[Central-Electronics] Drive a Johnson KW

[Jim Candela]

Hi All,

   In some ways a Johnson Thunderbolt is similar to the Johnson Desk KW using the linear SSB input. The link below is a thread on AMFONE of a TBolt restoration, and a few tweaks to enhance AM capability:

http://amfone.net/Amforum/index.php?topic=32464.0


  The final post on page 2 shows the station: Ranger + Tbolt, NC183D. I worked Clark last night on 20m AM to AM. He had 300 watts carrier into a Mosley 4 element at 55' from Arizona. The band was essentially dead, but Clark was S9 plus, and the station sounded excellent.

There is a Monday evening net at 7:45pm eastern time on 14.330Mhz. This time might need to be shifted back an hour since it was too-dark along the east coast last night, and propagation was better in the mid west.

Jim
WD5JKO

________________________________
 From: Jim Candela <jcandela at prodigy.net>
To: "central-electronics at mailman.qth.net" <central-electronics at mailman.qth.net> 
Sent: Sunday, November 4, 2012 10:50 AM
Subject: Re: [Central-Electronics] Drive a Johnson KW
 


Good input thus far!

I think another option is to use the 200V as an all mode exciter, and run the desk KW in linear mode. This option does not use the Johnson AM modulator at all. Here all you need is 3 watts PEP drive to the 4-400's in linear mode, so this is even less drive than a CE 600L requires. So on AM, 750mw carrier modulated 100% gives 3 watts PEP.

I know the 200V has some sort of power reduction feature which might do fine. Otherwise a 10ish db attenuator might be needed.

A pair of 4-400's in linear mode will easily do 300-400 watts AM carrier output with headroom for 100% modulation so long as the output loading controls are set for 4X the carrier power. The efficiency will be around 30%, and therefore the 4-400's will be blushing orange. Make sure the fans are working or the RF deck will get real HOT.

Here is the Desk KW manual. Unfortunately the schematics are cropped.

http://www.k7jrl.com/pub/manuals/johnson/deskkw/vik-kw.pdf



Jim
WD5JKO


________________________________
From: Colin Lamb <k7fm at teleport.com>
To: Jimmy Green <tomcatham at hughes.net>; central-electronics at mailman.qth.net 
Sent: Sunday, November 4, 2012 9:26 AM
Subject: Re: [Central-Electronics] Drive a Johnson KW

I am assuming you are talking about the desk KW.  The rf drive is no problem for the CE.  However, the desk has a built in modulator but no audio driver, so you will need high level audio (probably at 500 ohms if I remember) to drive the modulators.  This may be on the order of 10 watts or so.  One of the rigs often used is the Johnson Ranger and that provides audio output from the modulators to drive the desk modulator.  Johnson also sold an accessory to use with the desk for those people who were going to drive the desk with a different rig, or a cw only rig.  That was simply a separate audio driver system.  The problem with the CE is that it uses low level audio and modulation into the linear finals, so there is no high level audio output.  

You could take low level audio output from the CE and use it to drive a pair of 6L6 drivers into a 500 ohm output (this is from memory and I did not look at the schematic).  You may want to get the old Johnson catalog out and read about the driver that was available for it.  You can probably find a schematic, too.  

73,  Colin  K7FM
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