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[AMPS] Why build high level AM?

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Subject: [AMPS] Why build high level AM?
From: (John Lyles)
Date: Fri, 13 Jul 2001 13:49:27 -0600
Using a 4 quadrant balanced modulator to create the AM signal does 
appeal in a simple way. It only requires a linear amplifier to follow 
it. In some of the small 20 watt carrier current AM transmitters that 
LPB (Low Power Broadcast Co) made, they used this approach. And, as 
Rich says, it is used in some of the rice boxes for both AM and SSB 

This approach can be had with the limitation that it reaches carrier 
cutoff and goes beyond, basically crossing over at 100% mod. Beyond 
this it continues to multiply the carrier and audio if you look on a 
scope. So to prevent this from causing distortion in the detectors, 
it must be limited in peak. With high level modulation such as 
collector, plate or drain modulation via audio voltage, the carrier 
reaches cutoff at near zero volts and can even go above 2 x DC for 
the positive peak, for assymetrical audio. This is utilized by AM 
broadcasters for loudness. Reaching -100% does cause pinch off of the 
carrier, and much distortion ensues in the detector, esp with 
envelope demodulation.

Building a 400 watt class AB1 or B audio amplifier and finding a 
tranny with the proper Z, along with a class C or D RF amp, is a lot 
more challenging than building a linear RF amplifier. If I am going 
to have to suffer with heating the shack to get a few hundred watts 
of RF out, I might as well do it with a box of  tubes. Otherwise I 
might just go use the Rice box and SB220. I think the same arguments 
are being made in the high end audio world, no?

R. L. Measures said:

>/\  John - I have done AM with plate-modulation and with low level
>modulation using an AN-612  modulator IC - followed by a >1499w-PEP
>linear amplifier.   With traditional plate/anode modulation, some IM
>distortion could be heard.  With the AN-612 modulator/linear set up, no
>distortion could be heard.  /  note:  the Trio-Kenwood TS-440, 930, 940,
>et cet. - as well as some other mfg's transceivers use the AN-612 for
>generating AM and SSB.  Motorola makes a similar modulator-IC.

Hg vapor rectifiers are purely esthetic here, but not in a technical 
context. The electrical noise and propensity to flash back, as well 
as the peak current limitations, preclude them from most power 
supplies today. I am "probably" going to use SCHS7500 7.5 kV PRV 2 
Amp diodes in FW CT arrangement to use the Stancor transformer I have 
to get 2100-2500 VDC. Not sure about the ozone comment however, as I 
would have thought that the purple haze tubes emit no more than other 
sealed devices such as diodes. I suppose that they have a high UV 
content in the radiated light, however, and that it ionizes the 
surrounding air. Hmmm.

>/\  Mercury-vapor rectifier hash is hardly low-noise.
>Ozone is not healthy for humans to breathe. 

>  >I have heard that some of those old CT secondary transformers can't
>>run FWB at full tilt without risking punch through near the center of
>>the winding.
>/\  The UTC S-type were infamous for this gripe.

Yes the old UTC catalogs make the comment that the center tap shall 
never be used by lifting ground to operate at low power or standby. 
The S-iron was also their economy stuff for ICCS operation, CG and 
others for commercial duty 24 hours a day.

>/\  The main problems with running AM these days is that it wastes watts,
>and virtually all AMers tend to take longish,  "lock to talk"
>transmissions that tend to be a sure-cure for insomnia.

Yep, isn't it wonderful that hams are relaxing again, yawn. Later....


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