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

[K6JEK]

All of this was from a single 1959 QST article?  They sound  
relatively unobtrusive.

Thanks,

Jon

On Apr 3, 2009, at 3:44 PM, Roy Morgan wrote:

>
> 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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