[Hallicrafters] SX-28A 1st RF tube replacement

[Carl]

Thats the W6SAI 6SB7Y article you may be referencing Roy??  CQ, sometime 
in 61 if I remember. Written for the NC-240D in particular but readily 
adaptable to others. The tube has 1/4 the noise resistance of the 
6SA7/6K8 and made a noticable improvement above 20M. It morphed into the 
6BA7 while the 6SA7 became the 6BE6.

Carl
KM1H



----- Original Message ----- 
From: "Roy Morgan" <k1lky at earthlink.net>
To: "Waldo Magnuson" <magnuson at mac.com>
Cc: <hallicrafters at mailman.qth.net>
Sent: Friday, April 03, 2009 6:44 PM
Subject: Re: [Hallicrafters] SX-28A 1st RF tube replacement


>
> On Apr 3, 2009, at 4:07 PM, Waldo Magnuson wrote:
>
>> A friend has a SX-28A in which the first RF tube (6AB7) has been
>> replaced with a miniature 6AK5 tube.
>
> The substitution that comes to mind, especially to improve performance
> on the highest frequencies, is to replace the 6SA7 mixer tube with the
> 6SB7.
> (Short pause to read the notes file.)
>
> Here it is.  Sorry, I do not have the original literature cite for any
> article.  (If anyone does, I'll add it to my notes file.)
>
> "tlogan7 at cox.net wrote:
>  ...
> My SX-28 has had a number of modifications that came from a 1959 QST
> article about improving a
> number of rigs of that era.
>
> One was to put in another IF stage (under the chassis) quite a neat 
> job.
>
> Another was to make a Q-9er, this is a cascode amplifier, using a dual
> triode. Much like the VHF
> TV tuners of the tube era. Realy perks up the performance and drops
> the receiver noise.
>
> The circuit is built on a 8 pin plug and is a plug replacement for the
> 1st RF amplifier tube, no
> other adjustments.
>
> Another improvement is replace the 6SA7 first mixer with a 6SB7. This
> is a pin for pin
> replacement, except it was designed for use in FM receivers, and has a
> top end of over 100MHz."
>
> I did not find any reference in my notes file to the 6AK5
> substitution, nor did a quick Google search turn up anything.  If I
> were going to make a substitution, I'd find an octal tube that would
> do the job and not add a miniature socket and so on.  If I wanted much
> improved sensitivity, I'd shut off the SX-28 and turn on the R-390A.
> HAH!
>
> Of course we can assume your friend is just interested in what has
> been done to his radio, and is not setting out to modify one.
>
> Roy
>
>
> Roy Morgan
> k1lky at earthlink.net
> 529 Cobb St.
> Groton NY, 13073
>
>
>
>
>
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