[Heathkit] Heath Warrior as an AM amplifier

[Richard Knoppow]

----- Original Message ----- 
From: "James M. Walker" <chejmw at buffalo.edu>
To: <heathkit at mailman.qth.net>
Sent: Tuesday, January 17, 2012 7:11 AM
Subject: Re: [Heathkit] Heath Warrior as an AM amplifier


> Or a classic case of "YOU CAN'T GET THERE FROM HERE" !
> Strange!
> Jim
> WB2FCN

    Linear amplfiers were used for many years by broadcast 
stations. One characteristic of these transmitters is low 
plate efficiency. A Class-B linear will have, in principle, 
no plate current in its quiescent state, i.e. no signal, and 
about 66% with maximum signal. When used for AM the carrier 
is there all the time and is well below the maximum output 
of the amplifier so its efficiency for the carrier is quite 
low. Since even processed speech and music has a high ratio 
of peak to average level the average or "all day" efficiency 
of AM linear amplfiers was not much better than the 
efficiency for the carrier only. While these transmitters 
had good signal quality the low efficiency made them 
expensive to operate especially since most high power 
transmitters required lower level linear amplifiers as 
drivers. There began to be a demand for something better. 
One answer was the Dougherty circuit of Western Electric, 
which had something like 65% all-day efficiency but was hard 
to keep tuned up and had rather high distortion. Other 
manufacturers began to offer plate modulated transmitters. 
These had fairly high efficiency but required expensive 
modulation transformers. There were modifications of the 
original Dougherty circuit that substantially improved its 
performance, particularly the ones patented by James O. 
Weldon, of Contenental Electronics. Transmitters of this 
type are currently made. Another approach was the pulse 
modulated transmitter (forgotten the inventor), capable of 
very high efficiency, this is the basis of the transmitters 
made by Harris, and I think now by others. There were some 
other approaches but amateurs has switched to SSB before 
these became available. The high efficiency linears have no 
advantage for carrier suppressed signals so are not found in 
ham gear.
     Using a linear amp for AM is a throwback to the 1930s. 
A good deal of technical literature of the time covers 
broadcast transmitters using them and may be of interest.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com