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

[Roy Morgan]

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