[ARC5] AM linear amplification

[Jim Wiley]

I am told by a broadcast engineer friend (Terry, AL7CE) that modern AM 
transmitters use a collection of dozens to hundreds of "building blocks" 
that are switched on and off at an audio rate, with the resulting RF 
being combined by a (hybrid?) network of some kind.  He converted 
(replaced) the KICY 50-KW  DX50 Harris (now Gates)  transmitter at Nome, 
Alaska this way and achieved something on the order of a 90% efficiency 
(AC power in to RF out) power after the change.   Failed RF modules can 
be "hot switched" with no loss of air time.  Transmitter has NO 
microprocessors controlling the process.

How effective is it?  The transmitter experiences a 3-degree rise in 
cooling air from outside air intake to exhaust, when running at full 
rated  power.

A different scheme used by Nautel (sp?) uses a pulse modulation 
technique with similar efficiency to the Harris system.  The combining 
system  is touchy - and as someone else here said, don't try this at home.


- Jim, KL7CC



On 12/19/2016 1:24 PM, Ian Wilson wrote:
> Good excuse to use a large tube (4-125, 813, etc). No need for air 
> cooling until you
> get to one size of plate dissipation higher :)
>
> 73, ian K3IMW
>
> On Mon, Dec 19, 2016 at 12:36 PM, Kenneth G. Gordon 
> <kgordon2006 at frontier.com <mailto:kgordon2006 at frontier.com>> wrote:
>
>     On 20 Dec 2016 at 9:05, AKLDGUY . wrote:
>
>     >
>     > My BC-AO-230 puts out about 5W and I'm interested in boosting
>     that with a 6146B Class AB1
>     > linear amp. The ARRL Handbooks say that AM linear
>     amplification is similar to controlled
>     > carrier modulation and caution that efficiency is low, about
>     33%, but don't give explicit
>     > operating conditions.
>     >
>     > I assume that the (6146B) stage cannot at any time be driven to
>     the same output that it is
>     > capable of in SSB amplification. That's because with no audio
>     applied, the level of the AM carrier
>     > drive must be carefully set so that the tube's plate is
>     dissipating half of its rated dissipation. The
>     > application of audio drives the stage to a level where the plate
>     dissipation is approached but not
>     > exceeded. This level is far below the SSB PEP input.
>     >
>     > Anyone have any comments?
>
>     I have nothing to add. You have about covered it.
>
>     Using any linear amp in AM service requires keeping the plate
>     dissipation at a "reasonable"
>     level.
>
>     You obviously understand that.
>
>     I would expect no more than about 25 watts output from your amp.
>     Still, that is 5X the
>     un-amplified output. I think that would be well worth the effort.
>
>     Actually, the 807 or 1625 has a slightly greater plate-dissipation
>     rating than the 6146. Some
>     books show it as having a PD rating about 5 watts greater than a 6146.
>
>     In addition, controlled-carrier modulation has greater AVERAGE
>     efficiency than straight AM,
>     because the carrier falls to a much lower level in the absence of
>     modulation, while it
>     remains steady for AM.
>
>     CC modulation of a 6146 results in around 35 watts output.
>
>     Ken W7EKB
>
>     ---
>     This email has been checked for viruses by Avast antivirus software.
>     https://www.avast.com/antivirus <https://www.avast.com/antivirus>
>
>     ______________________________________________________________
>     ARC5 mailing list
>     Home: http://mailman.qth.net/mailman/listinfo/arc5
>     <http://mailman.qth.net/mailman/listinfo/arc5>
>     Help: http://mailman.qth.net/mmfaq.htm
>     <http://mailman.qth.net/mmfaq.htm>
>     Post: mailto:ARC5 at mailman.qth.net <mailto:ARC5 at mailman.qth.net>
>
>     This list hosted by: http://www.qsl.net
>     Please help support this email list: http://www.qsl.net/donate.html
>
>
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20161219/edabc3ae/attachment.html>