, and you can run
> the carrier level back up.
> 
> At 1500wPEP output (as John so eloquently described in his
> article) with his rig and voice, he would have to keep his rig at
> 220w input (around 160w of carrier out) to keep within the 1500w
> limit.  Inverting the audio phase, he could probably run 1000w of
> carrier, with PEP audio to 1500w, still have the same QUALITY of
> audio, -and- probably be heard better, due to the lack of
> interference from the 160w carrier, to the 1000w carrier.
> It just wouldn't sound -as loud-.




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