[AMRadio] Re: looking for linear amplifier

[Donald Chester]

>From: "Jim Bromley" <k7jeb at qsl.net>

> > Continental Electronics, a long time manufacturer of
> > broadcast transmitters, never offered a plate modulated
> > product.  ...

>Yeah, but their linear amplifiers were Doherty types
>that achieved high carrier-level efficiency by use
>of a relatively complex two-tube (carrier and peak)
>scheme. They used that method to get up to the
>1 Megawatt level for BC transmitters...

That is true, but their lower-power models (250-1000 watts), of most 
interest for ham use, used more or less conventional low-level modulation.  
I remember their mid-50's 250 watt transmitter used a pair of 4-250's in the 
final.  They used a scheme that was a combination of control grid and screen 
grid modulaiton, along with negative feedback achieved by sampling the rf 
output from the transmitter, detecting it, and feeding the relulting audio 
signal back to an early af amplifier stage.  The DC input was 750 watts and 
carrier output was 250 watts - exactly what the theory says it should be.  
There was only one audio transformer in the whole rig - the 500-ohm line 
input transformer to the 1st audio stage.

I suspect the complexity of the Doherty system made it uneconomical for 
lower power levels.  BTW, there is another system similar to Doherty, the 
Terman-Woodward system.  It works on the same principle, but uses control 
grid modulation instead of linear amplification to achieve modulation.

>However, once
>PWM became widely accepted, they adopted that for
>most of their transmitter line.

As did most other manufacturers who had always relied on high level class-B 
plate modulation.  Of course, PWM is a form of high level plate modulation.  
Once a way around heavy audio transformers was achieved, Continental finally 
adopted it.

>Will it fit on my desktop and weigh less than 100 lbs?

If that's what you are  looking for, you might be interested in  "class-E".  
I saw a prototype built by Steve, WA1QIX, at the AM Forum at Dayton.  It 
would  run "legal limit", and was about the  size of a modern-day desktop 
linear, and weighed about 20 lbs.  The heavy part was the external power 
supply; the weight was typical for something of that power level, and it was 
about the same physical size as the  transmitter.  Instead of tubes, the 
unit uses a handful of power FET's that cost a couple of bucks each.  Much 
less expensive than tube(s), and without the weight and phase distortion of 
transformer coupled audio.  Check out Steve's web site at :

http://www.classeradio.org/

Good luck.

Don K4KYV

_________________________________________________________________
Get dial-up Internet access now with our best offer: 6 months @$9.95/month!  
http://join.msn.com/?page=dept/dialup