[Boatanchors] National SW-3 adventures

Sun Sep 27 16:53:55 EDT 2009

Which model of SW-3 do you have?

The commonest one used two '58's and a '27.  It was the ACSW-3 as I recall. 
It is MUCH easier to replace the '58's with 6D6's and use a '76 in place of 
the '27.  Anyone who has the "doghouse" supply can just remove the 2.5 volt 
winding leads and install a small 6.3 volt @ 1 amp transformer and hook it 
up to the places the 2.5 volt leads are removed in the supply.

The 6D6's are MUCH ,more plentiful than the '58's and 'ditto' the '76.  The 
Navy used oddles of those and still PLENTY available.

The latest version of the SW-3 was the "SW-3U" which used octal based tubes 
(6J7's and a 6C5 as I remember).  You could also use 2 volt tubes and work 
it on batteries.  (1D5GP and 1H4G).  I have NEVER seen one except in the old 
post WW2 ARRL handbook catalog sections.  I tred to find one for years with 
no luck.....ever.

The was also a battery version of the earlier SW-3 that used a pair of '34's 
and a '30.

The AC version with the 6D6's/6C5 was thebest I ever owned of the lot.  In 
my later "teen years I stupidly swapped off the ACSW-3 with a complete set 
of general coverage coils from 1.8- 30 Mhz and bandspread coils for 
160/80/40/20 and 15 meters along with a "doghouse" supply.  "Silly 
youth"!!!!

My battery version was sold in a weak moment to a Japanese ham who HAD to 
have one no matter the cost.  I was never as impressed with the battery 
version as I was with the AC tube version.  For 3 tubes it was a very 
excellent receiver.

As I recall, the SW-3 was designed by James Millen when he worked for the 
National Company in Malden, MA many years ago before he founded the J.W. 
Millen Co., who made some very excellently built products.

73,

Sandy W5TVW

----- Original Message ----- 
From: "Bill Cromwell" <wrcromwell at gmail.com>
To: <boatanchors at mailman.qth.net>
Sent: Sunday, September 27, 2009 11:39 AM
Subject: [Boatanchors] National SW-3 adventures

> Hi Everybody,
>
> I have a National SW-3 that was kindly loaned to me with some sample
> plug-in coils. The idea is that I am going to use this radio to make
> new coil forms as per sample and test them. The first step was
> assembling a power supply which is not all that difficult. The second
> step is getting the radio to receive with the pair of coils that were
> supplied. Friday night I finally completed step two! Over the years
> there have been numerous repairs and possibly modifications as
> evidenced by the many solder tracks and tacked together parts.I found
> the one healthy looking cold solder joint that was depriving the
> detector of plate voltage and then we had signals coming in. The radio
> may have been "inoperable" for a while or the solder joint may have
> opened during shipping to my house.
>
> The SW-3 is a 1930s receiver - a three tube regen that would be 1-V-1.
> I had nothing to do with the design or construction of this radio but
> the rush of getting that signal into the phones and all of the
> controls doing more or less what they should was about the same as my
> homebrew projects. I already have the first, hand made coil forms
> under construction and it will be great to get on with this project.
> The idea is to make new, plug-in coils and new, plug-in coil forms
> available. National used a non-standard six pin arrangement and those
> will be first but there is no reason other pinouts cannot be produced.
> People with fabricating skills can obviously make their own. I plan to
> make details available so that anybody who wants to can roll their
> own. The same approach will work for the standard four, five, six, and
> octal pinouts for homebrewers.
>
> 270 degree slf air variable caps anybody? That's what the SW-3 has and
> 270 degree rotation tends to spread the tuning range out over and
> above reduction drives. The straight line frequency label might be
> more aptly named straightER line frequency. The tendency to bunch up
> the stations at one end of the dial is greatly diminished. In 1933
> that probably was as "staright line" as it gets. "SLF" depends on too
> many other things to be an absolute expectation. I'm thinking through
> the process to make some of these at least for my own use. The ones in
> the SW-3 are *nice*.
>
> Radio operators accustomed to digital readouts and shaft encoders who
> have never used gear like this may be in for some surprises...
> hopefully all enjoyable ones. Radios like this have to be *operated*
> with both hands and your head - I've said that before. No computer is
> going to operate any part of it for you (even though some hams have
> used regens on PSK, etc). But those signals are there loud and clear.
>
> 73,
>
> Bill  KU8H
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