[ARC5] Off Topic, Hallicrafters SX-38

[Richard Knoppow]

----- Original Message ----- 
From: "Geoff" <geoffrey at jeremy.mv.com>
To: "D C *Mac* Macdonald" <k2gkk at hotmail.com>
Cc: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Friday, July 06, 2012 8:05 AM
Subject: Re: [ARC5] Off Topic, Hallicrafters SX-38


>I left that out due to its poor AM detector, its OK on CW.
>
>
> ----- Original Message ----- 
> From: "D C *Mac* Macdonald" <k2gkk at hotmail.com>
> To: "Geoff" <geoffrey at jeremy.mv.com>
> Cc: "Leslie Smith" <vk2bcu at operamail.com>; "ARC-5 List"
> <arc5 at mailman.qth.net>
> Sent: Friday, July 06, 2012 10:42 AM
> Subject: Re: [ARC5] Off Topic, Hallicrafters SX-38
>
>
> And don't overlook the Halli S-20R, pre-WW II predecessor 
> of the S-40
> series!
>
> Mac - K2GKK

     Both are surprisingly good receivers.  The detector in 
the S-20R can be easily modified to the type used in the 
S-40A. This eliminates the distortion on high level 
modulation.  I suspect the original detector can be improved 
by adjusting the bias on the detector/first audio section. 
This is established by a voltage divider. Because the 
detector uses a diode in the first audio tube with common 
cathode it is affected by the cathode bias for the amplifier 
section.  Many very old receivers had a "clarifier" which 
was a pot to adjust the bias on the detector. The voltage 
divider in the S-20R does the same thing.  The later 
receivers used grid leak bias for the audio which allows 
running the cathode at ground potential and results in much 
lower distortion.
     Hallicrafters used the same detector as in the S-20R in 
the first version of the S-40 but changed it during 
production.  They finally went to a separate detector tube 
in the S-40B.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com